JP5305831B2 - Imaging optics - Google Patents

Description

  The present invention relates to an imaging optical system suitable for a photographing lens used in a digital camera, a video camera, or the like.

  In recent years, with the widespread use of digital still cameras and video cameras, the number of pixels of image sensors is rapidly increasing, and an imaging lens with higher image quality is required.

  In a digital still camera and a video camera, generally, a space for inserting an optical low-pass filter for preventing moire and an IR cut filter for blocking infrared rays is required in front of the image sensor, and it is necessary to take a long back focus. In addition, image sensors widely used in digital still cameras and video cameras such as CCD and CMOS have a characteristic that the sensitivity decreases with respect to light having a large incident angle.

  For these reasons, it is said that a retrofocus imaging optical system having a long back focus and excellent telecentricity is suitable as an imaging lens for a digital still camera or a video camera.

  A retrofocus type imaging optical system is generally used for a wide-angle lens having a short focal length with respect to the diagonal length of the image sensor. However, the focal length substantially matches the diagonal length of the screen and is 50 ° to 60 °. The present invention can also be applied to a so-called standard lens having a diagonal angle of view. This retrofocus imaging optical system is superior in terms of telecentricity to a Gauss type or Tesser type often used for standard lenses, and has a feature that the peripheral light quantity ratio can be increased.

  In addition, since the retrofocus imaging optical system is asymmetrical, the rear focus method and inner focus method, which move a part of the optical system, improve the image quality at short distances rather than the overall extension method. Cheap. The retrofocus type imaging optical system is particularly suitable for the rear focus system because its angle tends to become gentler as it approaches the image plane side of the optical system because of its characteristics. By adopting the rear focus method, the focus lens group can be reduced in weight, so that the focus drive motor can be reduced in size, and as a result, the overall imaging apparatus can also be reduced in size.

Examples of a retrofocus imaging optical system designed for a digital still camera and excellent in telecentricity are disclosed in Patent Document 1 and Patent Document 2. Patent Document 3 discloses a retrofocus type wide-angle lens suitable for a large image sensor.
JP 2001-56433 A JP 2002-228925 A JP 2008-40033 A

  However, the retrofocus imaging optical system can increase the back focus, but it is difficult to downsize the entire optical system compared to the focal length and the size of the image sensor. The total length of the optical system becomes larger than that of a mold or a Tesser type optical system.

  The retrofocus imaging optical system described in Patent Document 1 also has very excellent telecentricity, but the overall length of the optical system is long compared to the image sensor and the lens center thickness is large. When applied to an image sensor, the entire optical system becomes large and heavy.

  In addition, the retrofocus imaging optical system described in Patent Document 2 has a longer overall optical system length than the image sensor, and has an image quality that can be used for a large image sensor with insufficient coma correction. I can't.

  Furthermore, since a small image sensor is inferior in S / N ratio and dynamic range, in recent years, there has been a demand for a camera that employs a large image sensor in order to obtain high image quality. However, many retrofocus imaging optical systems used in conventional digital still cameras are designed on the premise of a small image sensor, and the optical system is larger than the image sensor. For this reason, when applied to a large image sensor, the entire image pickup apparatus is increased in size.

  Although the retrofocus imaging optical system described in Patent Document 3 is smaller than the mounted image sensor, the total lens length is larger than the focal length, so that the angle of view is about 50 ° to 60 °. When applied to a lens having a lens, the entire image pickup apparatus is increased in size.

  The present invention has been made in view of such a situation, and in a retrofocus type imaging optical system having an angle of view of about 50 ° to 60 °, while realizing a performance compatible with a large image sensor. The purpose is to achieve sufficient miniaturization.

In order to achieve the above object, an image forming optical system embodying the present invention includes, in order from the object side, a first lens group having a positive refractive power as a whole and having an aperture stop therein, composed of a second lens group, the first lens group is the position in the optical axis direction is fixed, the second lens group performs focusing by moving in the optical axis direction, the object than the aperture stop of the first lens group A front group on the side and a rear group on the image plane side from the aperture stop, and the front group includes at least two negative lenses and at least one positive lens and has a positive refractive power as a whole, It is characterized by satisfying the following conditions.
(1) 0.40 <Sd1a / f <0.60
(2) Nd1ap> 1.77
(3) 1.2 <f1a / f <2.5
However, f: Focal length Sd1a at the time of photographing at infinity of the entire optical system: Full length of the front group Nd1ap: Refractive index d1 (wavelength 587.6 nm) of the positive lens in the front group f1a: Focal length of the front group

  Furthermore, the imaging optical system for carrying out the present invention is the above-described invention, wherein the second lens group is composed of one positive lens.

