JP7306687B2 - Imaging optical system - Google Patents

Description

The present invention relates to an imaging optical system suitable for use as an optical system of an interchangeable lens used in a digital camera or the like.

In general, interchangeable lenses used in digital cameras, etc., have become less restrictive in terms of back focus and rear lens diameter due to the shift to mirrorless cameras and large-diameter mounts. Due to larger size and higher performance (high definition), bright interchangeable lenses in the wide-angle to medium-telephoto range (shooting diagonal angle of view: 70-40°) have become common with f-numbers of 2.2 or less, and brighter lenses are becoming commonplace. It has been demanded. In addition, due to the miniaturization of the digital camera body, there is a demand for a more compact interchangeable lens, and furthermore, there is a demand for stabilization of aberration in the entire focusing photographing area according to consumer needs.

In order to meet this requirement, the front group (lens group) is arranged on the object side of the aperture stop in the optical axis direction, and the rear group (lens group) is arranged on the image side, so that each of the off-axis principal rays An optical system is also known that has a symmetrical arrangement type lens configuration that reduces the incident angle on the lens surface and suppresses the occurrence of astigmatism and coma on each lens surface. In particular, in this type of symmetrical arrangement type optical system, the closer the refractive power arrangement of the lenses is to the symmetry around the aperture stop, the more the coma aberration, distortion aberration, chromatic aberration of magnification, etc. cancel each other out between the front and rear groups. Therefore, it is possible to realize good aberration correction for the entire optical system. However, this kind of symmetrical arrangement type generally requires a high magnification, does not support objects at infinity, and has very different specifications such as image formation size, so it is mainly used as a copying lens. It is limited to specific fields such as lenses for photolithography and plate-making cameras. On the other hand, an imaging optical system has been proposed that can be used as a camera lens for a digital camera or the like by modifying this type of general symmetrical arrangement type lens configuration.

Conventionally, such an imaging optical system, in particular, a symmetrical arrangement type imaging optical system having an F number of 2.2 or less and three to five lenses arranged in front of and behind an aperture stop is known from Patent Documents 1 to 5. It is Patent document 1 is applied to a lens for a lens shutter type camera that becomes a quasi-wide-angle lens with an F number of 1.9 by configuring with five lenses, and patent document 2 has a quasi-angle of view of 32°. and is applied to a camera objective lens that is a quasi-wide-angle lens with an F number of 1.4. Further, Patent Documents 3 to 5 are applied to an imaging lens that is a camera lens with an F number of 2.8 in consideration of far point photography.

JP-A-62-183419 JP-A-5-80252 JP 2013-250534 A JP 2014-59466 A JP 2016-218486 A

However, the conventional lenses (imaging optical systems) disclosed in the aforementioned Patent Documents 1 to 5 have the following problems.

First, in the case of a symmetrical arrangement type imaging optical system with an F number of 2.2 or less and 3 to 5 lenses in front of and behind the aperture stop, it is basically possible to ensure optical performance on the long-distance side. Even so, it tends to be difficult to ensure sufficient optical performance on the short distance side. Thus, it is not easy to realize a bright interchangeable lens that covers the short distance side, specifically, the range from wide angle to medium telephoto range (shooting diagonal angle of view 70-40°) with sufficient optical performance. However, the current situation is that the needs of consumers who desire such interchangeable lenses are not fully met.

Second, in the case of a symmetrical arrangement type imaging optical system, there is a problem that astigmatism and chromatic aberration increase as a basic tendency when the F-number is made smaller or the imaging angle of view is changed. Therefore, from the standpoint of obtaining an interchangeable lens for use in a digital camera or the like that reduces and stabilizes fluctuations in various aberrations over the entire focusing area, is compatible with a large high-definition imaging device, and is designed to be compact and compact. , the problem could not be said to have been satisfactorily resolved, and there was room for further improvement.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an imaging optical system that solves the problems existing in the background art.

In order to solve the above problems, the present invention is a symmetrical arrangement type in which the front lens group 101 is arranged on the object OBJ side in the optical axis Dc direction with respect to the aperture stop STO, and the rear lens group 102 is arranged on the image IMG side. , a positive lens group Lf including two or three consecutive positive lenses L2, and a pair of negative lenses L1 and L4 arranged on both sides of the positive lens group Lf. A meniscus lens having a convex surface on the object OBJ side and having both surfaces facing the air space S is used as the negative lens L4 on the aperture diaphragm STO side, and a negative lens L1 with a concave surface on the object OBJ side is used as the negative lens on the object OBJ side. A front lens group 101 including at least one aspherical lens having both curved surfaces curved toward the object OBJ side of the optical axis Dc, and positive lenses L6 and L7 using two biconvex lenses are continuous. and a pair of biconcave lenses disposed on both sides of the positive lens group Lr. Two cemented lenses J56 are formed by cementing the positive lenses L6 and L7 and the negative lenses L5 and L8 by the negative lenses L5 and L8. J78, the negative lens L8 on the image side of the two cemented lenses J56 and J78 is arranged so that the image IMG side faces the air space S, and behind the negative lens L8 both sides are the air space S and a rear lens group 102 comprising a final lens L9 composed of an aspherical lens with both curved surfaces curved in the same direction of the optical axis Dc. When the focal length of the negative lens L8 on the image IMG side from the negative lens L5 is BFL, and the focal length of the entire lens system is EFL, the condition "0.7<BFL/EFL<1.6" is satisfied. characterized by

In this case, according to a preferred embodiment of the invention, when constructing the front lens group 101, the negative lens L1 is placed on the object OBJ side and is a biconcave lens with both curved surfaces curved toward the object OBJ side of the optical axis Dc. In addition, a positive lens L2 using an aspherical lens located on the object OBJ side, and a negative lens L4 disposed on the aperture stop STO side and having a convex surface on the object OBJ side can be provided. EGFL is the focal length of all the positive lenses in the front lens group 101 where the absolute value of the anomalous partial dispersion ΔθgF of the glass material in the g line and the F line is 0.015 or more. If AGMX is the maximum value of the focal lengths of all the positive lenses L2 in 101, it is desirable to configure to satisfy "0.6<EGFL/AGMX<1.2". Each of the front lens group 101 and the rear lens group 102 includes one aspherical lens, and each aspherical lens is formed to have the same shape. can be arranged symmetrically with respect to On the other hand, the front and rear partial lens groups (designated lens groups) Lp whose air gaps change during focus adjustment in the front lens group 101 and the rear lens group 102 can be configured so that a maximum of three designated lens groups Lp can be moved. In this case, the air gap that changes during focus adjustment is the air gap including the aperture stop STO, the air gap between the two positive lenses L2 . At least one of an air gap between L3 and an air gap between the two positive lenses L6 and L7 of the rear lens group 102 can be included. Furthermore, when both the focal length FFL of the front lens group 101 and the focal length RFL of the rear lens group 102 have positive power, the entire lens system is moved to the object OBJ side to change the air space on the image IMG side during focus adjustment. A configuration can be provided.

According to the imaging optical system C according to the present invention having such a configuration, the following remarkable effects can be obtained.

(1) The lens configuration is configured by a symmetrical arrangement type having symmetry with respect to the aperture stop STO, which is advantageous for a compact optical system, and the internal lens groups (partially symmetrical lenses) in the front lens group 101 and the rear lens group 102 are used. Since the power distribution of the lens groups 101 and 102 is also symmetrical, the generation and correction of aberrations in the lens groups 101 and 102 can be appropriately balanced. In particular, since the final lens L9 is an aspherical lens whose both surfaces face the air space S and whose curved surfaces are curved in the same direction of the optical axis Dc, the degree of separation between the on-axis ray and the off-axis ray increases, The effect of correcting residual aberration can be enhanced, and the effect of the symmetric arrangement type can be enhanced. This makes it possible to reduce fluctuations in various types of aberration over the entire focusing shooting range and improve their stability, while keeping the F-number below 2.2 to cover sufficient optical performance from the semi-wide-angle range to the standard lens range. A bright interchangeable lens can be obtained, and thus an optimal interchangeable lens for use in a digital camera or the like that is compatible with a large-sized high-definition imaging device and is intended to be downsized and compactized can be provided.

