JPH0437404B2 - - Google Patents

Description

[Detailed description of the invention]

"Field of Industrial Application" The present invention relates to a finder optical system used in single-lens reflex cameras and the like that uses a pentamirror instead of a pentaprism. ``Prior Art'' Hitherto, Japanese Utility Model Publication No. 48-32325 and Japanese Utility Model Publication No. 10424 have been proposed as viewfinder optical systems for single-lens reflex cameras that use a pentamirror instead of a pentaprism. ``Problems to be Solved by the Invention'' However, although these finder optical systems have less reduction in magnification than conventional finder optical systems for single-lens reflex cameras that use pentaprisms, they have a large number of lenses, so the eyepiece There was a problem in that the total length of the lens increased, which hindered miniaturization of the camera. Another problem was that the cost was high because expensive glass was used. The present invention was made to solve the above-mentioned problems, and although it has a simple structure of two elements in two groups, it has good visibility (good performance), is small in size, and has a high magnification. The object of the present invention is to provide an expensive and inexpensive single-lens reflex finder optical system for a pentamirror. "Means for Solving the Problems" In order to achieve the above object, the finder optical system of the present invention includes a first lens consisting of a positive single lens and a negative single lens from the viewfinder screen side of a single-lens reflex camera. The shape factors of the first and second lenses are SF1 and SF2, respectively, and the shape factor SF is the radius of curvature of the surface of each lens on the viewfinder screen side R _S and the eye When the radius of curvature of the surface on the point side is R _E , it is defined as SF=R _S +R _E /R _S −R _E , and each of (1) −2<SF1<0 (2) 0<SF3<3 It is characterized by satisfying the conditions. The finder optical system configured as described above is characterized in that at least one of the four surfaces forming the first lens and the second lens is an aspherical surface. Note that the aspherical shape is expressed by the following equation. Here, X: Distance taken from the vertex in the direction of the light ray in the direction of the optical axis Y: Height from the optical axis R: Radius of curvature of the reference sphere K: Shape coefficient of rotational quadratic surface A ₂ m: Higher-order aspheric coefficient Furthermore, the thickness of the first lens in the optical axis direction is d ₁ ,
When the distance between the first lens and the second lens on the optical axis is d ₂ , the thickness of the second lens in the optical axis direction is d ₃ , and the combined focal length of the finder optical system is f, (3) It is characterized by satisfying the condition of 0.05<(d ₁ +d ₂ +d ₃ )/f<0.3. Furthermore, it is characterized by satisfying the following condition: (4) 1.0<f _F /f<1.3, where f _F is the distance from the finder screen to the first surface of the first lens. In addition, the first lens is made of acrylic resin, and the second lens is made of polycarbonate resin, and the first and second lenses may be bonded. It should be noted that similar performance can be obtained even if the first lens is made of crown-based glass having a similar Abbé number to that of an acrylic resin, and the second lens is made of a flint-based glass having an Abbé number similar to that of a polycarbonate resin. Further, both the first and second lenses may be made of acrylic resin. "Effect" Condition (1) expresses the relationship between the shapes of the first lens. If SF1 exceeds the lower limit of this condition (1), comatic aberration can be easily corrected, but spherical aberration cannot be corrected. This is difficult and undesirable. On the other hand, if the upper limit is exceeded, spherical aberration can be easily corrected, but comatic aberration becomes difficult to correct, which is not preferable, and the distance between the peripheral parts of the first lens and the second lens increases, so that the first lens The effective diameter becomes large, which is not preferable. Condition (2) expresses the relationship between the shapes of the second lens, and if SF2 exceeds the lower limit of condition (2), it will be difficult to correct coma aberration, which is not preferable. On the other hand, if the upper limit is exceeded, the radius of curvature of the surface on the eyepoint side becomes smaller, which effectively shortens the eyepoint, which is not preferable. Moreover, in order to further maintain good performance, it is preferable to use an aspheric surface for at least one of the four surfaces forming the first lens and the second lens. That is, if an aspherical surface is not used, coma aberration and diopter change with respect to the center will occur significantly in the peripheral part of the visual field, which is not preferable. In particular, when the distance between the first lens and the second lens is widened in order to increase the magnification, the above-mentioned aberrations occur significantly. Condition (3) is related to the ratio of the overall length of the finder optical system to the combined focal length of the finder optical system.If the lower limit of this condition (3) is exceeded, high magnification cannot be obtained, which is undesirable.On the other hand, if the upper limit is exceeded, Although a high magnification can be obtained, the effective diameter of the first lens becomes large, which is not preferable. Condition (4) defines the distance from the finder screen to the first surface; if the lower limit is exceeded, the finder magnification becomes low and the field of view becomes extremely narrow, and in some cases, it cannot be constructed. On the other hand, if the upper limit is exceeded, the radius of curvature becomes tight (small).
This is not preferable because it forces the use of lenses and makes it difficult to manufacture lenses. Furthermore, from the viewpoint of chromatic aberration correction, it is preferable that the first lens is made of acrylic resin or crown glass, and the second lens is made of polycarbonate resin or flint glass.
These lenses may be cemented. In addition, both the first and second lenses may be made of acrylic resin, and in this case, workability is good. In addition, by using resin lenses, the weight is reduced,
Cost reduction can also be achieved. "Example" Next, Examples 1 to 13 of the present invention are shown. Here, r _i is the radius of curvature of the i-th surface, d _i is the distance between the i-th surface and the (i+1)-th surface, and n _j and V _j are the d-1ine refractive index of the j-th lens (glass material), respectively. and Atsube numbers, K _i , A _4i , and A _6i are the rotational quadratic curved shape coefficient, fourth-order aspherical surface coefficient, and sixth-order aspherical surface coefficient of the i-th surface, respectively.

