JPH07104181A - Wide-angle photographic lens - Google Patents

Abstract

PURPOSE:To obtain the wide-angle photographic lens having a large aperture and a wide view angle, in which both of overall length of the lens and SIGMAd of the lens are small, are suitable for a collapsible barrel system, and a focal length is 35mm and an F-number if a 2.8 class. CONSTITUTION:In this lens constituted of a front group and a rear group, the front group and the rear group both consists of a lens formed by cementing positive lenses L1, L3 and negative lenses L2, L4 in this order, and at least one aspherical surface is provided, a diaphragm is arranged between the front group and the rear group, at least one aspherical surface satisfies DELTA/phi<0 [phi=(n'-n)/r], and also, 1<r1/r3<2 is satisfied. In this regard, (r), (n) and n', DELTA, and r1 denote the radius of paraxial curvature of the aspherical surface, refractive indexes of mediums before and behind the aspherical surface, the aspherical surface amount in the effective radius, and the radius of curvature of an i-th face counted from an object side, respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photographic lens suitable for a lens shutter camera or the like, and more particularly to a large-diameter and high-performance wide-angle photographic lens having an F number of 2.8 class.

[0002]

2. Description of the Related Art Conventionally, a wide-angle lens having an angle of view of about 60 ° to 64 ° has been standardly adopted in a lens shutter camera equipped with a monofocal lens. The most popular lens used for these is the triplet type, but the F number is limited to about 3.5.

By the way, as the wide-angle lens capable of increasing the aperture up to the F2.8 class, two types, a telephoto type and a tesser type, are often used. The former telephoto type is disclosed in JP-A-56-59217,
JP-A-57-116313 and JP-A-59-14731
No. 2 and the like are known, but many proposals have been made in addition to these. Both are composed of a positive front group and a negative rear group, which is an application of the type originally developed for telephoto lenses, but since the principal point position can be set to the object side,
The total lens length can be shortened. A telephoto ratio of around 1 is possible. However, this type has a large amount of astigmatism, field curvature, and distortion that are difficult to correct, so
In the above prior art example, the final lens is made aspherical to deal with it. Similarly, in other preceding examples, aberrations are corrected by using aspherical surfaces in the front and rear groups.

The latter tesser type is disclosed in Japanese Patent Laid-Open No.
Nos. 0-176011 and JP-A-2-208616 are known. Both of them use a behind diaphragm configuration to simplify the feeding mechanism when focusing. Since the principal point position is inside the lens system, the total length of the lens is longer than that of the telephoto type, which is disadvantageous in making the camera compact.

On the other hand, as a standard lens of a single-lens reflex camera, a lens having an angle of view of about 47 ° is used, and the Gauss type is famous, but the number of constituent lenses is large. Therefore, as a type having a small number of constituents, Japanese Patent Laid-Open No. 3-212606 has been proposed. It is composed of 4 lenses in 2 groups, positive / negative cemented lens and negative / positive cemented lens. By using an aspherical surface of 1 to 3 surfaces, the focal length is 50 m.
A lens of m, F number 2 class is obtained with good aberration correction.

[0006]

Conventionally, in order to miniaturize a lens shutter camera, it has been indispensable to reduce the total length of the taking lens (the length from the first surface to the film surface). The shorter the overall length of the taking lens, the thinner the camera thickness, and a camera with good portability was realized. Therefore, it can be said that most of the many proposed examples have been working on shortening the total lens length.

However, in recent years, the precision of the lens frame has been improved,
Along with the development of higher functionality, a method of not only simply shortening the lens frame length as in the past but also retracting the lens system to achieve miniaturization has been used. At this time, the thickness of the camera when retracted is
It is determined by d (the length from the first surface to the last surface). In the end, for downsizing, it becomes necessary to shorten both the total lens length and the lens system Σd.

From this point of view, since the telephoto type has a large Σd, it is not suitable for collapsing.

On the other hand, the tesser type is very advantageous because it has a small Σd, but as a drawback of this type, astigmatism and field curvature are largely generated, and even if an aspherical surface is used, these aberrations can be corrected. There is a problem in performance because it cannot be fully performed.