(2) The front lens group 101 is configured by providing, on the object OBJ side, a negative lens L1 having a concave surface on the object OBJ side, and at least one aspherical lens having both curved surfaces curved toward the object OBJ side of the optical axis Dc. , so that the convex surface is frequently used as the lens surface, and the angle of incidence and the angle of refraction can be moderated. As a result, even if the front lens group 101 enhances the convergence of incident light, it is possible to suppress the occurrence of aberrations and contribute to the overall compactness. Moreover, since the angle difference between the passing points of the rays of each angle of view on each lens surface can be reduced, it is possible to advantageously correct the aberration of a large light flux.

(3) The rear lens group 102 is provided with a negative lens L5 which is arranged on the side of the aperture stop STO and has a concave surface on the side of the aperture stop STO, is arranged on the side of the image IMG, and the side of the image IMG faces the air space S. Since the negative lens L8 is provided, it is possible to reduce the diameter of the lens used. As a result, lens processing can be performed advantageously, and a symmetrical arrangement type lens configuration can be easily constructed.

(4) The rear lens group 102 is composed of two cemented lenses J56 and J78 obtained by cementing negative lenses L5 and L8 using biconcave lenses and positive lenses L6 and L7 using biconvex lenses. The positive lenses L6 and L7 of J78 are opposed to each other, and the biconcave lens of one cemented lens J56 is configured as the negative lens L5 on the aperture stop STO side. In addition, by providing a difference in refractive index, correction of spherical aberration can be strengthened. As a result, residual aberration of the front lens group 101 can be corrected in a well-balanced manner, and aberration fluctuations can be reduced by using an adjustment space between the cemented lenses J56 and J78. Moreover, since the lens surface of the rear lens group 102 near the aperture stop STO can be opposed to the aperture stop STO, the effectiveness of the lens configuration using the symmetrical arrangement type can be further enhanced.

(5) When BFL is the focal length from the negative lens L5 on the aperture stop STO side of the rear lens group 102 to the negative lens L8 on the image IMG side, and EFL is the focal length of the entire lens system, then 0.7<BFL/ Since the configuration satisfies the condition "EFL<1.6", an appropriate refractive power balance in the rear lens group 102 can be ensured. As a result, an appropriate back focus, overall length, and brightness can be obtained, and good image performance can be obtained without placing too much load on aberration correction of the front lens group 101 and the final lens L9.

(6) In constructing the front lens group 101, according to a preferred embodiment, the negative lens L1 is placed on the object OBJ side and is a biconcave lens. If a positive lens L2 using a spherical lens located on the object OBJ side and a negative lens L4 arranged on the aperture stop STO side and having a convex surface on the object OBJ side are provided, then the front lens group 101 has a lens on the object OBJ side. Since it can be configured to include a negative biconcave lens and an aspherical lens that are resistant to , it is possible to enhance the effect of correcting aberration over a wide angle of incidence in the field angle from the quasi-wide-angle region to the standard region.

(7) According to a preferred embodiment, the focal lengths of all the positive lenses L2 in the front lens group 101 are set such that the absolute value of the anomalous partial dispersion ΔθgF of the glass material in the g-line and F-line is 0.015 or more. If EGFL and AGMX are the maximum focal lengths of all the positive lenses L2 in the front lens group 101, the front lens group can be constructed so as to satisfy "0.6<EGFL/AGMX<1.2". For 101, a glass material having anomalous partial dispersion with little wavelength dispersion is used, and two or more low-dispersion lenses are continuously used, so various aberrations such as spherical aberration, coma, and astigmatism can be corrected satisfactorily. can.

(8) According to a preferred embodiment, each of the front lens group 101 and the rear lens group 102 includes one aspherical lens, each aspherical lens is formed in the same shape, and two lenses are arranged in the direction of the optical axis Dc. If they are arranged symmetrically between the cemented lenses J56 and J78, it is sufficient to prepare the same lens, so the number of types of lenses that require processing can be reduced. can. Moreover, by symmetrically arranging the lenses, it is possible to suppress the aberration, and by changing the air gap according to the object distance within the symmetrical lens arrangement, it is possible to reduce the aberration fluctuation.

(9) According to a preferred embodiment, a maximum of three designated lens groups Lp are provided for front and rear partial lens groups (designated lens groups) Lp whose air gaps in the front lens group 101 and rear lens group 102 change during focus adjustment. is movable, it is possible to utilize the merit of canceling out aberration by the front lens group 101 and the rear lens group 102 in the symmetrical arrangement type lens configuration. can.

(10) According to a preferred embodiment, the air gap including the aperture stop STO, the two positive lenses L2 . If at least one of the air space between L3 and the air space between the two positive lenses L6 and L7 of the rear lens group 102 is included, optimum focus adjustment for the symmetrical arrangement type imaging optical system C according to this embodiment can be achieved. mechanism can be constructed.

(11) According to a preferred embodiment, when both the focal length FFL of the front lens group 101 and the focal length RFL of the rear lens group 102 have positive power, the entire lens system is moved toward the object OBJ side to the image IMG side during focus adjustment. By providing a configuration that changes the air gap between , the overall structure of the focus adjustment mechanism can be simplified, and aberration fluctuations due to changes in the distance to the object OBJ can be reduced.

A configuration diagram of an imaging optical system according to Reference Example 1 for the principle of the present invention, Longitudinal aberration diagrams of the imaging optical system according to Reference Example 1 at infinity and focusing methods F11-F12 shown in FIG. Longitudinal aberration diagrams of focus methods F13 and F14 shown in FIG. 1 of the imaging optical system according to Reference Example 1, A list of optical conditions in each example and each reference example according to the preferred embodiment of the present invention, A configuration diagram of an imaging optical system according to Example 1 of the same preferred embodiment, A configuration diagram of an imaging optical system according to Example 2, A configuration diagram of an imaging optical system according to Example 3, A configuration diagram of an imaging optical system according to Reference Example 2 related to the present invention, A configuration diagram of an imaging optical system according to Example 4 of the preferred embodiment of the present invention, FIG. 11 is a longitudinal aberration diagram of the imaging optical system according to Example 4 at infinity; FIG. 9 is a longitudinal aberration diagram of focus methods F21-F23 of the imaging optical system according to Example 4 shown in FIG. FIG. 9 is a longitudinal aberration diagram of the focus method F24-F26 shown in FIG. 9 of the imaging optical system according to Example 4; FIG. 9 is a longitudinal aberration diagram of the focus method F27 shown in FIG. 9 of the imaging optical system according to Example 4; A configuration diagram of an imaging optical system according to Example 5 of the same preferred embodiment, Longitudinal aberration diagrams of the imaging optical system according to Example 5 at infinity and focusing methods F31 and F32 shown in FIG. A configuration diagram of an imaging optical system according to Reference Example 3 related to the present invention, Longitudinal aberration diagram of the imaging optical system according to Reference Example 3 at infinity and focus method F41 shown in FIG. A configuration diagram of an imaging optical system according to Reference Example 4, Longitudinal aberration diagram at infinity of the imaging optical system according to Reference Example 4, Longitudinal aberration diagrams of focus methods F51-F53 shown in FIG. 18 of the imaging optical system according to Reference Example 4, A longitudinal aberration diagram of the focus method F54 shown in FIG. 18 of the imaging optical system according to Reference Example 4, A configuration diagram of an imaging optical system according to Reference Example 5,

Next, Examples 1-5 and Reference Examples 1-5, which are preferred embodiments of the present invention, will be cited and explained in detail with reference to the drawings.

Reference example 1

First, the imaging optical system C of Reference Example 1 showing the principle of the present invention will be specifically described with reference to FIGS. 1 to 4. FIG.

First, the configuration of an imaging optical system C according to Reference Example 1 will be described with reference to FIG. It should be noted that this imaging optical system C can be assumed to be applied to an interchangeable lens for a digital camera. In FIG. 1, OBJ indicates an object (subject), and IMG indicates an image (image sensor). Therefore, the object OBJ side is the front side in the optical axis Dc direction, and the image IMG side is the back side in the optical axis Dc direction.