  Furthermore, the imaging optical system for carrying out the present invention is the above-described invention, wherein the second lens group is a meniscus lens having a convex surface facing the image surface, and at least one surface is formed of an aspherical surface.

Furthermore, in the above-described invention, the imaging optical system for carrying out the present invention is configured such that the second lens group satisfies the following conditions.
(4) 0.60 <| β2 | <0.70
However,
β2: Imaging magnification when the second lens group is photographed at infinity

Further, the imaging optical system embodying the present invention is the above-described invention, wherein the front group is composed of two negative lenses and two positive lenses in order from the object side, and satisfies the following conditions: It is.
(5) Nd1an ≧ 1.69895
(6) νd1an ≧ 30.05
However,
Nd1an: Maximum value of the refractive index of d-line of the negative lens in the front group νd1an: Abbe number of the glass material having the highest refractive index among the negative lenses in the front group

  The imaging optical system embodying the present invention is compact compared to the size of the image sensor, has good optical performance, and is suitable for a digital still camera or video camera equipped with a large image sensor. And a compact imaging optical system can be provided.

  The best mode for carrying out the present invention will be described below with reference to the accompanying drawings. In addition, this invention is not limited by this embodiment.

  FIGS. 1, 5, 9, 13, and 17 show lens configurations of first to fifth examples in the imaging optical system of the present invention.

  The imaging optical system of the present invention has a field angle of about 50 ° to 60 °, and in order from the object side, a first lens group G1 having a positive refractive power and an aperture stop S, and a positive refractive power. The second lens group G2 is used to adjust the focus by moving the second lens group G2 in the optical axis direction. Further, the first lens group G1 includes a front group G1a having a positive refractive power closer to the subject than the aperture stop S and a rear group G1b having a positive refractive power closer to the image plane I than the aperture stop S. The front group G1a includes at least two negative lenses and at least one positive lens, and is configured to satisfy predetermined conditions described later.

  In order to ensure back focus and improve telecentricity, it is desirable to have negative refractive power on the object side. On the other hand, in order to shorten the overall length, it is desirable that the lens unit on the object side has a positive refractive power. For this reason, in the present invention, a negative lens is arranged in the front group G1a closer to the subject than the aperture stop S, and the front group G1a as a whole has a positive refractive power, thereby ensuring back focus and improving telecentricity. At the same time, the overall length of the optical system is reduced. Further, coma aberration is reduced by including at least two negative lenses in the front group G1a.

  Conditional expression (1) defines the ratio between the total length of the front group G1a and the focal length of the entire system. If the total length of the front group G1a is shortened beyond the lower limit of the conditional expression (1), it becomes impossible to secure the center thickness and edge thickness of the lens, it becomes impossible to set the refractive power of each lens appropriately, and it becomes difficult to correct aberrations. . On the other hand, when the upper limit of conditional expression (1) is exceeded, the total length of the front group G1a becomes long, and downsizing is hindered.

  Conditional expression (2) is a condition related to the refractive index of the positive lens in the front group G1a. If the range of the conditional expression (2) is exceeded, it is necessary to tighten the curvature of the positive lens in the front group G1a in order to maintain the refractive power, making it difficult to correct aberrations and increasing the center thickness to reduce the size. This causes problems such as obstructing or increasing the eccentricity sensitivity.

  Conditional expression (3) defines the ratio of the focal length of the front group G1a to the focal length of the entire system. If the focal length of the front group G1a is shortened beyond the lower limit of the conditional expression (3), the refractive power of the positive lens in the front group G1a needs to be very strong. Becomes larger. If the focal length of the front group G1a is increased beyond the upper limit of the conditional expression (3), the rear group G1b and the second lens group G2 require stronger refractive power, so that the entire length of the entire optical system becomes longer, resulting in a smaller size. Inhibits oxidization.

