JPH1130743A - Retrofocus type lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the retrofocus type lens which has excellent optical performance over the entire image plane. SOLUTION: This lens meets the requirements represented by 1.6<n1 N, 46<ν1 N, ν1 N-ν1 P<10, 75<ν2a NF, and 0.5<S2a NF/f<2.0, where D1 N is the mean value of the refractive index of a negative lens in a 1st lens group B1, ν1 N the mean value of the Abbe number of the negative lens in B1, ν1 P the mean value of the Abbe number of a positive lens in B1, ν2a NF the Abbe number of the negative lens F arranged on the most object side among the negative lenses in the front group B2a of a 2nd positive lens group B2, S2a NF the distance between the said negative lens and a stop S, and (f) the focal length of the whole system.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a retrofocus lens suitable for a wide-angle lens used in a single-lens reflex camera, a video camera, and the like.

[0002]

2. Description of the Related Art As a lens type advantageous for widening the angle of view, a so-called retrofocus type lens composed of a negative lens group and a positive lens group in order from the object side has been known. However, since it is easy to secure a sufficient back focus, it is often used for a wide-angle lens for a single-lens reflex camera.

For example, JP-A-51-72432, JP-A-55-143517, and JP-A-59-18530
7, JP-A-5-188294, JP-A-7-35
971 and the like.
-28336 and JP-A-62-78520 disclose an optical system to which the present technology is applied.

[0004]

However, the retrofocus wide-angle lens described above has the following disadvantages.

[0005] First, since the negative lens group is the leading lens type, negative distortion is likely to occur.
In order to solve this problem, the negative lens in the negative lens group may have a high refractive index. However, generally, a glass material having a high refractive index has a large dispersion, that is, a small Abbe number, and negative chromatic aberration of magnification occurs. Easier to do.

Means for simultaneously solving the above-mentioned problems of distortion and chromatic aberration of magnification by arranging a positive lens in the negative lens group has been proposed. However, in this case, a portion having a high image height is used. The chromatic aberration of magnification is excessively corrected to cause color distortion, and in order to balance the entire screen, the chromatic aberration of magnification is left as an under image at an intermediate image height and an over image at a maximum image height as shown in FIG. Not get.

An object of the present invention is to provide a retrofocus lens which overcomes the above-mentioned drawbacks and has good optical performance over the entire screen.

[0008]

According to a first aspect of the present invention, there is provided a retrofocus lens having a first negative lens unit and a second positive lens unit in order from the object side. The positive second lens unit has a front unit, a stop, and a rear unit in order from the object side, and satisfies the following condition. 1.6 <n _1N 46 <ν _1N ν _1N −ν _1P <10 75 <ν _2aNF 0.5 <S _2aNF /f<2.0

Here, n _1N is the average value of the refractive index of the negative lens in the negative first lens unit, ν _1N is the average value of the Abbe number of the negative lens in the negative first lens unit, ν _1P is the average value of Abbe numbers of the positive lenses in the negative first lens unit, ν
_2aNF is the Abbe number of the negative lens closest to the object side among the negative lenses in the front group of the positive second lens group, S _2aNF
Is the distance between the negative lens closest to the object side and the stop among the negative lenses in the front group in the positive second lens group, and f is the focal length of the entire system.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to the embodiments shown in FIGS. FIG. 1 is a lens configuration diagram. B1 is a negative first lens group, B2 is a positive second lens group, and a positive second lens group B2 is a front group B2a, a stop S, and a rear group in order from the object side. F2 is a negative lens closest to the object side in the front unit B2a. In addition, front group B2
A flare cutter C is arranged in the middle part of a.

In order to simultaneously achieve the correction of the distortion and the elimination of the chromatic aberration of magnification over the entire screen, the following conditional expression is satisfied. 1.6 <n _1N (1) 46 <ν _1N (2) ν _1N −ν _1P <10 (3) 75 <ν _2aNF (4) 0.5 <S _2aNF /f<2.0 (Five)

Here, n _1N is the average value of the refractive index of the negative lens in the negative first lens unit B1, ν _1N is the average value of the Abbe number of the negative lens in the negative first lens unit B1, and ν _{1N 1P} is the average value of the Abbe numbers of the positive lenses in the negative first lens unit B1, and ν
_2aNF is the Abbe number of the negative lens closest to the object side among the negative lenses in the front group B2a of the positive second lens group B2,
S _2aNF is the distance between the negative lens closest to the object side among the negative lenses in the front group B2a in the positive second lens group B2 and the stop, and f is the focal length of the entire system.

