JPH11337819A - Photographing optical system of front diaphragm - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a photographing optical system of a front diaphragm arranged on an eyepiece part of an observation optical system and suited for observing/recording the visual field of the observation optical system in the optical system of the front diaphragm sufficiently securing a back focus by constituting it with first to fifth lens groups and satisfying a specified condition. SOLUTION: This photographing optical system consists of a diaphragm S, a first lens group L1 of a positive meniscus lens, a second lens group L2 of a negative meniscus lens turning a strong concave surface toward an objective side, a third lens group L3 of the positive meniscus lens turning the strong concave surface to the objective side, a fourth lens group L4 of the positive lens turning a strong convex surface to an image side and a fifth lens group L5 of a joined lens of positive diffraction force joining a biconvex lens to the negative meniscus lens in order from the objective side, and an optical element L6 of a low-pass filter or an infrared cut filter is arranged on the image side. Then, the condition 0.7f<fB<1.1f is satisfied. Where, (f) is a focal distance of a whole photographing optical system and fB is the back focus of the photographing optical system.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photographic optical system provided with an aperture in front of a lens system used for a video camera or an electronic still camera. The present invention also relates to a photographing device provided with the photographing optical system.

[0002]

2. Description of the Related Art Usually, a photographing optical system has a stop disposed therein, and both an entrance pupil and an exit pupil are located inside the optical system. However, depending on the photographing apparatus, it may be necessary to arrange an aperture, a shutter, and the like in front of the optical system in order to reduce the cost of the mechanism or the apparatus.
In particular, in the case of a photographing apparatus that arranges in an eyepiece part of an observation optical system such as a binocular or a viewfinder provided in a still camera or a video camera and performs photographing, the exit pupil of the eyepiece of the observation optical system is at the aperture position. For this reason, as a photographing optical system used in a photographing apparatus, for example, an optical system of a front stop as described in Japanese Patent Application Laid-Open No. 49-53423 is generally used.

[0005] Video cameras and electronic still cameras are configured to capture a subject image formed by a photographic optical system using a solid-state image sensor. In this solid-state imaging device, a micro lens is formed on the imaging surface, thereby increasing the amount of received light. Therefore, it is preferable that the light beam incident on the imaging surface of the solid-state imaging device has a small incident angle. In other words, the exit pupil of the imaging optical system is located sufficiently far from the imaging surface. Is desirable.

Further, it is necessary to dispose a low-pass filter for preventing moire, an infrared cut filter for cutting infrared light, and the like between the photographing optical system and the solid-state image pickup device. The focus needs to be long enough.

[0005]

When the stop is arranged in front of the optical system, the optical system has a structure with poor symmetry with respect to the stop, distortion occurs, and a balance between on-axis aberration and off-axis aberration is obtained. And the occurrence of coma in the periphery becomes large, and the imaging performance deteriorates.

When photographing is performed by attaching a photographing optical system to an eyepiece of an observation optical system such as a binocular or a viewfinder provided in a camera or the like, the exit pupil diameter of the observation optical system and the incident light of the photographing optical system. It is desired to make the pupil diameter equal or larger. In this case, as is well known, in the front stop type optical system, the entrance pupil diameter corresponds to the stop diameter. The observation optical system is designed in consideration of the fact that the exit pupil diameter is observed by a human. Therefore, the entrance pupil diameter of the photographing optical system used in the photographing apparatus is 3 mm corresponding to the pupil diameter of the human eye regardless of the optical specifications of the photographing optical system.
About 8 mm is desired.

[0007] In response to such a demand, a low-cost front stop optical system composed of a small number of lenses, such as a triplet-type triplet type and a four-element Tessar type, has been proposed. However, in recent years, due to the demand for miniaturization and high image quality of video cameras and electronic still cameras,
Solid-state imaging devices have also been reduced in size and improved in image quality. Accordingly, the photographing optical system is required to firstly improve the high frequency performance by increasing the number of pixels of the solid-state imaging device, and secondly to shorten the focal length by reducing the size of the solid-state imaging device. Since the size cannot be reduced in order to correspond to the pupil diameter, a large aperture ratio is required. Therefore, it is difficult for the above-mentioned type of optical system to satisfy the above requirements.