The imaging optical system C according to Reference Example 1 has, as a basic configuration, a front lens group 101 arranged on the object OBJ side in the optical axis Dc direction with respect to the aperture stop STO, and a rear lens group 102 arranged on the image IMG side. It has a symmetrical arrangement type lens configuration.

The front lens group 101 includes a positive lens group Lf including two or three consecutive positive lenses L2, a negative lens L1 placed in front of the positive lens group Lf, and a negative lens L4 placed behind the positive lens group Lf. It is composed of lenses L1 to L4. More specifically, a negative lens L1 using a biconcave lens, a positive lens L2 using a biconvex lens, and a positive lens L3 using a positive meniscus lens, both surfaces (surface number i=7, 8) are surfaces in the air space S. and a negative lens L4 on the aperture stop STO side using a negative meniscus aspherical lens with a convex surface on the object OBJ side. +) (-).

As described above, when constructing the front lens group 101, the negative lens L1 having a concave surface on the object OBJ side is disposed on the object OBJ side, and both curved surfaces are at least curved toward the object OBJ side of the optical axis Dc. If one aspherical lens is included, the lens surface can be made convex and the angle of incidence and the angle of refraction can be moderated. , it is possible to suppress the occurrence of aberrations and contribute to making the entire system compact. Moreover, since the angle difference between the passing points of the rays of each angle of view on each lens surface can be reduced, it is possible to advantageously correct the aberration of a large light flux.

The rear lens group 102 includes a positive lens group Lr in which positive lenses L6 and L7 using two biconvex lenses are continuous, a negative lens L5 placed in front of the positive lens group Lr, and a negative lens L5 placed behind the positive lens group Lr. A total of four lenses L5-L8 of the lens L8 are arranged behind the negative lens L8, both surfaces (i=18, 19) face the air space S, and both curved surfaces (i=18, 19 ) are curved in the same direction of the optical axis Dc (in the example, the direction toward the image IMG side), and the final lens L9 is an aspherical lens. In this case, a biconcave lens whose image IMG side faces the air space S is used as the negative lens L8 on the image IMG side, and a biconcave lens is used as the negative lens L5 on the aperture stop STO side. The rear lens group 102 is composed of a total of five lenses L5-L9, and in particular, the four lenses L5-L8 constitute a partially symmetrical lens group, so that the power of each lens is (-) (+ ) (+) (-).

Thus, when constructing the rear lens group 102, the negative lens L5 is arranged on the side of the aperture stop STO and has a concave surface on the side of the aperture stop STO. If the negative lens L8 facing S is provided, the diameter of the lens to be used can be reduced (reduced diameter), so lens processing can be performed advantageously, and a symmetrical arrangement type lens configuration can be constructed. can be easily done.

Therefore, the imaging optical system C according to Reference Example 1 has a basic form in which the aperture stop STO is sandwiched and partially symmetrical lens groups are arranged in front and behind the aperture stop STO. Further, the nine lenses L1 to L9 in the entire lens system are all separated by an air space S. FIG.

The final lens L9 has the same shape as the negative lens L4, and is a negative meniscus aspherical lens having a concave surface on the object OBJ side reversed in the direction of the optical axis Dc. As will be described later, when the rear lens group 102 includes two cemented lenses J56 and J78, they are arranged symmetrically with respect to the two cemented lenses J56 and J78 in the direction of the axis Dc. As described above, each of the front lens group 101 and the rear lens group 102 includes one aspherical lens, and each aspherical lens is formed in the same shape and arranged symmetrically in the direction of the optical axis Dc. For example, it is sufficient to prepare the same lens, so the number of types of lenses that require processing can be reduced, and in particular, the cost of aspherical lenses to be molded and manufactured can be reduced. Moreover, by symmetrically arranging the lenses, it is possible to suppress the aberration, and by changing the air gap according to the object distance within the symmetrical lens arrangement, it is possible to reduce the aberration fluctuation.

Table 1 shows lens data of the entire lens system in the imaging optical system C of Reference Example 1. The imaging optical system C at an infinite object point has a focal length of 50.00 mm, an F number of 1.95, and a half angle of view of 23.4°.

In the "surface data" of Table 1, i indicates the surface number of the lens surface counted from the object OBJ side, and this surface number i corresponds to the code (number) shown in FIG. Correspondingly, the radius of curvature R(i) of the lens surface, the distance D(i) between the axial surfaces, the refractive index nd(i) of the lens, the Abbe number νd(i) of the lens, and the value of anomalous partial dispersion dPgF(i) , respectively. nd(i) and vd(i) are numerical values for the d-line (586.56 [nm]). The anomalous partial dispersion value dPgF(i) is synonymous with the anomalous partial dispersion ΔθgF(i). That is, dPgF(i) is a value indicating the anomalous partial dispersion of the glass material for the g-line and F-line of the lens, and is obtained by dPgF=θgf−(−1.61783×e−3)×νd+0.6414625. indicates θgf indicates the partial dispersion ratio between g-line and F-line. Further, the axial top surface distance D(i) indicates the lens thickness or the air space between the opposing surfaces. Furthermore, the unit of the radius of curvature R(i) and the surface distance D(i) is [mm]. The surface number OBJ indicates the object, STO the aperture stop, and IMG the image position. Infinity of the curvature radius R(i) is a plane, and a surface with A after the surface number i indicates that the surface shape is an aspherical surface. Blanks in the refractive index nd(i) and the Abbe number νd(i) indicate air.

In addition, the "aspherical surface coefficient" in Table 1 is defined as: Z is represented by equation (1). In Equation 1, R is the central radius of curvature, K is the conic constant, A4, A6, A8, and A10 are the 4th, 6th, 8th, and 10th aspheric coefficients, respectively, and H is the distance from the origin on the optical axis. Distance. In addition, in Table 2, "E" means "x10".

Furthermore, the "focus variable interval" in Table 1 corresponds to the focus methods [F11]-[F14] shown in FIG. That is, in the case of Reference Example 1, focus adjustment when the distance of object OBJ changes from infinity to short distance can be performed by any one of the four types of focus methods [F11] to [F14]. [F11] moves each of the three designated lens groups Lp, which are integrated with "L1-L4", "L5-L6", and "L7-L9" respectively, to the object OBJ side by different amounts, and the aperture stop STO [F12] changes "L1-L4" and "L5-L9" respectively. [F13] is a method in which the integrated two designated lens groups Lp... are moved toward the object OBJ by different amounts to change the air spacing including the aperture stop STO and the air spacing on the image IMG side. L1-L6" and L7-L9", respectively, are moved toward the object OBJ by different amounts, and the air gap between the positive lenses L6 and L7 of the rear lens group 102, The method [F14] for changing the air gaps on the image IMG side is a method in which all the lens groups "L1-L9" are moved together toward the object OBJ side to change the air gaps on the image IMG side.

2 and 3 show longitudinal aberration diagrams corresponding to the focusing methods [F11] to [F14] in the imaging optical system C of Reference Example 1. FIG. [F1s] in FIG. 2 indicates a longitudinal aberration diagram at infinity. Each longitudinal aberration diagram shows, from the left, spherical aberration (656.27 nm, 586.56 nm, 435.83 nm), astigmatism (586.56 nm), and distortion (586.56 nm). Each scale is ±0.50 mm, ±0.50 mm, ±3.0%.

In the case of the focus methods [F1s], [F11]-[F13], it is possible to shoot a flat subject perpendicular to the plane optical axis without blurring. In particular, when changing either the air space of the aperture diaphragm STO or the air space between the two positive lenses L6 and L7 in the rear lens group 102, which is the symmetrical point of the lens configuration, any focusing method can be used. It can be confirmed that good and equivalent imaging performance is obtained. In addition, since the adjustment interval at which imaging performance is good for a subject at a short distance is known, it can also be used to control the degree of curvature of field. On the other hand, in the case of the focusing method [F14], the field curvature (astigmatism) on the in-focus surface is negatively curved. It can be seen that the curve is focused on the subject, and the image or video that is farther away than the subject becomes more blurred.

