JPH0414323B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a photographic lens that is an improved modified Tetsusar type lens equipped with a lens shutter. As a photographing lens for a camera having a lens shutter, a so-called rear aperture-type Tetsuser type lens in which an aperture is disposed at the rear of the lens is most commonly used. Various photographic lenses of this type have been proposed, such as JP-A No. 53-97824 and JP-A No. 53-Sho.
130021, JP-A-56-75611, etc. However, in general, including these conventional examples, when looking at the distance from the first lens surface to the focal point, which is a standard for the compactness of a photographic lens, that is, the ratio of the total lens length L to the focal length f, the F number is FNo.
For a 2.8 class photographic lens, L/f is approximately 1.09, and when a photographic lens is miniaturized to approximately L/f approximately 1.06, the F number has to be relatively dark at FNo. = 3.5. The purpose of the present invention is to provide a photographic lens with a bright F number of 2.9 and a wide angle of view that is compact and has a wide angle of view. ° shooting lens has been achieved. The features of the lens configuration of the photographic lens for achieving the object of the present invention are, in order from the object side, a first lens having a positive refractive power and a meniscus shape with its convex surface facing toward the object side, a second lens having both lens surfaces concave; It consists of three lens groups: a laminated lens in which a third lens whose both lens surfaces are convex and a fourth lens with negative refractive power are laminated together.A diaphragm is provided on the image plane side of the laminated lens, and Ri is Let Di be the radius of curvature of the i-th lens surface in order from the object side, Di be the thickness and air gap of the i-th lens in order from the object side, and Ni and νi be the refractive index of the glass of the i-th lens in order from the object side. Atsbe number, when the combined focal length of all lens systems is f (1) 0.25<N ₃ −N ₄ <0.4 (2) 0.25<R ₁ /f<0.33 (3) 2<|R ₃ |/f However, R ₃ ≦0 (4) 0.26＜R ₄ /f＜0.35 (5) 0.07＜D ₁ /f＜0.1 (6) 0.06＜D ₃ +D ₄ /f＜0.15 (7) 20＜ν ₂ ＜32 (8 ) 10<ν ₄ −ν ₃ <30. The photographic lens according to the present invention focuses on the following points in order to simultaneously widen the photographic field of view and reduce the overall length of the lens. (b) Reduce the Petzval sum. (b) Shorten the back focus of the lens. (c) Reduce the total thickness of the lens. Regarding item (a), the third laminated lens counting from the object side of a normal Tetsusar type lens is a laminated lens in which lenses with negative and positive refractive powers are laminated together, but the present invention The photographic lens according to the above has a lens structure in which lenses with positive and negative refractive powers are bonded together. This is because when a configuration is made in which lenses with negative and positive refractive powers are bonded together, ν ₃ and ν ₄ need to be approximately equal due to the requirements for correcting chromatic aberration. On the other hand, if you use a structure in which lenses with positive and negative refractive powers are bonded together, ν ₃
<ν ₄ . This has the advantage that N ₃ >N ₄ can be satisfied under the condition that ν ₃ <ν ₄ when selecting the glass material. (B) can be achieved by increasing the refractive power of the first lens; however, if the refractive power is made too strong, it tends to worsen spherical aberration and distortion. If (c) is attempted, spherical aberration and comatic aberration are likely to worsen, and field curvature at the periphery of the screen is likely to increase. The photographic lens according to the present invention has been made in consideration of the above matters (a), (b), and (c), and the photographic lens according to the present invention has the above-mentioned matters in each lens in the above lens configuration. The object of the present invention is achieved by setting conditions (1) to (8). Next, each of the above conditional expressions will be explained. Conditional expression (1)
is the one that increases the refractive index difference between both lenses in the bonded lens of the third lens group and reduces the Petzval sum.If the lower limit of the same conditional expression is exceeded, the Petzval sum becomes large and the sagittal image plane is reduced in the middle of the screen. When the curvature increases in the negative direction and exceeds the upper limit, the sagittal field curvature increases in the positive direction at the periphery of the screen, and at the same time, inward coma aberration occurs in the middle of the screen, making it difficult to widen the angle of view. becomes difficult. Conditional expression (2) improves the sagittal field curvature in the positive direction at the periphery of the screen, which tends to occur within the range of conditional expression (1), by making R ₁ relatively small, and is the upper limit of the conditional expression. If the lower limit is exceeded, it becomes difficult to satisfactorily correct sagittal curvature of field in the positive direction at the periphery of the screen, and if the lower limit is exceeded, it becomes difficult to correct spherical aberration. Conditional expression (3) indicates the range in which |R ₃ | is set relatively large to suppress the sagittal curvature of field at the periphery of the screen from increasing in the positive direction and further correct distortion aberration satisfactorily. If the limit of the same conditional expression is exceeded, the sagittal curvature of field at the periphery of the screen increases, which is undesirable. Furthermore, the reason why conditional expression (3) is set as R ₃ ≦0 is because when R ₃ >0, positive distortion increases, which is not preferable. Conditional expression (4) is intended to shorten the back focus by making _R4 relatively small, and to effectively correct spherical aberration, which is difficult to correct if the total lens thickness is made thin. If the upper limit of the conditional expression is exceeded, it becomes difficult to correct the spherical aberration, and the sagittal curvature of field at intermediate angles of view increases in the negative direction, and if the lower limit is exceeded, the annular spherical aberration increases, which is preferable. do not have. conditional expression
(5) is intended to reduce the total lens thickness by regulating the thickness of D ₁ and to reduce the lens outer diameter of the first lens, thereby reducing the size of the entire lens. Furthermore, improvements have been made in field curvature. If the upper limit of the conditional expression is exceeded, the outer diameter of the first lens becomes large, making it difficult to downsize the photographic lens, and at the same time, the sagittal image plane at intermediate angles of view will be greatly curved in the negative direction. At the periphery of the screen, the sagittal image plane is significantly curved in the positive direction. Moreover, if the lower limit is exceeded, it becomes difficult to correct spherical aberration, which is not preferable. Conditional expression (6) is intended to reduce the size of the photographic lens in a well-balanced manner while correcting inward coma aberration at intermediate angles of view, which tends to occur within the range of conditional expression (1), and is the upper limit of the conditional expression. If it exceeds the lower limit, the outer diameter of the first lens increases, making it difficult to downsize and increases distortion, which is not preferable. If it exceeds the lower limit, inward coma aberration occurs at intermediate angles of view, which is not preferable. Conditional expression (7) is for good correction of longitudinal chromatic aberration and lateral chromatic aberration.If the upper limit of the conditional expression is exceeded, the axial chromatic aberration will be undercorrected and the lateral chromatic aberration in the positive direction will increase, and the lower limit If the value exceeds this value, axial chromatic aberration will be overcorrected, and lateral chromatic aberration will occur in the negative direction, which is undesirable. Conditional expression (8), along with conditional expression (7), is used to properly correct longitudinal chromatic aberration.If the upper limit of the conditional expression is exceeded, the longitudinal chromatic aberration will be undercorrected, and if the lower limit is exceeded, the longitudinal chromatic aberration will be insufficiently corrected. Axial chromatic aberration is overcorrected, which is not desirable. As explained above, by satisfying the above conditions, the photographic lens according to the present invention can be bright, have a wide angle of view, and be compact. For example, in the example described later, L/f=1.06,
2ω = 64 degrees and focal length f = 34mm, L
= 36mm, achieving an extremely compact photographic lens with excellent aberration correction. Further, in the photographic lens according to the present invention, focusing may be performed by extending the entire lens, or by extending only the first lens, or by extending the first lens and the second lens as a unit. Next, numerical examples of the present invention will be shown. In the numerical examples, Ri is the radius of curvature of the i-th lens surface from the object side, Di is the thickness and air gap of the i-th lens from the object side, and Ni and νi are the i-th lens surface from the object side, respectively. are the refractive index and Atsube number of the glass. Further, Table 1 shows the third-order aberration coefficients of Numerical Example 1. As is clear from Table 1, the Petzval sum is usually about 0.25 in a Tetsusar type lens with a rear diaphragm system, whereas in the photographic lens according to the present invention, the Petzval sum is about 0.25.
It is possible to reduce it to about 0.16. For reference, the values of the conditional expressions for each numerical example are shown below.

