JP3352253B2 - Rear focus type reverse telephoto shooting lens - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rear-focusing reverse telephoto lens having a shooting angle of view of about 94 degrees and an F-number of about 2.8 suitable for a 35 mm camera or a video camera, and a camera using the same. In particular, when focusing by moving the rear lens group in the lens system on the optical axis, a rear focus type reverse telephoto shooting lens with an appropriately configured lens system so that good optical performance can be obtained over the entire object distance And a camera using the same.

[0002]

2. Description of the Related Art Conventionally, a so-called reverse telephoto (retro-focus) photographing lens, in which a lens unit having a negative refractive power precedes a wide-angle photographing lens having a back focus longer than the focal length, is well known. Have been.

[0003] In this reverse telephoto type photographing lens, a rear focus type in which a rear lens group in a lens system is moved to perform focusing is disclosed in, for example, Japanese Patent Laid-Open Publication No. Sho.
JP-A-5-147607, JP-A-57-35821, JP-A-58-202414, JP-A-59-2
No. 16114, Japanese Patent Application Laid-Open No. 61-140910, and the like.

In general, in the rear focus type, the amount of extension of the focusing lens group is smaller than that of the whole focusing system in which the entire lens system is extended, and the focusing lens group is relatively small and light, so that focusing can be performed with a small driving force. Therefore, it is suitable for a camera or the like having an automatic focus detection device. In addition, since the entire length of the lens is always constant even when focusing is performed, there is an advantage that the photographing device is easily held and camera shake hardly occurs.

[0005]

Generally, the reverse telephoto type photographing lens has an asymmetric lens configuration as a whole, in which a lens unit having a negative refractive power is disposed in front of a lens unit having a positive refractive power and a lens unit having a positive refractive power is disposed in the rear. As a result, the amount of various aberrations such as spherical aberration, coma, distortion, and astigmatism increases, and the absolute value of the negative refractive power of the front lens group is increased to secure a long back focus. Therefore, the amount of occurrence of various aberrations is further increased, and there has been a problem that it is generally very difficult to satisfactorily correct these aberrations in a well-balanced manner.

On the other hand, among the focus methods, the rear focus method has the above-mentioned advantages, however, compared to the whole focus method in which the entire lens system is extended, when the focus lens group is moved, aberration fluctuation increases, and the entire object distance increases. It becomes difficult to properly perform aberration correction over the entire range.

This tendency is particularly remarkable in a so-called reverse telephoto photographing lens in which the front group includes a lens group having a negative refractive power and the rear group includes a lens group having a positive refractive power. Outward coma increases, astigmatism also worsens, and the optical performance decreases significantly.

According to the present invention, a photographing angle of view 94 having high optical performance in which aberration fluctuations during focusing are well corrected over the entire object distance from an object at infinity to a close object while maintaining the advantages of the rear focus system. Degree, F-number about 2.8, back focus is 1.7 ~ of focal length
It is an object of the present invention to provide a 1.9x reverse telephoto imaging lens and a camera using the same.

[0009]

According to the first aspect of the present invention, there is provided a rear focus type reverse telephoto taking lens having a first lens unit having a negative refractive power and a second lens unit having a positive refractive power in order from the object side. The first group includes three lenses: a meniscus negative eleventh lens having a convex surface facing the object side, a meniscus negative twelfth lens having a convex surface facing the object side, and a positive lens. And the second group has a convex surface facing the object side in order from the object side.
Digit meniscus negative 21st lens, positive 22nd lens
Lens, positive 23rd lens with convex surface facing the image side, both lenses
A negative 24th lens with a concave surface and a positive lens with a convex surface facing the object side
Lens, both lenses joined to the 25th lens
The negative 26th lens whose surface is concave and the positive 26th lens where both lens surfaces are convex
A laminated lens with the 27th lens
The lens unit has a positive 28th lens with a convex surface, and the second unit is moved on the optical axis to perform focusing. The focal length of the first unit and the total length of the lens are respectively f1, DT1, and the focal length of the entire system. Where f and DT are the total lens length and S is the air gap between the first and second groups when focusing on an object at infinity, 4 <| f1 / f | <6.5 (1) 0.1 <S / f <0.5 (2) 0.25 <DT1 / DT <0.5 (3) The condition is satisfied.