In this way, if a maximum of three designated lens groups Lp are movable with respect to the designated lens groups Lp before and after the air gap that changes during focus adjustment in the front lens group 101 and the rear lens group 102, symmetry can be achieved. Since the advantage of canceling out aberrations by the front lens group 101 and the rear lens group 102 in the arrangement type lens configuration can be utilized, a flexible focus adjustment mechanism can be constructed using this advantage.

In particular, the air gap including the aperture stop STO, the two positive lenses L2 . At least one of the air space between L3 and the air space between the two positive lenses L6 and L7 of the rear lens group 102 can be included, and is the optimum for the symmetrical arrangement type imaging optical system C according to Reference Example 1. A focus adjustment mechanism can be constructed.

That is, in the case of the imaging optical system, since imaging is performed by changing the distance to the object OBJ (object), imaging performance is ensured while slightly changing the symmetry of the optical system (changing the air gap). Therefore, the imaging optical system C in Reference Example 1 having symmetry utilizes the effect of canceling aberrations before and after the position of the symmetry point to utilize the air space (air space S) including the aperture stop STO, or the front lens Designated lens groups Lp on both sides of the air space (air space S) between the two biconvex lenses (positive lenses L2, L3/L6, L7) in the group 101 and the rear lens group 102 are moved.

For example, when the entire optical system is moved for a short distance and the curvature of field changes negatively on the image IMG plane, the focal plane curves from the screen center to the periphery on the image IMG plane toward the lens side. Therefore, when focusing on the center of the screen, there is a tendency to blur toward the periphery. When this phenomenon is replaced with the object OBJ side, by focusing on the object at the center of the screen, a curve is formed in which the near object is focused as it goes to the periphery, and further distant images and videos become more blurred. Therefore, it is common to reduce the aberration by narrowing down the aperture pattern STO and widen the depth of field to make the plane object into a plane image plane. That is, the focus adjustment method for moving the lens group to reduce aberration is a photographing method in which the subject on the short distance plane (the plane perpendicular to the optical axis) is made a flat image plane so as not to be blurred.

On the other hand, as a shooting technique, by focusing on a part of the subject on the screen, the entire optical system is moved at a short distance in order to create an image that blurs the space in front of and behind it to create a photographic effect. There is also a method in which aberration is generated by moving the lens group. As in Reference Example 1, when the adjustment interval that allows obtaining good imaging performance for a subject at a short distance is known, it can also be used to control the degree of curvature of field.

In the case of a wide-angle lens in which aberration fluctuation is small even when the distance to the object OBJ is changed and the amount of movement of the lens group is small, only the air gap on the image IMG side is changed by moving the entire imaging optical system. In some cases, the focus is adjusted by moving a single lens group without changing the air gap on the image IMG side, such as the inner focus method and the front focus method.

When both the focal length FFL of the front lens group 101 and the focal length RFL of the rear lens group 102 have positive power, the entire lens system is moved to the object OBJ side to change the air space on the image IMG side during focus adjustment. can be adopted. In this case, while simplifying the overall structure of the focus adjustment mechanism, it is possible to reduce aberration fluctuations due to changes in the distance from the object OBJ. FIG. 4(c) shows the values of the focal lengths FFL and RFL in Example 1-3 and Reference Examples 1-2 and 4-5.

Since such an overall extension system can be configured with a compact and simple mechanism, it is adopted in many other imaging optical systems. Among the various focusing methods described above, if the moving distance of all lens groups is small, a focusing method [F14] that moves the entire lens system toward the object OBJ can be adopted. performance can be ensured. That is, in the overall extension method, when the distance to the object OBJ changes from infinity to a short distance, the position of the image IMG changes, so focusing is performed by moving the entire lens toward the object OBJ. In the imaging optical system C according to Reference Example 1, the lens configuration is of a symmetrical arrangement type having a symmetrical lens group or a partially symmetrical lens group with respect to the aperture stop STO, and aberration fluctuations due to changes in the distance from the object OBJ are reduced. In the case of a wide-angle lens configuration in which the amount of lens movement is small, it is possible to employ a full extension system.

In addition, the imaging optical system C according to Reference Example 1 is set so as to satisfy predetermined optical conditions. The first optical condition (applied to Reference Example 1 and Examples 1-5) is that the focal length from the negative lens L5 on the aperture stop STO side of the rear lens group 102 to the negative lens L8 on the image IMG side is BFL, and the entire lens is When the focal length of the system is EFL,
0.7<BFL/EFL<1.6 (optical condition 1)
It is set as a condition to satisfy

In the case of an imaging optical system (imaging lens), the refractive power of the front lens group 101 can allow a wide range of numerical values from "positive" to "negative", but the refractive power of the rear lens group 102 allows the entire lens system to be at a predetermined focal point. It should be set to "Positive" in order to be imaged at the image IMG position at a distance. In the case of the front lens group 101, which has five or less lenses, the entire lens group can be configured as a partially symmetrical lens group. , the refractive power cannot be increased, and almost all the refractive power is borne by the rear lens group 102 except for the final lens L9.

Therefore, by setting so as to satisfy the optical condition 1, an appropriate refractive power balance in the rear lens group 102 can be ensured. As a result, an appropriate back focus, overall length, and brightness can be obtained, and good image performance can be obtained without placing too much burden on aberration correction of the front lens group 101 and the final lens L9. Therefore, if the optical condition 1 is not satisfied, an appropriate back focus, total length, and brightness cannot be obtained, and an excessive load is applied to the aberration correction of the front lens group 101 and the final lens L9, resulting in a good image. performance cannot be obtained. In the case of Reference Example 1, as shown in FIG. 4A, EFL is 50.00 and BFL is 40.40. Therefore, BFL/AFL is 0.81, which satisfies the optical condition 1.

The second optical condition (applied to Reference Examples 1, 2, 4-5 and Example 1-3) is the abnormal partial dispersion of the glass material at the g-line and F-line of the positive lens L2 in the front lens group 101. Let EGFL be the focal length of all the positive lenses whose absolute value of ΔθgF is 0.015 or more, and let AGMX be the maximum focal length of all the positive lenses L2 in the front lens group 101.
0.6<EGFL/AGMX<1.2 (optical condition 2)
It is set as a condition to satisfy

By setting in this manner, the front lens group 101 uses a glass material having anomalous partial dispersion with little wavelength dispersion, and two or more low-dispersion lenses are continuously used. It is possible to satisfactorily correct various aberrations such as

In general, as the angle of view becomes narrower, more chromatic aberrations such as longitudinal chromatic aberration and chromatic aberration of magnification occur, which tends to degrade optical performance. It is known that chromatic aberration can be corrected by using a glass material. If the above-mentioned optical condition 2 is satisfied and correction is performed to reduce chromatic aberration on the object OBJ side of the front lens group 101, the burden of aberration on the subsequent lens group, that is, the rear lens group 102 can be reduced. In addition, by continuously using two or more low-dispersion lenses, the power per lens can be weakened. As a result, the number of refracting surfaces increases, and various aberrations such as spherical aberration, coma, and astigmatism can be reduced by the correction effect.

Moreover, when the front lens group 101 is constructed as a partially symmetrical lens group, the refractive power of each lens can be weakened, so that the incident angle of the incident light with respect to the lens surface through which the incident light passes can be kept low, and the occurrence of aberrations and errors. Sensitivity can be suppressed. In a low-refractive-index lens having anomalous dispersion, if the curvature is weakened, the desired refractive power cannot be obtained, so the angle of incident light on the lens surface tends to increase. Therefore, continuous use of the low-dispersion lens becomes effective, and if the refractive power is increased by adding the high-refractive-index lens to the front lens group 101, it can be used as an anomalous-dispersive low-refractive-index lens capable of correcting chromatic aberration. can be used effectively.

In the case of Reference Example 1, as shown in FIG. 4B, EGFL is 57.05 and AGMX is 58.54. Therefore, EGFL/AGMX is 0.97, which satisfies the optical condition 2.