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[Table] "Effects of the Invention" As explained above, according to the present invention, despite the simple configuration of two elements in two groups, it has high magnification without using a pentaprism, is compact, and has low aberrations. As is clear from the figure, a finder optical system with good performance can be obtained. Especially the first lens and the second lens.
When resin is used for the lens, it has the advantage of being extremely low cost.

[Brief explanation of drawings]

1 to 13 are cross-sectional views of optical systems according to embodiments 1 to 13 of the present invention, respectively. 1st
4 to 26 respectively show Example 1 of the present invention.
FIG. 7 is a diagram of various aberrations when the eye ring diameter is φ4 in Examples 1 to 13.

Claims (1)

[Scope of Claims] 1. From the viewfinder screen side of a single-lens reflex camera, the system is composed of a first lens consisting of a positive single lens and a second lens consisting of a negative single lens, and the system of the first and second lenses is Let the ape factors be SF1 and SF2, respectively, and the shape factor SF is SF=R _{S +} R when the radius of curvature of the surface on the viewfinder screen side of each lens is R S and the radius of curvature of the surface on _the eye point side is R _E. A finder optical system defined by _E /R _S -R _E , and characterized by satisfying the following conditions: (1) -2<SF1<0 (2) 0<SF2<3. 2. The finder optical system according to claim 1, wherein at least one of the four surfaces forming the first lens and the second lens is an aspherical surface. 3 The thickness of the first lens in the optical axis direction is _d1 , and the distance between the first lens and the second lens on the optical axis is
d ₂ , the thickness of the second lens in the optical axis direction is d ₃ , and the composite focal length of the finder optical system is f, (3) 0.05<(d ₁ +d ₂ +d ₃ )/f<0.3. 3. The finder optical system according to claim 1, wherein the finder optical system satisfies the following. 4. The finder according to claim 1, wherein the finder satisfies the following condition: (4) 1.0<f _F /f<1.3, where f _F is the distance from the finder screen to the first surface of the first lens. Optical system. 5. The finder optical system according to claim 1, wherein the first lens is made of acrylic resin and the second lens is made of polycarbonate resin. 6. The finder optical system according to claim 5, wherein the first lens and the second lens are cemented. 7. The finder optical system according to claim 1, wherein the first lens and the second lens are both made of acrylic resin.