For this reason, an optical system which satisfies the above-mentioned requirements by increasing the number of lenses of the photographing optical system is disclosed in Japanese Patent Laid-Open Publication No.
An optical system described in, for example, Japanese Patent No. 78443 is known.

Among these conventional examples, Japanese Patent Application Laid-Open No. 52-323
The optical system described in Japanese Patent Publication No. 23 is a bright photographing optical system having a front stop having five elements in seven groups, an entrance pupil diameter of about 7 mm, and an F-number of about 1.2. However, the back focus is short, and there is no space for disposing an optical filter such as a low-pass filter or an infrared cut filter. These filters cannot be disposed, so that moire occurs or the contrast is reduced due to flare by infrared light. .

The optical system described in Japanese Patent Application Laid-Open No. 8-278443 is a small photographing optical system having a four-group, five-panel front aperture, but has an F number of about 3.4 and is dark.
The aperture diameter is as small as about 2.2 mm. for that reason,
When this optical system is used by placing it on the eyepiece of the observation optical system, the exit pupil diameter of the observation optical system is reduced to be smaller than the pupil diameter of the human eye by the aperture of the imaging optical system. The image formed by the method is darker than the state observed by a human, and the observation field of view tends to be blurred, and the depth of field becomes deep.

The present invention is directed to an optical system of a front diaphragm, which is suitable for use in a video camera or an electronic still camera and has a sufficient back focus, and which is disposed at an eyepiece of the observation optical system to provide a visual field of the observation optical system. The present invention is to provide a photographing optical system having a front aperture suitable for observing and recording the image.

[0012]

The photographing optical system of the front diaphragm according to the present invention comprises, in order from the object side, a first lens comprising a positive lens.
A second lens group consisting of a lens group and a negative lens having a strong concave surface facing the object side, a third lens group of a meniscus lens having a strong concave surface facing the object side, and a fourth lens being a positive lens having a strong convex surface facing the image side Fifth lens consisting of a group and a cemented lens having a positive refractive power
The zoom lens is composed of a lens group and satisfies the following condition (1). (1) 0.7f <f _B <1.1f where f is the focal length of the entire photographing optical system, and f _B is the back focus of the photographing optical system.

The photographing optical system according to the present invention comprises six elements in five groups.
The first lens group to the fourth lens group have a substantially retrofocus type lens configuration to ensure the back focus of the optical system. The fifth lens group is a cemented lens to correct chromatic aberration, particularly chromatic aberration of magnification.

As is well known, a retrofocus type optical system has a configuration in which an optical element having a negative refractive power is arranged on the object side and an optical element having a positive refractive power is arranged on the image side.

In order to make such a configuration in the first to fourth lens units of the optical system according to the present invention, the second lens unit is replaced by a negative lens having strong negative power on the object side and the third lens unit. Is a meniscus lens having strong negative power on the object side, and the fourth lens group is a positive lens having strong positive power on the image side. In this manner, the third lens group is substantially an optical element having a negative refractive power up to the object-side surface and the optical element having a positive refractive power after the image-side surface of the third lens group. Configured. Further, a positive lens is arranged in the first lens group to correct spherical aberration and coma of the optical system. Making the third lens group a meniscus lens so that the object side surface and the image side surface of the third lens group cancel out Petzval sum each other;
Thus, the change in performance due to the eccentricity of the third lens group can be reduced despite the relatively strong power of each lens surface.

Further, by setting the fifth lens group to have a positive power, the exit pupil position of the photographing optical system is made far from the image plane, and the optical system is made close to telecentric so that the size of the image by focusing is reduced. The optical system does not change or changes little. This is effective for use as an optical system of a photographing apparatus that performs photographing by arranging it on the eyepiece of the observation optical system.

The photographing optical system according to the present invention satisfies the above condition (1). In the condition (1), if the lower limit of 0.7f is not reached, an optical filter such as a low-pass filter or an infrared cut filter cannot be disposed between the photographing optical system and the imaging surface, and the use is limited. Not preferred. When the value exceeds the upper limit of 1.1f in the condition (1), the diameter of the rear unit (the lens unit on the image side) becomes large, and off-axis aberrations are remarkably generated. If the total length of the optical system is to be shortened, the power of the second lens group is increased, and accordingly, the amount of generated aberration is increased, and it is particularly difficult to correct spherical aberration and Petzval sum.