In Reference Example 1, the power distribution of each lens in the partially symmetrical lens group of the front lens group 101 is (negative)-(positive)-(positive)-(negative), and positive lenses L2 and L3 are divided by negative lenses L1 and L4. The power distribution of each lens in the partially symmetrical lens group of the rear lens group 102 is set to (negative)-(positive)-(positive)-(negative), as in the front lens group 101. have symmetry such that the positive lenses L6 and L8 are sandwiched between the negative lenses L5 and L8, the negative lenses L5 and L8 are biconcave lenses, and the positive lenses L6 and L7 are biconvex lenses, Since the two positive lenses L6 and L7 are opposed to each other with an air gap between them, the generation and correction of aberrations in each group are balanced. As a result, it is possible to ensure good mutual aberration balance as a whole.

In addition, the final lens L9 has an aspherical surface that can partially change the surface shape by utilizing the fact that the on-axis ray and the off-axis ray pass separately, thereby correcting the residual aberration. Furthermore, by providing an air space between the partially symmetrical lens group of the rear lens group 102 and the final lens L9, the degree of separation between the on-axis ray and the off-axis ray is increased, the effect of the aspherical lens is increased, and the symmetrical arrangement is achieved. In order to maintain the effect of the type, we used an aspherical lens with both sides facing the same direction. In the imaging optical system C, since the distance between the space on the object OBJ side and the space on the image IMG side is not symmetrical, good aberration correction cannot be achieved with a completely symmetrical lens configuration. For this reason, the front lens group 101 has a strong convergence effect so as to converge light rays from a large object.

Next, the imaging optical system C of Example 1 according to the present embodiment will be described with reference to FIGS. 4 and 5. FIG.

In the imaging optical system C of Example 1, the negative lens L5 using a biconcave lens and the positive lens L6 using a biconvex lens in the rear lens group 102 of Reference Example 1 described above are configured as a cemented lens J56. The positive lens L7 used and the negative lens L8 using a biconcave lens are composed of a cemented lens J78. The biconcave lens of the lens J56 is configured as the negative lens L5 on the aperture stop STO side. The configuration of the front lens group 101, that is, the lenses L1, L2, L3, L4 and the final lens L9 are the same as those of the first reference example.

Table 2 shows lens data of the entire lens system of Example 1. The imaging optical system C at an infinite object point has a focal length of 49.80 mm, an F-number of 1.95, and a half angle of view of 23.5°.

As described above, the rear lens group 102 is composed of two cemented lenses J56 and J78 obtained by cementing negative lenses L5 and L8 using biconcave lenses and positive lenses L6 and L7 using biconvex lenses. If the positive lenses L6 and L7 of the cemented lenses J56 and J78 are arranged to face each other and the biconcave lens of one of the cemented lenses J56 is configured as the negative lens L5 on the aperture stop STO side, a difference in Abbe number can be obtained by Off-axis chromatic aberration can be easily and effectively corrected, and spherical aberration can be more strongly corrected by providing a difference in refractive index. As a result, residual aberration of the front lens group 101 can be corrected in a well-balanced manner, and aberration fluctuations can be reduced by using an adjustment space between the cemented lenses J56 and J78. Moreover, since the lens surface of the rear lens group 102 near the aperture stop STO can be opposed to the aperture stop STO, the effectiveness of the lens configuration using the symmetrical arrangement type can be further enhanced.

Also, there are four types of focus adjustment for a short-distance object, which are the same as in the first reference example. In the case of Example 1, as shown in FIG. 4A, EFL is 49.80 and BFL is 39.40. Therefore, BFL/AFL is 0.79, which satisfies the optical condition 1 described above. Furthermore, as shown in FIG. 4(b), EGFL is 57.05 and AGMX is 58.54. Therefore, EGFL/AGMX is 0.97, which satisfies the optical condition 2 described above. In addition, as shown in FIG. 4(c), FFL has a positive power of 108.49 and RFL has a positive power of 75.31.

As described above, even if the lens configuration in the partially symmetrical lens group in the rear lens group 102 is configured as cemented lenses J56 and J78 with respect to the basic form of Reference Example 1, it is configured by a combination of single lenses. Imaging performance equivalent to that of the basic form of Reference Example 1 can be ensured.

Next, an imaging optical system C of Example 2 according to the present embodiment will be described with reference to FIGS. 4 and 6. FIG.

In Example 2, as shown in FIG. 6, the front lens group 101 includes five lenses L1, LS, L2, L3, and L4, and the rear lens group 102 includes five lenses L5, L6, L7, and L8. , L9. In this case, a biconcave lens is used as the negative lens L1 of the front lens group 101, and a meniscus lens having a convex surface on the object OBJ side is used as the positive lens LS, and the negative lens L1 and the positive lens LS are constructed as a cemented lens. The positive lenses L3 and L4 have the same shape, and are composed of positive meniscus lenses having both surfaces convex toward the object OBJ. In this way, if a meniscus lens having both surfaces convex toward the object OBJ side is used as the positive lens of the front lens group 101, the overall length can be reduced, so that the refractive power can be made nearly uniform, and the front lens group 101 can be made more compact. Since the device can be made compact and the incident light angle of light rays can be made small, the occurrence of aberration and error sensitivity can be suppressed.

Further, the glass material of the positive lenses L3 and L4 has an abnormal partial dispersion value of 0.0375. Further, the negative lens L5 is a negative meniscus lens having both surfaces convex on the object OBJ side, and is composed of an aspherical lens. The front lens group 101 is configured as a partially symmetrical lens group, and the powers of the lenses L1 to L4 are (-) (+) (+) (+) (-).

The rear lens group 102 includes a cemented lens J56 composed of a negative lens L5 using a biconcave lens and a positive lens L6 using a biconvex lens, and a positive lens L7 using a biconvex lens and a negative lens L8 using a biconcave lens. The positive lenses L6 and L7 of the two cemented lenses J56 and J78 are opposed to each other, and the biconcave lens of one of the cemented lenses J56 is used as the negative lens L5 on the aperture stop STO side. Configured. As a result, the four lenses L5 to L8 are configured as a partially symmetrical lens group having powers of (-) (+) (+) (-). In Example 2, too, the aperture stop STO is interposed therebetween, and symmetrical partial lens groups are arranged in front of and behind the aperture stop STO. The final lens L9 has the same shape as the lens L4, and is a negative meniscus aspheric lens having a concave surface on the side of the object OBJ reversed on the optical axis Dc.

Table 3 shows lens data of the entire lens system of Example 2. The imaging optical system C at an infinite object point has a focal length of 48.25 mm, an F number of 1.74, and a half angle of view of 24.2°.

There are four types of focus adjustment for a short-distance object, which are the same as in the first reference example. In the case of Example 2, as shown in FIG. 4A, EFL is 48.25 and BFL is 40.77. Therefore, BFL/AFL is 0.85, which satisfies the optical condition 1 described above. Further, as shown in FIG. 4B, the lenses L2 and L3 both have an EGFL of 94.26 and an AGMX of 94.26. Therefore, EGFL/AGMX is 1.00, which satisfies the optical condition 2 described above. In addition, as shown in FIG. 4(c), FFL has a positive power of 131.90 and RFL has a positive power of 60.82.

As described above, even when the number of each lens group and the lens configuration are changed from the basic configuration of Reference Example 1, the same imaging performance as that of the basic configuration of Reference Example 1 can be ensured.

Next, an imaging optical system C of Example 3 according to the present embodiment will be described with reference to FIGS. 4 and 7. FIG.

Example 3 is a type in which the angle of view of Example 1 is further widened, and as shown in FIG. Configured. In this case, the powers of the lens groups 101 and 102 are configured as partially symmetrical lens groups of (-) (+) (+) (-). As the final lens L9, a negative meniscus aspherical lens having the same shape as the lens (negative lens) L4 of the front lens group 101 and having a concave surface on the object OBJ side reversed on the optical axis Dc is used.

Table 4 shows lens data of the entire lens system of Example 3. The imaging optical system C at the point of infinity has a focal length of 37.00 mm and an F number of 1.95.

There are four types of focus adjustment for a short-distance object, which are the same as in the first reference example. In the case of Example 3, as shown in FIG. 4A, EFL is 37.00 and BFL is 31.85. Therefore, BFL/AFL is 0.86, which satisfies the optical condition 1 described above. Furthermore, as shown in FIG. 4B, the lens L3 has an EGFL of 41.12 and an AGMX of 45.23. Therefore, EGFL/AGMX is 0.91, which satisfies the optical condition 2 described above. In addition, as shown in FIG. 4(c), FFL has a positive power of 97.17 and RFL has a positive power of 51.20.