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【table】 [Brief explanation of drawings]

FIG. 1 is a sectional view of a lens according to Numerical Example 1 of the present invention, and FIGS. 2 to 5 are various aberration diagrams of Numerical Example 1 to Numerical Example 4 of the present invention, respectively.
In the figure, S is a sagittal image plane, and M is a meridional image plane.

Claims (1)

[Claims] 1. A first lens in the form of a meniscus with a positive refractive power with its convex surface facing the object side in order from the object side, a second lens with both lens surfaces concave, and a third lens with both lens surfaces convex. and a fourth lens with negative refractive power are bonded together, a diaphragm is provided on the image plane side of the bonded lens, and Ri is set to the i-th lens surface in order from the object side. radius of curvature,
Di is the thickness and air gap of the i-th lens in order from the object side, Ni and νi are the refractive index and Atsube number of the glass of the i-th lens in order from the object side, and f is the composite focal length of the lens system. When 0.25<N ₃ −N ₄ <0.4 0.25<R ₁ /f<0.33 2<|R ₃ |/f However, R ₃ ≦0 0.26<R ₄ /f<0.35 0.07<D ₁ /f<0.1 0.06<D ₃ +D ₄ /f<0.15 20<ν ₂ <32 10<ν ₄ −ν ₃ <30 A photographic lens that satisfies the following conditions.