In the photographing optical system of the present invention, it is desirable that the following condition (2) is satisfied. (2) −0.3 <R _2F / R _2R <0.3 where R _2F and R _2R are the radii of curvature of the object-side surface and the image-side surface of the second lens unit, respectively.

Condition (2) is a condition relating to bending of the second lens group. When the second lens group, which is a negative lens having a strong concave surface facing the object side, exceeds the upper limit of 0.3 in the above condition (2), the refractive power of this lens group becomes weak, and the optical system is configured as a retrophy-type. Or the back focus cannot be secured, or the refractive power of each surface becomes extremely strong, and the amount of aberrations increases, making correction difficult. If the lower limit of -0.3 is not reached, coma is generated on each surface of the second lens unit, and the peripheral performance is greatly deteriorated when decentered.

In the photographing optical system according to the present invention, the following condition (3) can be satisfied. (3) 10 <ν ₁ −ν ₂ <25 where, ν ₁ and ν ₂ are Abbe numbers of the first lens unit and the second lens unit, respectively.

The condition (3) defines the Abbe number of the first lens unit and the second lens unit, whereby the axial chromatic aberration of the optical system can be corrected well.

In condition (3), if the value is below the lower limit of 10, axial chromatic aberration will be undercorrected, and if the value exceeds the upper limit of 25, axial chromatic aberration will be overcorrected.

In the photographing optical system of the present invention, it is desirable that the following condition (4) is satisfied. (4) 1.57 < _nd2 <1.75 where _nd2 is the refractive power of the second lens group.

The condition (4) defines the refractive power of the second lens unit, and the upper limit of the condition (4) is 1.75.
Exceeds the lower limit of 1.57, the radius of curvature of the surface of the second lens unit must be reduced, and spherical aberration occurs, making it difficult to correct the aberration.

Any one of the above conditions (2) to (4) may be satisfied, but it is particularly preferable that all of them are satisfied. Further, any two of the conditions (2) to (4) may be satisfied.

The photographing optical system according to the present invention has the following conditions (1-1) instead of conditions (1) to (4).
It is more desirable to satisfy (2-1), (3-1), and (4-1). _{(1-1) 0.8f <f B <} 1.0f (2-1) -0.3 <R 2F / R 2R <-0.1 (3-1) 15 <ν 1 -ν 2 <25 ( 4-1) 1.60 < _nd2 <1.70

These conditions may satisfy one or a part of them.

It is more preferable that the following condition (3-2) is satisfied instead of the condition (3) or the condition (3-1). (3-2) 18 <ν ₁ −ν ₂ <23

[0029]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the photographing optical system of the present invention will be described based on the following examples.

Example 1 f = 11.588, F / 2.318 r ₁ = ∞ (aperture) d ₁ = 1.000 r ₂ = -18.987 d ₂ = 1.616 n ₁ = 1.640 ν ₁ = 60.07 r ₃ = -8.320 d ₃ = 2.643 _{r 4 = -4.312 d 4 = 0.936} n 2 = 1.581 ν 2 = 40.75 r 5 = -15.234 d 5 = 1.419 r 6 = -7.241 d 6 = 3.945 n 3 = 1.816 ν 3 = 46.62 r 7 = -7.171 d 7 = 0.200 r ₈ = -97.887 d ₈ = 2.154 n ₄ = 1.816 ν ₄ = 46.62 r ₉ = -18.640 d ₉ = 0.200 r ₁₀ = 24.836 d ₁₀ = 4.641 n ₅ = 1.741 ν ₅ = 52.64 r ₁₁ = -9.897 d _{11 = 0.988 n 6 = 1.847 ν} 6 = 23.78 r 12 = -88.212 d 12 = 7.210 r 13 = ∞ d 13 = 4.500 n 7 = 1.516 ν 7 = 64.14 r 14 = ∞ d 14 = 2.375 f B /f=1.083 _{, r 2F / r 2R = 0.283} , ν 1 -ν 2 = 19.32 n d2 = 1.581