In this way, when adopting the same four types of focus adjustment as in Reference Example 1 (Embodiment 1), if the basic form of Reference Example 1 is maintained, the specific configuration of details will be different. Even in this case, imaging performance equivalent to that of the basic configuration of Reference Example 1 can be ensured.

Reference example 2

Next, an imaging optical system C of Reference Example 2 related to the present embodiment will be described with reference to FIGS. 4 and 8. FIG.

In Reference Example 2, the angle of view is widened in the same manner as in Example 3, and in particular, the F number is increased to 1.26. As shown in FIG. , the number of lenses in the rear lens group 102 is five. That is, the front lens group 101 includes a biconcave negative lens L1, a biconvex positive lens L2, a positive meniscus lens (positive lens) L3 having a convex surface on the object OBJ side, and a negative meniscus lens (positive lens) L3 having a convex surface on the object OBJ side. The power of each lens is (-) (+) (+) (-), and is configured as a partially symmetrical lens group.

The rear lens group 102 includes a cemented lens J56 composed of a negative lens L5 using a biconcave lens and a positive lens L6 using a biconvex lens, and a positive lens L7 using a biconvex lens and a negative lens L8 using a biconcave lens. The positive lenses L6 and L7 of the two cemented lenses J56 and J78 are opposed to each other, and the biconcave lens of one of the cemented lenses J56 is used as the negative lens L5 on the aperture stop STO side. Configured. As a result, the four lenses L5 to L8 are configured as a partially symmetrical lens group having powers of (-) (+) (+) (-). In Reference Example 2 as well, the aperture stop STO is interposed, and partially symmetrical lenses are arranged in front and behind it. The final lens L9 is a negative meniscus aspherical lens having a concave surface on the object OBJ side.

Table 5 shows lens data of the entire lens system of Reference Example 2. The imaging optical system C at an infinite object point has a focal length of 51.14 mm, an F-number of 1.26, and a half angle of view of 22.9°.

There are four types of focus adjustment for a short-distance object, which are the same as in the first reference example. In the case of Reference Example 2, as shown in FIG. 4A, EFL is 51.14 and BFL is 51.43. Therefore, BFL/AFL is 1.01, which satisfies the optical condition 1 described above. Also, as shown in FIG. 4(c), FFL has a positive power of 128.46, and RFL has a positive power of 65.65.

Therefore, even when the imaging optical system C is widened and the F number is increased to 1.26 as in Reference Example 2, the basic form of Reference Example 1 is the same as in Example 1-3. It is possible to secure the same imaging performance as

Next, an imaging optical system C of Example 4 according to the present embodiment will be described with reference to FIGS. 4 and 9 to 13. FIG.

In the fourth embodiment, the front lens group 101 is composed of five lenses, and the rear lens group 102 is composed of five lenses. That is, the front lens group 101 includes a negative lens L1 using a biconcave lens, a positive lens LS using an aspheric lens having a positive meniscus with both surfaces convex toward the object OBJ side, and a positive lens L2 using a biconvex lens. L3, composed of a negative meniscus lens (negative lens) L4 having a convex surface on the object OBJ side, and configured as a partially symmetrical lens group in which the power of each lens is (-) (+) (+) (+) (-). . The lenses L2 and L3 have substantially the same refractive power, the abnormal partial dispersion value of the glass material in the lens L2 is 0.037, and the abnormal partial dispersion value of the glass material in the lens L3 is 0.0194. When constructing the front lens group 101, a negative lens L1 which is arranged on the object OBJ side and which uses a biconcave lens, both curved surfaces curved toward the object OBJ side of the optical axis Dc and which uses an aspherical lens is arranged on the object OBJ side. and a negative lens L4 having a convex surface on the object OBJ side. Since a spherical lens can be included in the configuration, it is possible to enhance the aberration correction effect for a wide angle of incidence in the field angle from the semi-wide-angle region to the standard region.

The rear lens group 102 includes a cemented lens J56 composed of a negative lens L5 using a biconcave lens and a positive lens L6 using a biconvex lens, and a positive lens L7 using a biconvex lens and a negative lens L8 using a biconcave lens. The positive lenses L6 and L7 of the two cemented lenses J56 and J78 are opposed to each other, and the biconcave lens of one of the cemented lenses J56 is used as the negative lens L5 on the aperture stop STO side. Configured. As a result, the four lenses L5 to L8 are configured as a partially symmetrical lens group having powers of (-) (+) (+) (-). The final lens L9 uses a negative meniscus aspherical lens having a concave surface on the object OBJ side.

As for the focus method, that is, the method for adjusting the focus when the distance of the object OBJ changes from infinity to short, seven types of focus methods [F21] to [F27] are applied as shown in FIG. That is, [F21] moves the two designated lens groups Lp . and L3, and the air space on the image IMG side. [F23] is ``L1, LS'' and ``L2-L4''. , and "L5-L9" are moved toward the object OBJ by different amounts. [F24] is a method of changing the space and the air space on the image IMG side. by different amounts to change the air gap between the positive lenses L2 and L3 of the front lens group 101, the air gap between the positive lens L3 and the negative lens L4 of the front lens group 101, and the air gap on the image IMG side, [F25] moves each of the three designated lens groups Lp . [F26] is "L1-L2", Each of the three designated lens groups Lp . [F27] is a system that changes the air gap including the aperture stop STO and the air gap on the image IMG side. In this method, the specified lens group Lp is moved toward the object OBJ by different amounts to change the air gap including the aperture stop STO, the air gap between the positive lenses L6 and L7 of the rear lens group 102, and the air gap on the image IMG side. .

Table 6 shows lens data of the entire lens system of Example 4. The imaging optical system C at an infinite object point has a focal length of 49.17 mm, an F-number of 1.94, and a half angle of view of 23.7°.

10 to 13 show longitudinal aberration diagrams corresponding to the focusing methods [F21] to [F27] in the imaging optical system C of Example 4. FIG. [F2s] in FIG. 10 shows a longitudinal aberration diagram at infinity. Each longitudinal aberration diagram shows, from the left, spherical aberration (656.27 nm, 586.56 nm, 435.83 nm), astigmatism (586.56 nm), and distortion (586.56 nm). Each scale is ±0.50 mm, ±0.50 mm, ±3.0%.

In the case of Example 4, as shown in FIG. 4A, EFL is 49.28 and BFL is 75.15. Therefore, BFL/AFL is 1.52, which satisfies the optical condition 1 described above. Further, as shown in FIG. 4B, the EGFL of the lens L2 is 60.00, the AGMX is 74.53, and the EGFL/AGMX is 0.81. The EGFL of L3 is 55.00, the AGMX is 74.53, and the EGFL/AGMX is 0.74, which satisfies the optical condition 2 described above.

In this way, the designated lens groups Lp before and after the air gap that changes during focus adjustment in the front lens group 101 and the rear lens group 102 are configured so that a maximum of three designated lens groups Lp can be moved. By varying the positions of Lp..., it is possible to ensure good imaging performance even when selecting one of the seven focusing methods [F21]-[F27], and to utilize the merits of the symmetrical arrangement type lens configuration. Various focus adjustment mechanisms can be constructed.

Next, an imaging optical system C of Example 5 according to the present embodiment will be described with reference to FIGS. 4, 14 and 15. FIG.

The basic lens configuration in Example 5 is the same as in Example 4. That is, in terms of the number of lenses, the front lens group 101 is composed of five lenses, and the rear lens group 102 is composed of five lenses. That is, the front lens group 101 includes a negative lens L1 using a biconcave lens, a positive lens LS using an aspheric lens having a positive meniscus with both surfaces convex toward the object OBJ side, and a positive lens L2 using a biconvex lens. L3, composed of a negative meniscus lens (negative lens) L4 having a convex surface on the object OBJ side, and configured as a partially symmetrical lens group in which the power of each lens is (-) (+) (+) (+) (-). . The lenses L2 and L3 have substantially the same refractive power, the abnormal partial dispersion value of the glass material in the lens L2 is 0.037, and the abnormal partial dispersion value of the glass material in the lens L3 is 0.0194.