  Since the second lens group G2 of the imaging optical system of the present invention is composed of only one positive lens, the focus lens can be reduced in weight, and focusing can be speeded up or the drive motor can be downsized. This contributes to the downsizing of the entire imaging device. Further, by forming one positive lens of the second lens group G2 into a meniscus shape with a convex surface facing the image plane I, the angle at which off-axis light rays enter one positive lens can be reduced. . Therefore, the occurrence of aberration in one positive lens can be suppressed, and fluctuations in aberration due to movement of the second lens group G2 in the optical axis direction and eccentricity can be suppressed.

  In general, when a positive lens is arranged behind the aperture stop S, negative distortion is generated. Therefore, the convergence action becomes weaker as at least one surface of one positive lens of the second lens group G2 goes to the lens periphery, or Negative distortion can be reduced by using an aspherical surface with a strong diverging action. At the same time, by adopting such an aspherical shape, it is possible to reduce the variation in spherical aberration associated with the movement of the second lens group G2 due to focusing. This effect can be obtained even if one of the object side surface and the image surface side surface of the second lens group G2 is an aspheric surface, and the both surfaces are also aspheric.

  By forming the second lens group G2 functioning as a focus lens as described above, it is possible to maintain performance at a short distance even if the second lens group G2 is composed of only one lens. .

  The second lens group G2 is further configured to satisfy the conditional expression (4). Conditional expression (4) defines the imaging magnification of the second lens group G2. If the lower limit of conditional expression (4) is exceeded, the refractive power of the second lens group G2 increases, aberration fluctuations accompanying focusing increase, and the weight of the second lens group G2 increases, so focus drive is performed at the same speed. In order to do this, a large focus drive motor must be used, which hinders downsizing of the entire imaging apparatus. If the upper limit of conditional expression (4) is exceeded, the amount of movement of the focus lens required for focus adjustment to a short distance increases, which also hinders downsizing of the optical system.

  The front group G1a of the first lens group G1 is composed of two negative lenses and two positive lenses, and it is particularly desirable to satisfy a predetermined condition. As widely used in small digital cameras, when the photographic lens is retracted and stored when not in operation, the photographic lens cannot be stored compactly if the number of lenses in the front group G1a is increased. On the other hand, if the number of both the negative lens and the positive lens in the front group G1a is reduced, aberration correction becomes insufficient and decentration sensitivity also increases.

  In addition, it is preferable to select at least one glass material among the negative lenses in the front group G1a so as to satisfy the conditional expressions (5) and (6). If the refractive index is lowered beyond the range of the conditional expression (5), the curvature of the negative lens in the front group G1a becomes tight and hinders downsizing, and correction of coma becomes insufficient. If the Abbe number decreases beyond the range of conditional expression (6), it becomes difficult to correct lateral chromatic aberration at the periphery of the screen.

  The imaging optical system according to the present invention has a characteristic that is particularly suitable for a large-sized solid-state imaging device by being configured to satisfy each of the above conditions, but is not limited to the type and size of the image sensor. It is possible to provide high imaging characteristics.

  Tables 1 to 5 show specific numerical examples according to the above-described embodiments. In each numerical example (overall specifications), f is a focal length, Fno is an F number, and 2ω is a diagonal angle of view. In (lens specifications), the number indicates the surface number of the lens, r is the radius of curvature of each surface, d is the surface spacing, nd is the refractive index with respect to the d-line, and νd is the Abbe number based on the d-line. Further, * (asterisk) attached to the surface number indicates that the surface shape is an aspherical surface. Furthermore, (conditional expression) shows the values of conditional expressions (1) to (6) in each numerical example.

  The shape of the aspheric surface is y for the displacement from the optical axis in the direction perpendicular to the optical axis, z for the displacement (sag amount) from the intersection of the aspheric surface and the optical axis in the optical axis direction, and r for the radius of curvature of the reference spherical surface. When the conic coefficient is K, 4, 6, 8, and the 10th-order aspheric coefficient is A4, A6, A8, and A10, the coordinates of the aspheric surface are expressed by the following equations.

2 to 4 are aberration diagrams showing the respective aberrations at the respective photographing distances (infinity, 1000 mm, 500 mm) of the numerical example 1, and FIG. 2 shows the aberrations at the infinity photographing distance. Shows the aberration at the shooting distance of 1000 mm, and FIG. 4 shows the aberration at the shooting distance of 500 mm.
In the longitudinal aberration diagram, d-line, g-line, and C-line indicate aberrations for the respective wavelengths, ΔS indicates the sagittal image plane, and ΔM indicates the aberration in the meridional image plane. In the lateral aberration diagram, Y indicates the image height.