Conditional expression (1) is a condition for defining the average value of the refractive index of the negative lens in the negative first lens unit B1, and out of this range, the negative distortion becomes particularly large.

Conditional expression (2) is a condition for defining the average value of the Abbe numbers of the negative lenses in the negative first lens unit B1. If the average value is outside this range, the lateral chromatic aberration will be insufficiently corrected in the under direction.

Condition (3) is a condition for appropriately setting the average value of the Abbe number of the negative lens and the average value of the Abbe number of the positive lens in the negative first lens unit B1.

The positive lens in the negative lens group closest to the object side of the retrofocus type lens (the negative first lens group B1 in this embodiment) mainly has negative distortion and negative distortion generated in the lens group B1. It has a function of correcting magnification chromatic aberration.

[0017] However in the case of wide-angle lens, the intermediate angle and maximum angle, since the different heights greater from the optical axis of the light beam passing through the negative lens group, i.e. maximum angle h ₂ and intermediate 1 since the magnitude of the angle h ₁ is largely different,
If the dispersion of the positive lens group is too large, that is, if the Abbe number is too small, the chromatic aberration of magnification is excessively corrected in the over direction at the maximum angle of view as shown in FIG. Distortion occurs.

Conditional expression (3) suppresses such a phenomenon. If the conditional expression (3) is exceeded, the color distortion will be excessive.

Conditional expression (4) defines the Abbe number of the negative lens F located closest to the object side among the negative lenses in the front group B2a of the positive second lens unit B2. Conditional expression (5) ) Are conditions for defining the distance between the negative lens F and the stop S.

Although the chromatic distortion is reduced by the conditional expression (3), the negative lateral chromatic aberration remains in this state, so that the first lens group B1 and subsequent lens groups have the first lens group. It is necessary to make it easy to correct the chromatic aberration of magnification remaining in B1. Therefore, among the negative lenses in the front unit B2a located on the object side of the stop S of the positive second lens unit B2, the dispersion of the negative lens F located closest to the object side among the negative lenses in the front unit B2a is small, that is, the Abbe number is large. Further, by defining the distance of the negative lens F from the stop S, the occurrence of negative chromatic aberration is reduced and the correction of chromatic aberration of magnification is facilitated.

Conditional expressions (4) and (5) are the above conditions. If the condition (4) is not satisfied, the negative lateral chromatic aberration generated in the negative lens becomes large, and the correction of the lateral chromatic aberration in the entire system is performed. Becomes difficult.

If the upper limit of the condition (5) is exceeded, the height of the off-axis light beam passing through the negative lens F from the optical axis becomes too large, and chromatic aberration of magnification is apt to occur. Even if the condition (4) is satisfied, it is difficult to correct lateral chromatic aberration. On the other hand, if the lower limit is exceeded, the height of the off-axis light beam passing through the negative lens F from the optical axis becomes small and overlaps too much with the on-axis light beam, so that the degree of freedom to control each light beam separately decreases. As a result, it becomes difficult to correct astigmatism and coma flare.

As described above, conditional expressions (1) to (5)
Is satisfied, it is possible to obtain good optical performance in which negative distortion and lateral chromatic aberration are eliminated.

It is preferable that conditional expressions (2) to (5) have the following numerical ranges. 49 <ν _1N (2) ′ 5.0> ν _1N −ν _1P (3) ′ 80 <ν _2aNF (4) ′ 0.7 <S _2aNF /f<1.5 (5) ′

In order to further enhance the effects of the present invention, it is preferable that the negative lens F satisfies the following conditional expression. Here, ν is the Abbe number of the negative lens F, and ng, nF and nc are the refractive indices of the g-line, F-line and c-line, respectively. (Ng−nF) / (nF−nc)> − 0.0016ν + 0.645 (6)

This conditional expression (6) is a condition for defining the partial dispersion and the Abbe number of the g-line and the F-line of the negative lens F. When this condition is satisfied, the difference between the chromatic aberration of magnification of the F-line and the g-line becomes small. Further better optical performance is obtained.