The rear lens group 102 includes a cemented lens J56 composed of a negative lens L5 using a biconcave lens and a positive lens L6 using a biconvex lens, and a positive lens L7 using a biconvex lens and a negative lens L8 using a biconcave lens. The positive lenses L6 and L7 of the two cemented lenses J56 and J78 are opposed to each other, and the biconcave lens of one of the cemented lenses J56 is used as the negative lens L5 on the aperture stop STO side. Configured. As a result, the four lenses L5 to L8 are configured as a partially symmetrical lens group having powers of (-) (+) (+) (-). The final lens L9 uses a negative meniscus aspherical lens having a concave surface on the object OBJ side.

In the fifth embodiment, the focusing method is changed when the distance to the object OBJ changes from infinity to short. That is, one air gap between two positive lenses (both convex lenses) L2 and L3 and between L6 and L7 constituting the partially symmetrical lens group is changed, and at the same time, the amount of movement of the air gap on the image IMG side is minimized. By doing so, a so-called front focus system is employed in which the lenses L7-L9 are kept at a constant distance from the image IMG plane. As shown in FIG. 14, two types of focus systems [F31] and [F32] are applied. At [F31], two designated lens groups Lp, which are formed by fixing "L7-L9" and integrating "L1-L2" and "L3-L6" respectively, are moved toward the object OBJ by different amounts, and are moved forward. [F32] is a method of changing the air space between the positive lenses L2 and L3 of the lens group 101 and the air space between the positive lenses L6 and L7 of the rear lens group 102. [F32] fixes "L7-L9" and "L1- L6" is moved toward the object OBJ by different amounts to change the air gap between the positive lenses L6 and L7 of the rear lens group 102. FIG.

Table 7 shows lens data of the entire lens system of Example 5. The imaging optical system C at an infinite object point has a focal length of 48.73 mm, an F number of 1.94, and a half angle of view of 23.9°.

FIG. 15 shows longitudinal aberration diagrams corresponding to the focus methods [F31]-[F32] in the imaging optical system C of Example 5. In FIG. [F3s] in FIG. 15 shows a longitudinal aberration diagram at infinity. Each longitudinal aberration diagram shows, from the left, spherical aberration (656.27 nm, 586.56 nm, 435.83 nm), astigmatism (586.56 nm), and distortion (586.56 nm). Each scale is ±0.50 mm, ±0.50 mm, ±3.0%.

In the case of Example 5, as shown in FIG. 4A, EFL is 48.73 and BFL is 71.22. Therefore, BFL/AFL is 1.46, which satisfies the optical condition 1 described above. Further, as shown in FIG. 4B, the EGFL of the lens L3 is 60.03, the AGMX is 62.49, and the EGFL/AGMX is 0.96, which satisfies the optical condition 2 described above.

Thus, the focus method [F31] in which two lens groups "L1-L2" and "L3-L6" are used as moving groups, and the focus method in which one lens group "L1-L6" is integrated as a moving group are used. As in Examples 1 to 4 and Reference Example 2, any focusing method can ensure the same imaging performance as the basic form of Reference Example 1, and utilizes the merits of the symmetrical arrangement type lens configuration. Various focus adjustment mechanisms can be constructed.

Reference example 3

Next, an imaging optical system C of Reference Example 3 related to the present embodiment will be described with reference to FIGS. 4 and 16 to 17. FIG.

In Reference Example 3, the number of lenses is such that the front lens group 101 is composed of four lenses, and the rear lens group 102 is composed of five lenses. That is, the front lens group 101 is composed of a negative lens L1 using a biconcave lens, positive meniscus lenses (positive lenses) L2 and L3 having a convex surface on the object OBJ side, and a negative meniscus lens (negative lens) L4 having a convex surface on the object OBJ side. It is configured as a partially symmetrical lens group in which the power of each lens is (-) (+) (+) (-).

The rear lens group 102 includes a cemented lens J56 composed of a negative lens L5 using a biconcave lens and a positive lens L6 using a biconvex lens, and a positive lens L7 using a biconvex lens and a negative lens L8 using a biconcave lens. The positive lenses L6 and L7 of the two cemented lenses J56 and J78 are opposed to each other, and the biconcave lens of one of the cemented lenses J56 is used as the negative lens L5 on the aperture stop STO side. Configured. As a result, the four lenses L5 to L8 are configured as a partially symmetrical lens group having powers of (-) (+) (+) (-). The final lens L9 uses a negative meniscus aspherical lens having a convex surface on the object OBJ side.

In Reference Example 3, the focusing method is changed when the distance to the object OBJ changes from infinity to short distance. That is, as shown in FIG. 16, the method of focus adjustment is such that two designated lens groups Lp each integrating lenses L4 to L6 and L7 to L9 constituting the designated lens group Lp are placed on the object OBJ side. A focus method [F41] is adopted in which the air gap between the lenses L3 and L4 and the air gap between the lenses L6 and L7 are changed by moving the lenses L1-L3 at a fixed distance from the image IMG surface. A rear focus system was used to maintain the position.

Table 8 shows lens data of the entire lens system of Reference Example 3. The imaging optical system C at an infinite object point has a focal length of 49.80 mm, an F number of 1.94, and a half angle of view of 23.5°.

FIG. 17 shows a longitudinal aberration diagram corresponding to the focus method [F41] in the imaging optical system C of Reference Example 3. As shown in FIG. [F4s] in FIG. 17 shows a longitudinal aberration diagram at infinity. Each longitudinal aberration diagram shows, from the left, spherical aberration (656.27 nm, 586.56 nm, 435.83 nm), astigmatism (586.56 nm), and distortion (586.56 nm). Each scale is ±0.50 mm, ±0.50 mm, ±3.0%.

In the case of Reference Example 3, as shown in FIG. 4A, EFL is 49.80 and BFL is 47.73. Therefore, BFL/AFL is 0.96, which satisfies the optical condition 1 described above.

In this way, the two designated lens groups Lp, which are integrated with the lenses L4 to L6 and L7 to L9 constituting the designated lens groups Lp, are moved toward the object OBJ by different amounts, and the distance between the lenses L3 and L4 is increased. Even in the case of a rear focus system in which the air gap between the lenses L6 and L7 is changed and the lenses L1-L3 are kept at a constant distance from the image IMG plane, the results of Examples 1-5 and Reference As in Example 2, it is possible to secure imaging performance equivalent to that of the basic form of Reference Example 1, and to construct various focus adjustment mechanisms that utilize the advantages of the symmetrical arrangement type lens configuration.

Reference example 4

Next, an imaging optical system C of Reference Example 4 related to the present embodiment will be described with reference to FIGS. 4 and 18 to 21. FIG.

The configuration of the imaging optical system C of Reference Example 4 is the same as the basic configuration of Reference Example 1 described above. That is, the front lens group 101 includes a negative lens L1 using a biconcave lens, a positive meniscus aspherical lens (positive lens) L2 having both surfaces convex toward the object OBJ side, a positive lens L3 using a biconvex lens, and a positive lens L3 using a biconvex lens. It is composed of a negative meniscus lens (negative lens) L4 having a convex surface, and is configured as a partially symmetrical lens group in which the power of each lens is (-) (+) (+) (-).

The rear lens group 102 includes a cemented lens J56 composed of a negative lens L5 using a biconcave lens and a positive lens L6 using a biconvex lens, and a positive lens L7 using a biconvex lens and a negative lens L8 using a biconcave lens. It is composed of a cemented lens J78. The positive lenses L6 and L7 of the two cemented lenses J56 and J78 are arranged to face each other, and the biconcave lens of one cemented lens J56 is configured as the negative lens L5 on the aperture stop STO side. As a result, the power of each lens is configured as a partially symmetrical lens group of (-) (+) (+) (-). The final lens L9 is configured as a negative meniscus aspherical lens having a convex surface on the object OBJ side.

Table 9 shows lens data of the entire lens system of Reference Example 4. The imaging optical system C at an infinite object point has a focal length of 36.01 mm, an F number of 1.27, and a half angle of view of 31.4°.