[0010]

1 and 2 are sectional views of lenses according to numerical examples 1 and 2 of the present invention, which will be described later. In the figure, L1 denotes a first group (front group) having a negative refractive power, L2 denotes a second group (rear group) having a positive refractive power, SP denotes an aperture, and IP denotes an image plane. Focusing from an object at infinity to a close object uses a rear focus system in which the second lens unit L2 is moved toward the object side on the optical axis.

According to the present invention, a negative meniscus eleventh lens unit having the first lens unit L1 having a negative refractive power with the convex surface facing the object side is provided.
Similarly, a negative meniscus twelfth lens having a convex surface facing the object side
The lens is composed of three lenses, that is, a positive thirteenth lens whose both lens surfaces are convex.

In the present invention, as described above, the two eleventh and twelfth lenses on the object side of the first lens unit L1 are each formed in a meniscus shape with the convex surface facing the object side, so that the luminous flux reaching the periphery of the screen That is, when an off-axis light beam having a large incident angle to the first lens surface passes through the eleventh lens and the twelfth lens, the light beam is prevented from having an extremely large incident angle and an exit angle with respect to each lens surface. Is gradually bent in a direction parallel to the optical axis. This reduces the front lens diameter,
Good aberration correction.

By arranging two lenses having a negative refractive power closest to the object side, a lens configuration advantageous for securing a predetermined back focus is obtained. Further, a positive thirteenth lens is arranged on the image plane side of the eleventh lens and the twelfth lens,
The thirteenth lens corrects the distortion generated by the eleventh and twelfth lenses, thereby reducing the burden of the distortion corrected by the second group, thereby facilitating the correction of other aberrations. Then, the second lens group has a convex surface facing the object side.
Skull-shaped negative 21st lens, positive 22nd lens, image plane
Positive 23rd lens with convex surface facing the side, both lens surfaces concave
Negative 24th lens and positive 25th lens with convex surface facing the object side
Laminated lens with lens joined, both lens surfaces concave
Negative 26th lens and positive 27th lens with both lens surfaces convex.
Lens, and both lens surfaces
It consists of a convex positive twenty-eighth lens. The second group
Also the meniscus with the 21st lens on the object side convex toward the object side
The lens configuration of the second group can be changed by using a cas-shaped negative lens.
The lens configuration is advantageous for securing the back focus,
Make sure that the off-axis light beam enters each lens surface at a gentle angle.
To reduce the aberration generated on the first lens surface of the second group.
You. Its to the image plane side of the meniscus-shaped negative of the 21 lens
A positive second lens is disposed, and the first lens is disposed on the 22nd lens.
Has the effect of correcting large negative distortion
And the next positive twenty-third lens produces distortion and
The spherical aberration generated by the negative twenty-first lens having a meniscus shape
Corrected well. Next, both lens surfaces are concave second negative.
Fourth lens and positive 25th lens with strong convex surface facing the object side
With a positive cemented lens joined to
Tal halo and chromatic aberration are well corrected. Also joining
It has the function of correcting sagittal halo on the lens surface.
Thus, more favorable aberration correction is performed. In addition, 26th
Lens and the 27th lens
Balancing spherical aberration and coma with 28 lenses
Lens construction that is
It has been done.

In the present invention, the first lens unit and the second lens unit are set as described above, and the focal length of the first lens unit and the total lens length (from the first lens surface of the first lens unit to the final lens surface of the first lens unit) Optical constants such as the focal length of the entire system and the overall length of the lens (the length from the first lens surface to the final lens surface when focusing on an object at infinity) are conditional expressions (1). ) To (3). In this way, aberration fluctuations when the second unit is moved to perform focusing are favorably corrected, and an inverse telephoto imaging lens having high optical performance over the entire object distance is configured.

Next, the technical meaning of each of the above conditional expressions will be described.

Conditional expression (1) satisfactorily corrects various aberrations while lengthening the back focus, and furthermore, the sensitivity of the second unit as a focus lens, that is, the amount of movement of the image plane with respect to the amount of movement of the second unit. This is for setting the ratio appropriately. If the negative refractive power of the first lens unit becomes too weak beyond the upper limit of conditional expression (1), the sensitivity of the focus lens (second lens unit) tends to increase. For example, when used in an autofocus camera, Although this is advantageous for the torque of the focusing motor, the back focus is shortened, and the diameter of the front lens and the overall length of the lens are increased. If the negative refractive power of the first lens unit becomes too strong beyond the lower limit of conditional expression (1),
Although it is advantageous to lengthen the back focus, distortion is significantly increased, and it is difficult to correct this by the second lens unit.