It is also possible to apply the above-mentioned Japanese Patent Laid-Open No. 3-212606 to a lens shutter camera. At this time, good aberration correction is possible, and Σd
However, since the optical system originally has a symmetrical shape, the principal point position is near the center of the lens, and the total lens length cannot be shortened. Therefore, this type also has a difficulty in size.

The present invention has been made in view of the above-mentioned drawbacks of the prior art, and an object thereof is to provide a lens system suitable for a collapsible system because both the total lens length and the lens Σd are small. , To achieve a high-performance wide-angle photographic lens. In addition, the specifications include a focal length of 35 m.
It provides a wide-angle photographic lens with a large aperture and a wide angle of view in the m, F number 2.8 class.

[0012]

A wide-angle photographic lens of the present invention which achieves the above object is a lens composed of a front group and a rear group in order from the object side, wherein the front group and the rear group are both
A positive lens and a negative lens are cemented in this order, and have at least one aspherical surface.

In this case, it is desirable to arrange a diaphragm between the front group and the rear group.

Further, in these cases, it is desirable that at least one aspherical surface satisfies the following conditional expression (1). Δ / φ <0 [φ = (n′−n) / r] (1) where r is the paraxial radius of curvature of the aspherical surface, n and n ′ are the refractive indices of the media before and after the aspherical surface, Δ is the amount of aspherical surface at the effective radius.

Further, in any of the above cases, it is desirable that the following conditional expression (2) is satisfied. 1 <r ₁ / r ₃ <2 (2) where r _i is the radius of curvature of the i-th surface counted from the object side.

[0016]

The function and operation of adopting the above configuration will be described below. A triplet type or tesser type lens system is composed of three to four lenses, but the first lens of positive refracting power and the second lens of negative refracting power are separated from each other. Large aberrations are generated on the front and back surfaces of the lens, enabling aberration correction for the entire system. Therefore, there is a drawback in that the decentration accuracy of these lenses naturally becomes severe.

On the other hand, in the present invention, the first positive refractive power
The lens and the second lens of negative refractive power, and the third lens of positive refractive power
Since the lens and the fourth lens having a negative refractive power are both cemented lenses, the aberration correction capability is not sufficient as it is. In particular, the smaller the F number and the larger the diameter, the larger the amount of spherical aberration generated, and the more difficult it becomes to correct. Therefore, it is necessary to introduce at least one aspherical surface to enable correction of spherical aberration. At this time, it is desirable that at least one aspherical surface satisfies the following conditional expression (1). Δ / φ <0 [φ = (n′−n) / r] (1) where r is the paraxial radius of curvature of the aspherical surface, n and n ′ are the refractive indices of the media before and after the aspherical surface, Δ is the amount of aspherical surface at the effective radius.

The above conditional expression (1) defines the shape of the aspherical surface, and the aspherical shape is such that the negative refractive power is gradually strengthened or the positive refractive power is weakened as the distance from the optical axis increases. Is shown. By satisfying conditional expression (1), it becomes possible to correct spherical aberration. In particular, it is desirable to make at least the third surface (the image-side surface of the second lens) aspheric. At this time, various aberrations of the entire system can be corrected in the best balance.

Since the lens system of the present invention is composed of a front group having a positive refracting power and a rear group having a positive refracting power with a diaphragm interposed therebetween,
The overall arrangement is symmetrical. Therefore, it is possible to satisfactorily correct distortion and lateral chromatic aberration.
However, unlike the above-mentioned prior art JP-A-3-212606, by constructing the rear lens group by cementing a positive lens and a negative lens in this order, the negative lens arranged closest to the image side causes Like the telephoto type, the principal point position is set to the object side, and the total lens length can be shortened.
In order to make full use of such effects, it is desirable that the image-side surface of the fourth lens be concave on the image side.

Further, it is desirable to satisfy the following conditional expression (2) for better aberration correction. 1 <r ₁ / r ₃ <2 (2) where r _i is the radius of curvature of the i-th surface counted from the object side.