  From the respective aberration diagrams, it can be seen that in the numerical value example 1 of the present invention, various aberrations are satisfactorily corrected and the imaging performance is excellent.

6 to 8 are aberration diagrams showing the respective aberrations at the respective photographing distances (infinity, 1000 mm, 500 mm) of the numerical example 2, and FIG. 6 shows the aberrations at the infinity photographing distance. Shows the aberration at the shooting distance of 1000 mm, and FIG. 8 shows the aberration at the shooting distance of 500 mm.
In the longitudinal aberration diagram, d-line, g-line, and C-line indicate aberrations for the respective wavelengths, ΔS indicates the sagittal image plane, and ΔM indicates the aberration in the meridional image plane. In the lateral aberration diagram, Y indicates the image height.

  From each aberration diagram, it can be seen that, in Numerical Example 2 of the present invention, various aberrations are corrected well and the imaging performance is excellent.

10 to 12 are aberration diagrams showing the respective aberrations at the respective photographing distances (infinity, 1000 mm, 500 mm) of the numerical example 3, and FIG. 10 shows the aberrations at the infinity photographing distance. Shows the aberration at the shooting distance of 1000 mm, and FIG. 12 shows the aberration at the shooting distance of 500 mm.
In the longitudinal aberration diagram, d-line, g-line, and C-line indicate aberrations for the respective wavelengths, ΔS indicates the sagittal image plane, and ΔM indicates the aberration in the meridional image plane. In the lateral aberration diagram, Y indicates the image height.

  From each aberration diagram, it can be seen that in Numerical Example 3 of the present invention, various aberrations are corrected favorably and the imaging performance is excellent.

14 to 16 are aberration diagrams showing aberrations at the respective photographing distances (infinity, 1000 mm, 500 mm) of the numerical example 4, and FIG. 14 shows aberrations at the infinity photographing distance. Shows the aberration at the shooting distance of 1000 mm, and FIG. 16 shows the aberration at the shooting distance of 500 mm.
In the longitudinal aberration diagram, d-line, g-line, and C-line indicate aberrations for the respective wavelengths, ΔS indicates the sagittal image plane, and ΔM indicates the aberration in the meridional image plane. In the lateral aberration diagram, Y indicates the image height.

  From each aberration diagram, it can be seen that in the numerical value example 4 of the present invention, various aberrations are satisfactorily corrected and the imaging performance is excellent.

18 to 20 are aberration diagrams showing the respective aberrations at the respective photographing distances (infinity, 1000 mm, 500 mm) of the numerical example 5, and FIG. 17 shows the aberrations at the infinity photographing distance. Shows the aberration at the shooting distance of 1000 mm, and FIG. 19 shows the aberration at the shooting distance of 500 mm.
In the longitudinal aberration diagram, d-line, g-line, and C-line indicate aberrations for the respective wavelengths, ΔS indicates the sagittal image plane, and ΔM indicates the aberration in the meridional image plane. In the lateral aberration diagram, Y indicates the image height.

  From each aberration diagram, it can be seen that in the numerical value example 5 of the present invention, various aberrations are satisfactorily corrected and the imaging performance is excellent.

  As described above, according to the imaging optical system of the present invention, the retrofocus imaging optical system having an angle of view of about 50 ° to 60 ° realizes performance capable of handling a large image sensor. However, sufficient miniaturization can be achieved.