Conditional expression (2) holds that the first lens unit is fixed and the second lens unit is fixed.
This is for good focusing in a group. If the distance between the first lens unit and the second lens unit is excessively increased beyond the upper limit of conditional expression (2), the moving space of the second lens unit that moves toward the object side during focusing from an object at infinity to a close object. However, there is an advantage that the distance between the diaphragm and the first lens unit is long, but the outer diameter of the lens of the first lens unit for securing the off-axis light flux increases. If the lower limit of conditional expression (2) is exceeded and the distance between the first unit and the second unit becomes too short, the lens outer diameter can be made compact and the height of the off-axis light beam from the optical axis can be reduced. Although it is advantageous for correction, it becomes difficult to shorten the close distance.

Conditional expression (3) represents the ratio of the total length of the first lens unit and the entire system, that is, the ratio to the distance from the lens surface closest to the object side to the lens surface closest to the image plane in the first lens unit and the entire system. It is specified. If the total length of the first lens unit is too long beyond the upper limit of conditional expression (3), the tendency of reverse telephoto (retro type) in the first lens unit becomes strong, and the back focus is secured as a whole lens system. However, since the distance between the stop in the second group and the first lens surface becomes longer, the outer diameter of the first group for securing off-axis light flux increases. Exceeding the lower limit of conditional expression (3) and shortening the entire length of the first lens unit is advantageous for making the outer diameter of the first lens unit compact, but is not preferable because the back focus becomes short.

The reverse telephoto taking lens aimed at by the present invention can be achieved by satisfying the above conditions. To obtain high optical performance over the entire object distance, the following conditions must be satisfied.
It is good to satisfy the matter.

[0020]

[0021]

[0022]

[0023] ◎ is to satisfy the first 24 lens and each refractive index of the material of the 25 lens N24, N25 and to 0.1 when the <N25-N24 ······ (4) following condition.

Conditional expression (4) is mainly for favorably correcting sagittal halo by the cemented lens surface of the cemented lens. Deviating from conditional expression (4) makes it easy to correct chromatic aberration, but makes it difficult to satisfactorily correct sagittal halo.

Next, numerical examples of the present invention will be described. In the numerical examples, Ri is the radius of curvature of the i-th lens surface in order from the object side, Di is the i-th lens thickness and air spacing from the object side, and Ni and νi are the i-th lens surfaces in order from the object side. The refractive index and Abbe number of glass. Table 1 shows the relationship between the above-described conditional expressions and various numerical values in the numerical examples. (Numerical Example 1) f = 20.0 fNO = 1: 2.9 2ω = 94.5 ° R 1 = 39.49 D 1 = 2.00 N 1 = 1.62299 ν 1 = 58.2 R 2 = 21.74 D 2 = 6.72 R 3 = 49.41 D 3 = 1.70 N 2 = 1.60311 ν 2 = 60.7 R 4 = 25.16 D 4 = 6.50 R 5 = 133.95 D 5 = 7.30 N 3 = 1.58313 ν 3 = 59.4 R 6 = -83.60 D 6 = Variable (4.09 ~) R 7 = 26.63 D 7 = 1.00 N 4 = 1.78650 ν 4 = 50.0 R 8 = 10.93 D 8 = 3.06 R 9 = 374.81 D 9 = 2.80 N 5 = 1.80518 ν 5 = 25.4 R10 = -48.07 D10 = 1.27 R11 = 194.64 D11 = 11.60 N 6 = 1.51633 ν 6 = 64.2 R12 = -14.38 D12 = 0.73 R13 = (aperture) D13 = 3.50 R14 = -23.20 D14 = 1.10 N 7 = 1.62096 ν 7 = 35.9 R15 = 12.92 D15 = 6.70 N 8 = 1.80518 ν 8 = 25.4 R16 = 90.10 D16 = 0.57 R17 = -134.18 D17 = 1.10 N 9 = 1.84666 ν 9 = 23.9 R18 = 19.03 D18 = 7.60 N10 = 1.48749 ν10 = 70.2 R19 = -17.58 D19 = 0.10 R20 = 234.45 D20 = 3.50 N11 = 1.77250 ν11 = 49.6 R21 = -45.07 (Numerical Example 2) f = 20.49 fNO = 1: 2.9 2ω = 93.1 ° R 1 = 39.03 D 1 = 2.00 N 1 = 1.62299 ν 1 = 58.2 R 2 = 23.24 D 2 = 6.06 R 3 = 52.92 D 3 = 1.70 N 2 = 1.60311 ν 2 = 60.7 R 4 = 24.56 D 4 = 6.30 R 5 = 115.45 D 5 = 10.80 N 3 = 1.58313 ν 3 = 59.4 R 6 = -93.19 D 6 = Variable (3.97 ~) R 7 = 25.08 D 7 = 1.00 N 4 = 1.77250 ν 4 = 49.6 R 8 = 10.72 D 8 = 3.34 R 9 = -1747.11 D 9 = 2.80 N 5 = 1.80518 ν 5 = 25.4 R10 = -45.10 D10 = 1.27 R11 = 149.79 D11 = 11.60 N 6 = 1.51633 ν 6 = 64.2 R12 = -14.37 D12 = 0.73 R13 = (Aperture) D13 = 3.50 R14 = -23.50 D14 = 1.10 N 7 = 1.62096 ν 7 = 35.9 R15 = 13.05 D15 = 6.70 N 8 = 1.80518 ν 8 = 25.4 R16 = 83.36 D16 = 0.66 R17 = -145.33 D17 = 1.10 N 9 = 1.84666 ν 9 = 23.9 R18 = 19.16 D18 = 7.60 N10 = 1.48749 ν10 = 70.2 R19 = -18.23 D19 = 0.15 R20 = 283.36 D20 = 3.40 N11 = 1.77250 ν11 = 49.6 R21 = -43.99