Conditional expression (2) is a conditional expression for better correcting spherical aberration and coma, and it is the third surface from the first surface.
It indicates that the radius of curvature of the surface is smaller. The aberration generated in the first surface and the aberration generated in the rear group are added to deteriorate the overall aberration. Since these aberrations must be corrected by the third surface, stronger power is given to this surface and larger aberrations are generated to correct the aberrations of the entire system. Therefore, if the lower limit of 1 to condition (2) is exceeded, this aberration correction capability will not be exhibited, and good aberration correction will be difficult only with an aspherical surface. On the other hand, when the upper limit of 2 is exceeded, the aberration generated on the third surface becomes too large, and even if an aspherical surface is used, it cannot be balanced with the aberrations generated on other surfaces.

As is generally known, it is effective to use a positive lens having a large Abbe number and a negative lens having a small Abbe number in combination for correcting chromatic aberration.
Therefore, it is desirable that the glass used for the negative lens satisfy the following conditional expression (3). ν _N <50 (3) where ν _N is the Abbe number of the negative lens. If the Abbe number of the negative lens exceeds 50 beyond the range of the conditional expression (3), it becomes difficult to satisfactorily correct chromatic aberration.

With the above-mentioned structure, it is possible to obtain a large-diameter wide-angle photographic lens having good performance while shortening both the total lens length and Σd of the lens system. Furthermore, the decentering accuracy, which was a problem with the triplet type and the tesser type, can be changed to a cemented lens as in the present invention.
The accuracy can be loosened. Further, in the triplet type or the tesser type, a ghost is likely to occur between the separated first lens and second lens, but the present invention solves such a problem.

[0024]

EXAMPLES Examples 1 to 4 of the wide-angle photographic lens of the present invention will be described below with reference to the drawings. FIG. 1 shows Example 1.
2 is a lens cross-sectional view of Example 3, and FIGS.
Has a cross-sectional shape similar to that of the first embodiment and is not shown.
Although the lens data will be described later, the focal lengths of all the examples are 35 mm, and the F number is 2.8.

In any of the embodiments, in order from the object side, a cemented lens of a positive meniscus lens L1 having a convex surface facing the object side and a negative meniscus lens L2 having a convex surface facing the object side, a diaphragm, a positive lens L3 and a negative lens. It is composed of a cemented lens with L4, and the cemented surface of the rear group is a cemented lens with a concave surface facing the object side. In the third embodiment, the final surface is convex on the image side, but in the other embodiments, the final surface is concave on the image side.

As for the aspherical surface, the first embodiment has the first surface,
In Example 3, the first surface and the third surface are provided on the third surface and the fifth surface.
Surface, 5th surface, 7th surface, 4th surface, Example 3 has 3rd surface, 7th surface, 2nd surface, Example 4 has 1st surface, 3rd surface, 7th surface Used for.

The lens data of each embodiment are shown below. In addition to the above, the symbols are f, the focal length of the entire system, F _NO is the F number, 2ω is the angle of view, f _B is the back focus, r ₁ ,
r ₂ ... curvature radius of each lens surface, d _1, d ₂ ... the spacing between the lens surfaces, n _d1, n _d2 ... d-line refractive index of each lens, ν _d1, ν _d2 ... Each lens Is the Abbe number. In addition,
The aspherical shape is represented by the following equation, where x is the traveling direction of light on the optical axis and y is the direction orthogonal to the optical axis. x = (y ² / r) / [1+ {1-P (y /
r) ² } ^1/2 ] + A ₄ y ⁴ + A ₆ y ⁶ + A ₈ y ⁸ + A ₁₀ y ¹⁰ +
A ₁₂ y ¹² where r is the paraxial radius of curvature, P is the conic coefficient, A ₄ , A ₆ ,
A ₈ , A ₁₀ , and A ₁₂ are aspherical coefficients.