Example 2 f = 11.596, F / 1.611 r ₁ = ∞ (aperture) d ₁ = 1.000 r ₂ = 23.067 d ₂ = 2.319 n ₁ = 1.755 ν ₁ = 52.32 r ₃ = -15.169 d ₃ = 1.882 r _{4 = -6.600 d 4 = 0.842 n} 2 = 1.689 ν 2 = 31.07 r 5 = 22.490 d 5 = 2.156 r 6 = -6.380 d 6 = 3.443 n 3 = 1.834 ν 3 = 37.16 r 7 = -7.551 d 7 = 0.200 _{r 8 = 45.820 d 8 = 3.511} n 4 = 1.816 ν 4 = 46.62 r 9 = -14.726 d 9 = 0.200 r 10 = 13.804 d 10 = 4.736 n 5 = 1.603 ν 5 = 65.44 r 11 = -12.411 d 11 = 0.986 _{n 6 = 1.847 ν 6 = 23.78} r 12 = 44.835 d 12 = 3.060 r 13 = ∞ d 13 = 4.500 n 7 = 1.516 ν 7 = 64.14 r 14 = ∞ d 14 = 2.375 f B /f=0.725,r 2F / _{r 2R = -0.293, ν 1 -ν} 2 = 21.25 n d2 = 1.689

[0032] Example 3 f = 11.593, F / 2.321 r 1 = ∞ ( _{stop) d 1 = 1.000 r 2 =} 105.876 d 2 = 2.200 n 1 = 1.757 ν 1 = 47.82 r 3 = -9.777 d 3 = 1.500 r _{4 = -5.005 d 4 = 1.000 n} 2 = 1.620 ν 2 = 36.26 r 5 = 47.216 d 5 = 1.700 r 6 = -6.680 d 6 = 3.067 n 3 = 1.834 ν 3 = 37.16 r 7 = -7.270 d 7 = 0.200 _{r 8 = -412.744 d 8 = 3.021} n 4 = 1.772 ν 4 = 49.60 r 9 = -11.261 d 9 = 0.200 r 10 = 16.022 d 10 = 3.879 n 5 = 1.603 ν 5 = 65.44 r 11 = -12.229 d 11 = _{1.000 n 6 = 1.847 ν 6 =} 23.78 r 12 = 103.295 d 12 = 4.980 r 13 = ∞ d 13 = 4.500 n 7 = 1.516 ν 7 = 64.14 r 14 = ∞ d 14 = 2.375 f B /f=0.890,r 2F _{/ r 2R = -0.106, ν 1} -ν 2 = 11.56 n d2 = 1.620 However, r _1, r _2, ··· is the radius of curvature of each lens surface, d
₁ , d ₂ ,... Are the thickness of each lens and the air gap,
, n ₁ , n ₂ ,.
₁ , ν ₂ ,... Are Abbe numbers of the respective lenses.

In the first embodiment, the stop S and the first meniscus lens of the positive meniscus lens are arranged in order from the object side with the configuration shown in FIG.
A lens unit L1, a second lens unit L2 of a negative meniscus lens having a strong concave surface facing the object side, a third lens unit L3 of a positive meniscus lens having a strong concave surface facing the object side, and a strong convex surface facing the image side And a fifth lens unit L5 of a cemented lens having a positive refractive power in which a biconvex lens and a negative meniscus lens are cemented. A low-pass filter or red The optical element L6 of the outer cut filter is arranged.

Embodiment 2 is an optical system having the structure shown in FIG.
In order from the object side, a stop S, a first lens unit L1 of a biconvex lens, a second lens unit L2 of a biconcave lens with a strong concave surface facing the object side, and a positive meniscus lens with a strong concave surface facing the object side A third lens unit L3, a fourth lens unit L4 of a biconvex lens having a strong convex surface facing the image side, and a fifth lens unit L5 of a cemented lens having a positive refractive power in which the biconvex lens and the biconcave lens are joined. An optical element L6 is arranged between the fifth lens unit L5 and the image plane.