[0026]

[Table 1]

[0027]

According to the present invention, by setting each element as described above, aberrations during focusing over the entire object distance from an object at infinity to an object at a short distance, while maintaining the advantages of the rear focus type. 1. A shooting angle of view of about 94 degrees, which has high optical performance with fluctuations satisfactorily corrected, F number 2.
Approximately 8, the back focus is 1.7 to 1.9 of the focal length
It is possible to achieve a reverse telephoto shooting lens of about twice and a camera using the same.

[Brief description of the drawings]

FIG. 1 is a sectional view of a lens according to a numerical example 1 of the present invention.

FIG. 2 is a sectional view of a lens according to a numerical example 2 of the present invention.

FIG. 3 is an aberration diagram of an object at infinity according to Numerical Embodiment 1 of the present invention.

FIG. 4 is an aberration diagram at an object distance of 2500 in Numerical Example 1 of the present invention.

FIG. 5 is an aberration diagram of an object at infinity according to Numerical Example 2 of the present invention.

FIG. 6 is an aberration diagram at an object distance of 2500 in Numerical Example 2 of the present invention.

[Explanation of symbols]

 L1 First lens unit L2 Second lens unit SP Aperture IP Image plane dd line gg line S. C Sine condition ΔS Sagittal image plane ΔM Meridional image plane

Claims (3)

(57) [Claims] 1. A lens system comprising: a first lens unit having a negative refractive power and a second lens unit having a positive refractive power in order from the object side. The first unit has a meniscus shape having a convex surface facing the object side. It has a negative eleventh lens, a meniscus negative twelfth lens having a convex surface facing the object side, and a positive lens . The second lens unit has a meniscus having a convex surface facing the object side in order from the object side.
Cas-shaped negative 21st lens, positive 22nd lens, image surface side
Positive 23rd lens with the convex surface facing the
The negative 24th lens and the positive 25th lens with the convex surface facing the object side
Lens with lens attached, both lens surfaces concave
A negative 26th lens and a positive 27th lens having both lens surfaces convex.
Lens and lens surfaces are convex
A second lens unit having a positive surface having a positive 28th lens, and moving the second unit on the optical axis to perform focusing;
The focal length and the total lens length of the group are f1 and DT1, respectively, the focal length and the total lens length of the entire system are f and DT, respectively, and the air gap between the first and second groups when focusing on an object at infinity is S. In this case, the following condition is satisfied: 4 <| f1 / f | <6.5 0.1 <S / f <0.5 0.25 <DT1 / DT <0.5 Telephoto shooting lens. 2. The rear focus system according to claim 1 , wherein a condition of 0.1 <N25−N24 is satisfied when the refractive indices of the materials of the 24th lens and the 25th lens are N24 and N25, respectively. Inverted telephoto lens. 3. A camera comprising the rear focus type reverse telephoto type photographing lens according to claim 1 .