Example 1 f = 35.0, F _NO = 2.9, 2ω = 63.4 °, f _B = 22.2 r ₁ = 10.3590 (aspherical surface) d ₁ = 4.000 n _d1 = 1.77250 ν _d1 = 49.66 r ₂ = 48.6590 d ₂ = 1.000 n _d2 = 1.67270 ν _d2 = 32.10 r ₃ = 8.8120 (aspherical surface) d ₃ = 3.600 r ₄ = ∞ (aperture) d ₄ = 2.400 r ₅ = 117.0460 (aspherical surface) d ₅ = 5.800 n _d3 = 1.77250 ν _d3 = 49.66 r ₆ = -9.4290 d ₆ = 1.000 n _d4 = 1.56444 ν _d4 = 43.78 r ₇ = 109.1830 Aspheric coefficient 1st surface P = 1.0000 A ₄ = 0.11240 × 10 ^-4 A ₆ = 0.15627 × 10 ^-6 A ₈ = 0.23392 × 10 ^-9 A ₁₀ = 0.92155 × 10 ^-11 A ₁₂ = 0 3rd surface P = 1.0000 A ₄ = 0.14149 × 10 ^-3 A ₆ = 0.21898 × 10 ^-5 A ₈ = 0.49961 × 10 ^-7 A ^{_{10 = 0.60708 × 10 -9 A 12}} = 0 fifth surface _{P = 1.0000 A 4 = -0.33016 ×} 10 -4 A 6 = 0.76904 × 10 -6 A 8 = -0.13092 × 10 -7 A 10 = 0.10222 × 10 - ⁹ A ₁₂ = 0
.

Example 2 f = 35.0, F _NO = 2.9, 2ω = 63.4 °, f _B = 23.8 r ₁ = 11.0190 (aspherical surface) d ₁ = 4.300 n _d1 = 1.77250 ν _d1 = 49.66 r ₂ = 36.8040 d ₂ = 1.000 n _d2 = 1.67270 ν _d2 = 32.10 r ₃ = 8.7740 (aspherical surface) d ₃ = 3.000 r ₄ = ∞ (aperture) d ₄ = 2.000 r ₅ = 76.8390 (aspherical surface) d ₅ = 6.000 n _d3 = 1.77250 ν _d3 = 49.66 r ₆ = -8.1640 d ₆ = 1.000 n _d4 = 1.56444 ν _d4 = 43.78 r ₇ = 90.9080 (aspherical surface) aspherical coefficient 1st surface P = 1.0000 A ₄ = 0.72931 × 10 ^-5 A ₆ = 0.18426 × 10 ^-7 A ₈ = -0.37198 x 10 ^-9 A ₁₀ = -0.13139 x 10 ^-10 A ₁₂ = 0 3rd surface P = 1.0000 A ₄ = 0.13404 x 10 ^-3 A ₆ = 0.18509 x 10 ^-5 A ₈ = 0.11778 × 10 ^-7 A ₁₀ = 0.10083 × 10 ^-8 A ₁₂ = 0 5th surface P = 1.0000 A ₄ = -0.26437 × 10 ^-5 A ₆ = -0.30450 × 10 ^-6 A ₈ = 0.14034 × 10 ^-7 A ^{_{10 = -0.14568 × 10 -9 A 12}} = 0 seventh surface _{P = 1.0000 A 4 = 0.28636 ×} 10 -4 A 6 = -0.15316 × 10 -6 A 8 = 0.90896 × 10 -9 A 10 = 0.19591 × 10 - ¹⁰ A ₁₂ = 0
.

Example 3 f = 35.0, F _NO = 2.9, 2ω = 63.4 °, f _B = 27.2 r ₁ = 15.5850 d ₁ = 4.400 n _d1 = 1.77250 ν _d1 = 49.66 r ₂ = 124.0040 d ₂ = 3.100 n _d2 = 1.63636 ν _d2 = 35.37 r ₃ = 10.4000 (aspherical surface) d ₃ = 2.800 r ₄ = ∞ (aperture) d ₄ = 0.700 r ₅ = -162.5130 d ₅ = 5.000 n _d3 = 1.72916 ν _d3 = 54.68 r ₆ =- 6.1800 d ₆ = 1.000 n _d4 = 1.54814 ν _d4 = 45.78 r ₇ = -51.2380 (aspherical surface) aspherical coefficient third surface P = 1.0000 A ₄ = 0.16032 × 10 ^-3 A ₆ = 0.39935 × 10 ^-5 A ₈ = 0.29 120 × 10 ^-6 A ₁₀ = -0.168 72 × 10 ^-7 A ₁₂ = 0.52011 × 10 ^-9 7th surface P = 0.2202 A ₄ = 0.39011 × 10 ^-4 A ₆ = -0.28026 × 10 ^-5 A ₈ = 0.11737 × 10 ^-6 A ₁₀ = -0.22 103 × 10 ^-8 A ₁₂ = 0.15966 × 10 ^-10
.