It is sectional drawing which shows the lens structure in the 1st Example of this invention. FIG. 6 is an aberration diagram showing spherical aberration, astigmatism, distortion aberration, and coma aberration at infinity shooting distance according to Numerical Example 1 in which specific numerical values are applied to the first example. FIG. 6 is an aberration diagram showing spherical aberration, astigmatism, distortion, and coma aberration at a shooting distance of 1000 mm according to Numerical Example 1 in which specific numerical values are applied to the first example. FIG. 6 is an aberration diagram showing spherical aberration, astigmatism, distortion, and coma aberration at a shooting distance of 500 mm according to Numerical Example 1 in which specific numerical values are applied to the first example. It is sectional drawing which shows the lens structure in the 2nd Example of this invention. FIG. 10 is an aberration diagram showing spherical aberration, astigmatism, distortion aberration, and coma aberration at infinity shooting distance according to Numerical Example 2 in which specific numerical values are applied to the second example. FIG. 10 is an aberration diagram showing spherical aberration, astigmatism, distortion, and coma aberration at a shooting distance of 1000 mm according to Numerical Example 2 in which specific numerical values are applied to the second example. FIG. 10 is an aberration diagram showing spherical aberration, astigmatism, distortion, and coma aberration at a shooting distance of 500 mm according to Numerical Example 2 in which specific numerical values are applied to the second example. It is sectional drawing which shows the lens structure in the 3rd Example of this invention. FIG. 10 is an aberration diagram showing spherical aberration, astigmatism, distortion aberration, and coma aberration at infinity shooting distance according to Numerical Example 3 in which specific numerical values are applied to the third example. FIG. 10 is an aberration diagram showing spherical aberration, astigmatism, distortion, and coma aberration at a shooting distance of 1000 mm according to Numerical Example 3 in which specific numerical values are applied to the third example. FIG. 10 is an aberration diagram showing spherical aberration, astigmatism, distortion, and coma aberration in a shooting distance of 500 mm according to Numerical Example 3 in which specific numerical values are applied to the third example. It is sectional drawing which shows the lens structure in the 4th Example of this invention. FIG. 10 is an aberration diagram showing spherical aberration, astigmatism, distortion, and coma aberration at an infinite distance, according to Numerical Example 4 in which specific numerical values are applied to the fourth example. FIG. 10 is an aberration diagram showing spherical aberration, astigmatism, distortion, and coma aberration at a shooting distance of 1000 mm, according to numerical example 4 in which specific numerical values are applied to the fourth example. FIG. 10 is an aberration diagram showing spherical aberration, astigmatism, distortion, and coma aberration in a shooting distance of 500 mm according to Numerical Example 4 in which specific numerical values are applied to the fourth example. It is sectional drawing which shows the lens structure in the 5th Example of this invention. FIG. 10 is an aberration diagram showing spherical aberration, astigmatism, distortion aberration, and coma aberration at infinity shooting distance according to Numerical Example 5 in which specific numerical values are applied to the fifth example. FIG. 10 is an aberration diagram showing spherical aberration, astigmatism, distortion, and coma aberration at a shooting distance of 1000 mm, according to numerical example 5 in which specific numerical values are applied to the fifth example. FIG. 10 is an aberration diagram showing spherical aberration, astigmatism, distortion, and coma aberration at a shooting distance of 500 mm, in Numerical Example 5 where specific numerical values are applied to the fifth example.

Explanation of symbols

G1: first lens group G1a: front group S: aperture stop G1b: rear group G2: second lens group I: image plane

Claims (5)

From the object side,
A first lens unit having a positive refractive power as a whole and having an aperture stop inside;
It consists of a second lens group having a positive refractive power as a whole ,
The first lens group has a fixed position in the optical axis direction,
The second lens group performs focus adjustment by moving in the optical axis direction,
The first lens group includes a front group on the object side from the aperture stop and a rear group on the image plane side from the aperture stop,
The front group includes at least two negative lenses and at least one positive lens and has a positive refractive power as a whole,
An imaging optical system characterized by satisfying the following conditions.
(1) 0.40 <Sd1a / f <0.60
(2) Nd1ap> 1.77
(3) 1.2 <f1a / f <2.5
However, f: Focal length Sd1a at the time of photographing at infinity of the entire optical system: Full length of the front group Nd1ap: Refractive index d1 (wavelength 587.6 nm) of the positive lens in the front group f1a: Focal length of the front group
  The imaging optical system according to claim 1, wherein the second lens group includes one positive lens.   3. The imaging optical system according to claim 1, wherein the second lens group is formed of a meniscus lens having a convex surface facing the image surface side, and at least one surface is an aspherical surface. The imaging optical system according to claim 1, wherein the second lens group satisfies the following condition.
(4) 0.60 <| β2 | <0.70
However,
β2: Imaging magnification when the second lens group is photographed at infinity 5. The front group is composed of two negative lenses and two positive lenses in order from the object side, and satisfies the following condition. 6. Imaging optical system.
(5) Nd1an ≧ 1.69895
(6) νd1an ≧ 30.05
However,
Nd1an: Maximum value of the refractive index of d-line of the negative lens in the front group νd1an: Abbe number of the glass material having the highest refractive index among the negative lenses in the front group

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