When f _2a is the focal length of the front unit B2a in the positive second lens unit B2 and f is the focal length of the entire system, it is desirable that the following conditional expression is satisfied. 2.0 <f _2a /f<5.0 (7)

Conditional expression (7) defines the focal length of the front group B2a of the positive second lens unit B2. If the upper limit is exceeded, the positive second lens unit B2 may obtain a sufficient refractive power. This makes it difficult to correct the positive spherical aberration generated in the negative first lens unit B1. Conversely, when the value goes below the lower limit, the spherical aberration becomes large in the under direction.

It is preferable that conditional expression (7) be set in the following numerical range. 2.2 <f _2a /f<4.0 (7) ′

Further, it is preferable that the negative first lens unit B1 is composed of a negative lens component composed of only a negative lens and a positive lens component in order from the object side, and satisfies the following condition. Here, f ₁ is the focal length of the negative first lens unit B1, and f _2b
Is the focal length of the front group B2a in the positive second lens group B2. −10 <f ₁ /f<−2.0 (8) 1.0 <f _2b /f<2.0 (9)

Condition (8) defines the focal length of the negative first lens unit B1. If the upper limit is exceeded, the front lens diameter increases, and if the lower limit is exceeded, it is particularly difficult to correct negative distortion. Becomes

Condition (9) is a condition for defining the focal length of the second positive lens unit B2. If the upper limit of the conditional expression (9) is exceeded, it becomes difficult to use a retrofocus type lens. It becomes difficult. vice versa,
If the lower limit is exceeded, it becomes difficult to correct negative distortion and negative spherical aberration.

It is more preferable that conditional expressions (8) and (9) be in the following numerical ranges. −8.0 <f ₁ /f<−3.0 (8) ′ 1.2 <f _2b /f<1.8 (9) ′

More preferably, the first negative lens unit B1 includes, in order from the object, a negative meniscus lens convex to the object side, a negative lens and a positive lens, or an aspherical surface having a negative refractive power weaker toward the periphery. If a negative meniscus lens convex to the object side and a positive lens are used, negative distortion and astigmatism can be corrected well without increasing the size of the entire optical system.

In addition to the conditional expressions (1) to (3), at least one of the positive lenses P in the rear unit B2b of the positive second lens unit B2 satisfies the following conditional expression. If you do
Further better optical performance can be obtained. 1.6 <n (10) 60 <ν (11) (ng−nF) / (nF−nc)> − 0.0016νd + 0.641 (12) where n and ν are refractions of the positive lens P, respectively. Value, Abbe number.

Conditional expression (10) is a condition for defining the refractive index of the positive lens P. If this condition is satisfied, negative distortion can be easily corrected.

Conditional expression (11) is a condition for defining the Abbe number of the positive lens P. If this condition is satisfied, it becomes easy to correct lateral chromatic aberration in the positive direction.

Conditional expression (12) is a condition for defining the partial dispersion of the positive lens P between the g-line and the f-line. It is preferable to satisfy this condition since the difference in the chromatic aberration of magnification between the g-line and the f-line becomes small.

The focusing to a short distance is performed by (a)
(B) a method in which the first negative lens group B1 is used and the positive second lens group B2 is moved close to the first lens group B2, and (b) a method in which the first negative lens group B1 is fixed and only the second positive lens group B2 is used.
(c) The front group B2a of the negative first lens group B1 and the positive second lens group B2 is fixed, and the rear group B of the positive second lens group B2 is fixed.
Any of the methods of extending only 2b may be used. However, in consideration of the balance between the load on the actuator when used as a photographic lens for autofocusing and the optical performance, the negative first lens group B1 is fixed and the positive first lens group B1 is fixed. Second lens group B2
The most desirable method is to feed out the front group B2a and the rear group B2b closer to each other.

FIGS. 2 to 8 show Examples 1 to 7 respectively.
FIG. Tables 1 to 7 show Example 1 respectively.
7 are Numerical Examples 1 to 7.

Ri is the radius of curvature of the i-th lens surface from the object side, di is the lens thickness or air gap, and ν is the Abbe number. d, g, C, and F are d line, g line, C line, and F, respectively.
The index of refraction for the line.