Example 4 f = 35.0, F _NO = 2.9, 2ω = 63.4 °, f _B = 23.6 r ₁ = 1.11.2670 (aspherical surface) d ₁ = 4.500 n _d1 = 1.77250 ν _d1 = 49.66 r ₂ = 43.2120 d ₂ = 1.000 n _d2 = 1.67270 ν _d2 = 32.10 r ₃ = 9.0610 (aspherical surface) d ₃ = 5.000 r ₄ = ∞ (aperture) d ₄ = 1.000 r ₅ = 59.6430 d ₅ = 4.000 n _d3 = 1.77250 ν _d3 = 49.66 r ₆ = -8.6880 d ₆ = 1.000 n _d4 = 1.56732 ν _d4 = 42.83 r ₇ = 67.6480 (aspherical surface) aspherical coefficient 1st surface P = 1.0000 A ₄ = 0.92931 × 10 ^-5 A ₆ = -0.26552 × 10 ^{-6 _{A 8 = 0.61038 × 10 -8 A}} 10 = -0.67672 × 10 -10 A 12 = 0 third surface _{P = 1.0000 A 4 = 0.13393 ×} 10 -3 A 6 = 0.26539 × 10 -6 A 8 = 0.77612 × 10 - ⁷ A ₁₀ = -0.58284 × 10 ^-9 A ₁₂ = 0 7th surface P = 1.0000 A ₄ = 0.33942 × 10 ^-4 A ₆ = -0.25675 × 10 ^-6 A ₈ = 0.18468 × 10 ^-8 A ₁₀ = 0.24893 × 10 ^-10 A ₁₂ = 0
.

Aberration diagrams showing spherical aberration, astigmatism, distortion, and chromatic aberration of magnification of Examples 1 to 4 upon focusing on infinity are shown in FIGS. 3 to 6, respectively. Further, the numerical values of the conditional expressions (1) to (3) in each example are shown in Table 1 below. In this table, the value of Y indicates the effective radius when calculating the aspheric amount Δ. Further, Ri represents the surface number (i-th surface).

[0033]

[0034]

As is apparent from the above description, the construction of the present invention shortens both the total lens length and the lens system Σd to make it a size suitable for the collapsible system, and has a large aperture and high performance wide-angle photographic lens. Could be achieved.

[Brief description of drawings]

FIG. 1 is a sectional view of a wide-angle photographic lens according to a first embodiment of the present invention.

FIG. 2 is a sectional view of a third embodiment.

FIG. 3 is an aberration diagram illustrating spherical aberration, astigmatism, distortion, and chromatic aberration of magnification when focusing on an object at infinity in Example 1;

FIG. 4 is an aberration diagram similar to FIG. 3 of Example 2.

FIG. 5 is an aberration diagram similar to FIG. 3 of Example 3;

FIG. 6 is an aberration diagram similar to FIG. 3 of Example 4.

[Explanation of symbols]

 L1 ... Positive meniscus lens L2 ... Negative meniscus lens L3 ... Positive lens L4 ... Negative lens

Claims (4)

[Claims] 1. In a lens composed of a front group and a rear group in order from the object side, both the front group and the rear group are composed of a lens in which a positive lens and a negative lens are cemented in this order, and at least one surface A wide-angle photographic lens having an aspherical surface. 2. The wide-angle photographic lens according to claim 1, further comprising a stop disposed between the front group and the rear group. 3. The wide-angle photographic lens according to claim 1, wherein at least one aspherical surface satisfies the following conditional expression (1). Δ / φ <0 [φ = (n′−n) / r] (1) where r is the paraxial radius of curvature of the aspherical surface, n and n ′ are the refractive indices of the media before and after the aspherical surface, Δ is the amount of aspherical surface at the effective radius. 4. The wide-angle photographic lens according to claim 1, wherein the following conditional expression (2) is satisfied. 1 <r ₁ / r ₃ <2 (2) where r _i is the radius of curvature of the i-th surface counted from the object side.