The third embodiment has a configuration as shown in FIG.
A first lens unit L1 of a biconvex lens, a second lens unit L2 of a biconcave lens having a strong concave surface facing the object side, and a third lens unit L3 of a positive meniscus lens having a strong concave surface facing the object side. A fourth lens unit L4 of a positive meniscus lens having a strong convex surface facing the image side, and a fifth lens unit L of a cemented lens having a positive refracting power obtained by cementing a biconvex lens and a biconcave lens
5, and an optical element L6 is arranged between the fifth lens unit L5 and the image plane.

The aberrations in the first, second, and third embodiments are as shown in FIGS. 4, 5, and 6, respectively, where IH is the image height.

In the photographing optical system described above, an optical system having the configuration described in each of the following items in addition to the optical system described in the claims can also achieve the object of the present invention.

(1) An optical system according to claim 1, 2 or 3, which satisfies the following condition (4): (4) 1.57 < _nd2 <1.75

(2) The optical system according to claim 1, 2, or 3, or the above item (1), wherein the following condition (1-1) is satisfied instead of the condition (1). An imaging optical system characterized in that: _{(1-1) 0.8f <f B <} 1.0f

(3) Claim 2 or 3 of the claims
Alternatively, the optical system described in the above item (1) or (2), wherein the following condition (2-1) is satisfied instead of the condition (2). (2-1) −0.3 <R _2F / R _2R <−0.1

(4) In the optical system described in claim 3 or (1), (2) or (3), the following condition (3-1) is used instead of condition (3). An imaging optical system characterized by satisfying (1). (3-1) 15 <ν ₁ −ν ₂ <25

(5) The optical system described in the above item (4), wherein the following condition (3-2) is satisfied instead of the condition (3-1). (3-2) 18 <ν ₁ −ν ₂ <23

(6) The above (1), (2), (3),
In the optical system described in the item (4) or (5), the condition (4) is satisfied.
A photographing optical system satisfying the following condition (4-1) instead of (4) 1.60 < _nd2 <1.70

(7) Claims 1, 2 or 3 of the claims or the above (1), (2), (3), (4),
(5) or a photographing apparatus provided with the optical system described in (6), wherein the optical system is mounted on an afocal optical system in which an exit pupil, which is another optical system, is located behind the final lens surface. An imaging device characterized in that observation or imaging is performed.

[0045]

The front aperture photographing optical system of the present invention has a long back focus and is suitable for a video camera or an electronic still camera, and is also suitable for observation and photographing by being attached to an eyepiece of an observation optical device.

[Brief description of the drawings]

FIG. 1 is a sectional view of a first embodiment of the present invention.

FIG. 2 is a sectional view of a second embodiment of the present invention.

FIG. 3 is a sectional view of a third embodiment of the present invention.

FIG. 4 is an aberration curve diagram according to the first embodiment of the present invention.

FIG. 5 is an aberration curve diagram according to the second embodiment of the present invention.

FIG. 6 is an aberration curve diagram according to a third embodiment of the present invention.

Claims (3)

[Claims] 1. A first lens group consisting of a positive lens, a second lens group consisting of a negative lens having a strong concave surface facing the object side, and a third lens of a meniscus lens having a strong concave surface facing the object side in order from the object side. A photographing optical system of a front stop, which includes a fourth lens unit of a positive lens having a strong convex surface facing the group and the image side and a fifth lens unit of a cemented lens having a positive refractive power, and satisfies the following condition (1). (1) 0.7f <f _B <1.1f where f is the focal length of the entire photographing optical system, and f _B is the back focus of the photographing optical system. 2. The photographing optical system according to claim 1, wherein the following condition (2) is satisfied. (2) -0.3 < _R2F / _R2R <0.3 where _R2F and _R2R are the radii of curvature of the object-side and image-side surfaces of the second lens unit, respectively. 3. The photographing optical system according to claim 1, wherein the following condition (3) is satisfied. (3) 10 <ν ₁ −ν ₂ <25 where, ν ₁ and ν ₂ are Abbe numbers of the first lens unit and the second lens unit, respectively.