The aspherical surface is represented by the following equation, where X is the direction of the optical axis and h is the distance from the optical axis. ^{X = (h 2 / r)} / [1+ {1- (h / r) 1/2] + Ah
² + Bh ⁴ + Ch ⁶ + Dh ⁸ + Eh ¹⁰

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

[Table 3]

[0046]

[Table 4]

[0047]

[Table 5]

[0048]

[Table 6]

[0049]

[Table 7]

The following table shows the conditional expressions in each of Numerical Examples 1 to 7.
Numerical values of (1) to (5) and (7) to (9). Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 (1) n _1N 1.6385 1.6385 1.6779 1.7347 1.6968 1.6968 1.6385 (2) ν _1N 55.4 55.4 55.3 52.6 55.5 55.5 55.4 (3) ν _1N -Ν _1D -14.8 -14.8 -14.9 3.0 0.2 1.68 -14.8 (4) ν _2aNF 81.6 81.6 95.1 81.6 81.6 81.6 81.6 (5) S _2aNF / F 1.024 1.025 1.181 1.070 1.060 1.049 1.039 (7) f _2a / f 3.43 3.15 2.83 3.45 3.26 3.31 3.34 (8) f 1 / f -5.94 -3.91 -3.87 -5.07 -4.56 -4.83 -5.93 (9) f 2b / f 1.47 1.47 1.51 1.48 1.48 1.47 1.49

9 to 15 are aberration diagrams of the first to seventh embodiments.

[0052]

As described above, the retrofocus type lens according to the present invention can favorably correct distortion and chromatic aberration of magnification in particular, while having a wide angle of about 2ω = 84 °.

[Brief description of the drawings]

FIG. 1 is a lens configuration diagram.

FIG. 2 is a cross-sectional view of the first embodiment.

FIG. 3 is a sectional view of a second embodiment.

FIG. 4 is a sectional view of a third embodiment.

FIG. 5 is a sectional view of a fourth embodiment.

FIG. 6 is a sectional view of a fifth embodiment.

FIG. 7 is a sectional view of a sixth embodiment.

FIG. 8 is a sectional view of a seventh embodiment.

FIG. 9 is an aberration diagram of the first embodiment.

FIG. 10 is an aberration diagram of the second embodiment.

FIG. 11 is an aberration diagram of the third embodiment.

FIG. 12 is an aberration diagram of the fourth embodiment.

FIG. 13 is an aberration diagram of the fifth embodiment.

FIG. 14 is an aberration diagram of the sixth embodiment.

FIG. 15 is an aberration diagram of the seventh embodiment.

FIG. 16 is a diagram showing color distortion.

[Explanation of symbols]

 B1 Negative first lens group B2 Positive second lens group B2a Front group of positive second lens group B2b Rear group of positive second lens group F Negative lens closest to object in front group M Meridional focal line S sagittal focal line g g line c c line FF line

Claims (3)

[Claims] 1. A retrofocus lens having a negative first lens group and a positive second lens group in order from the object side, wherein the positive second lens group includes a front group, an aperture, and a rear group in order from the object side. A retrofocus type lens having the following conditions. 1.6 <n _1N 46 <ν _1N ν _1N −ν _1P <10 75 <ν _2aNF 0.5 <S _2aNF /f<2.0 where n _1N is a negative lens in the negative first lens unit. Ν _1N is the average of the Abbe number of the negative lens in the negative first lens group, and ν _1P is the average of the Abbe number of the positive lens in the negative first lens group. , Ν _2aNF is the Abbe number of the negative lens closest to the object side among the negative lenses in the front group of the positive second lens group, and S _2aNF is the front group in the positive second lens group. Is the distance between the negative lens disposed closest to the object side and the stop, and f is the focal length of the entire system. 2. The retrofocus lens according to claim 1, wherein the following condition is satisfied. 2.0 <f _2a /f<5.0 where f _2a is the focal length of the front group in the positive second lens group. 3. The retro first lens group according to claim 2, wherein the first negative lens unit includes, in order from the object side, a negative lens component composed of only a negative lens and a positive lens component, and satisfies the following condition. Focus lens. −10 <f ₁ /f<−2.0 1.0 <f _2b /f<2.0 where f ₁ is the focal length of the negative first lens unit, f
_2b is the focal length of the front group in the positive second lens group.