There are four types of focusing methods, which are the same as those in the first reference example. [F51] to [F54] shown in FIG. 18 respectively correspond to [F11] to [F14] shown in FIG. In the case of Reference Example 4, as shown in FIG. 4A, EFL is 36.01 and BFL is 38.40. Therefore, BFL/AFL is 1.07, which satisfies the optical condition 1 described above. Also, as shown in FIG. 4(c), FFL has a positive power of 111.36, and RFL has a positive power of 40.58.

20 and 21 show longitudinal aberration diagrams corresponding to the focusing methods [F51] to [F54] in the imaging optical system C of Reference Example 4. FIG. [F5s] in FIG. 19 shows a longitudinal aberration diagram at infinity. Each longitudinal aberration diagram shows, from the left, spherical aberration (656.27 nm, 586.56 nm, 435.83 nm), astigmatism (586.56 nm), and distortion (586.56 nm). Each scale is ±0.50 mm, ±0.50 mm, ±3.0%.

Reference Example 4 is an example in which the angle of view is widened and the F-number is increased to 1.265, but each aberration ensures good imaging performance as in the other Reference Examples and Examples. In the focusing method that moves the entire lens, the focal plane of field curvature (astigmatism) curves to the positive side. There is a curve that fits, and images and videos that are closer to the subject can be obtained more blurred. In addition, since an adjustment interval that ensures good imaging performance for a subject at a short distance can be determined, it can be used to control the degree of curvature of field.

Reference example 5

Next, an imaging optical system C of Reference Example 5 related to the present embodiment will be described with reference to FIGS. 4 and 22. FIG.

In Reference Example 5, as shown in FIG. 22, the front lens group 101 includes a negative lens L1 using a biconcave lens, a positive lens L2 using a positive meniscus aspherical lens having both surfaces convex toward the object OBJ side, and an object lens L2. It consists of a positive meniscus lens (positive lens) L3 with a convex surface on the OBJ side and a negative meniscus lens (negative lens) L4 with a convex surface on the object OBJ side. ).

The rear lens group 102 includes a cemented lens J56 composed of a negative lens L5 using a biconcave lens and a positive lens L6 using a biconvex lens, and a positive lens L7 using a biconvex lens and a negative lens L8 using a biconcave lens. The positive lenses L6 and L7 of the two cemented lenses J56 and J78 are opposed to each other, and the biconcave lens of one of the cemented lenses J56 is used as the negative lens L5 on the aperture stop STO side. Configured. As a result, the four lenses L5 to L8 are configured as a partially symmetrical lens group having powers of (-) (+) (+) (-). The final lens L9 is a negative meniscus aspherical lens having a convex surface on the object OBJ side.

Table 10 shows lens data of the entire lens system of Reference Example 5. The imaging optical system C at an infinite object point has a focal length of 48.00 mm, an F number of 1.26, and a half angle of view of 24.4°.

There are four types of focusing methods, which are the same as those in Reference Examples 1 and 4. FIG. In the case of Reference Example 5, as shown in FIG. 4A, EFL is 48.00 and BFL is 45.40. Therefore, BFL/AFL is 0.95, which satisfies the optical condition 1 described above. Also, as shown in FIG. 4(c), FFL has a positive power of 126.00, and RFL has a positive power of 53.15.

Reference Example 5, in particular, has an imaging optical system C with a large aperture having an F-number of 1.261, but ensures good imaging performance as in the other Reference Examples and Examples.

Although the preferred embodiments including Examples 1-5 and Reference Examples 1-5 related to the same embodiments have been described above in detail, the present invention is not limited to such embodiments, and details Configurations, shapes, materials, quantities, numerical values, etc. can be arbitrarily changed, added, or deleted without departing from the gist of the present invention.

For example, when constructing the front lens group 101, a negative lens L1 that is arranged on the object OBJ side and that uses a biconcave lens, and an aspherical lens that has both curved surfaces curved toward the object OBJ side of the optical axis Dc, The positive lens L2 located second from the object OBJ side and the aperture stop STO side are arranged, and the object OBJ side has a convex surface, and the image IMG side curved surface is set to have a curvature smaller than a predetermined size. Although the lens L4 is provided and the shape of the surface in contact with the air space S from the aperture stop STO to the third is opposed to the aperture stop STO, the configuration of each lens used is another configuration. does not exclude In the front lens group 101, EGFL is the focal length of all lenses having an absolute value of 0.015 or more for the anomalous partial dispersion ΔθgF of the glass material in the g-line and the F-line of the positive lenses L2 and L3. Assuming that the maximum value of all the positive lenses in the rear lens group 102 is AGMX, it is desirable to satisfy the condition "0.6<EGFL/AGMX<1.2". does not exclude

INDUSTRIAL APPLICABILITY The imaging optical system according to the present invention can be used as a dedicated lens or an interchangeable lens in various optical devices such as digital cameras and video cameras.

C: imaging optical system, 101: front lens group, 102: rear lens group, STO: aperture stop, OBJ: object, IMG: image, Dc: optical axis, L1: negative lens, L2...: positive lens, L4: negative Lens, L5: negative lens, L6...: positive lens, L7: positive lens, L8: negative lens, L9: final lens, Lf: positive lens group, Lr: positive lens group, Lp...: designated lens group, J56: cemented lens, J78: cemented lens, S: air space

Claims (7)

In an imaging optical system having a symmetrical arrangement type lens configuration in which the front lens group is arranged on the object side in the optical axis direction with respect to the aperture stop and the rear lens group is arranged on the image side, two or three consecutive positive lenses are used. It has a positive lens group including a lens, and a pair of negative lenses arranged on both sides of this positive lens group, and uses a meniscus lens with a convex surface on the object side, both sides of which face the air space, as the negative lens on the side of the aperture stop. and the front lens group including a negative lens having a concave surface on the object side as the negative lens on the object side, and at least one aspherical lens having both curved surfaces curved toward the object side of the optical axis; Two cemented lenses in which a positive lens and a negative lens are cemented by a positive lens group in which positive lenses using two biconvex lenses are continuous, and negative lenses using a pair of biconcave lenses arranged on both sides of this positive lens group. wherein the negative lens on the image side of the two cemented lenses is arranged so that the image side faces the air space, and behind the negative lens, both surfaces face the air space and both curved surfaces are the optical axis and a rear lens group configured by arranging a final lens composed of an aspherical lens curved in the same direction, wherein BFL is the focal length from the negative lens on the aperture stop side to the negative lens on the image side in the two cemented lenses, When the focal length of the entire lens system is EFL,
0.7<BFL/EFL<1.6
An imaging optical system characterized by being configured to satisfy the following conditions. The front lens group comprises: the negative lens that is arranged on the object side and that uses a biconcave lens; the positive lens that has both curved surfaces curved toward the object side of the optical axis and is located on the object side using an aspherical lens; 2. An imaging optical system according to claim 1, further comprising said negative lens disposed on said aperture diaphragm side and having a convex surface on its object side. Let EGFL be the focal length of all the positive lenses in the front lens group where the absolute value of the anomalous partial dispersion ΔθgF of the glass material in the g-line and the F-line of the positive lens is 0.015 or more; When the maximum value of the focal length of the positive lens of is AGMX,
0.6<EGFL/AGMX<1.2
2. The imaging optical system according to claim 1, wherein the following condition is satisfied. Each of the front lens group and the rear lens group includes one aspherical lens, each aspherical lens is formed in the same shape, and is symmetrical with respect to the two cemented lenses in the optical axis direction. 2. The imaging optical system according to claim 1, wherein the optical system comprises: 1. A maximum of three designated lens groups are movable in front and rear partial lens groups (designated lens groups) having an air gap that changes during focus adjustment in the front lens group and the rear lens group. The imaging optical system described. The air gap that changes during the focus adjustment is at least one of the air gap including the aperture stop, the air gap between the two positive lenses of the front lens group, and the air gap between the two positive lenses of the rear lens group. 6. The imaging optical system of claim 5, comprising: When both the focal length FFL of the front lens group and the focal length RFL of the rear lens group have positive power, the entire lens system is moved toward the object side to change the air space on the image side during focus adjustment. An imaging optical system according to claim 1, 5 or 6, characterized by

Images