JP2984506B2 - Viewfinder optical system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a finder optical system used for an imaging device such as a camera.

[0002]

2. Description of the Related Art Conventionally, a viewfinder optical system used in an imaging device such as a camera has a configuration in which an image formed by an objective lens is observed by an eyepiece through an optical system for erect image formation. The eyepiece at this time is
There were many simple configurations such as a positive single lens or a positive / negative cemented lens taking correction of chromatic aberration into account.

However, in recent years, it has become necessary to incorporate a variety of display devices and photometric devices while keeping the distance from the eyepiece lens to the eye point sufficiently long. And the optical path length tends to be long. Also, when the finder device itself is reduced in weight by combining the optical system for erect image formation from a prism made of optical glass with a reflecting mirror, the optical path length of the optical system for erect image formation is inversely proportional to its refractive index. Become longer. When the finder optical system has such a configuration,
If a conventional simple eyepiece is used and the finder diopter is substantially constant, the focal length of the eyepiece becomes longer, and as a result, the finder is expressed by the ratio of the focal length of the objective lens to the focal length of the eyepiece. Magnification will necessarily decrease.

Therefore, as a finder optical system which overcomes this drawback and prevents a decrease in the finder magnification, Japanese Utility Model Publication No. 48-10424, Japanese Patent Application Laid-Open No. 57-54931, Japanese Patent Application Laid-Open No. 1-142521, Japanese Patent Application Laid-Open No. 2-18
No. 1713, Japanese Unexamined Patent Publication No. 4-26815 and the like have been proposed.

[0005]

However, in the first, third and fourth conventional examples, the lens surface closest to the eye point of the eyepiece is a concave surface which is relatively strong with respect to the eye point. Therefore, when the holding member for holding the lens is arranged, the distance from the holding member to the eye point, that is, the substantial eye relief becomes short. Further, the second and fifth conventional examples propose an eyepiece having a two-group structure of a positive lens group and a negative lens group in order from the object side, but the former mainly has a weak refractive power of the negative lens group. In the latter case, the distance between the positive lens group and the negative lens group is not sufficiently large, and as a result, the front principal point of the eyepiece is located much closer to the object side than the actual position of the eyepiece. Therefore, a decrease in the finder magnification cannot be sufficiently prevented.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a finder optical system for observing an image formed by an objective lens with an eyepiece through an erect image forming optical system.
The eyepiece comprises, in order from the object side, a biconvex positive lens having a strong convex surface facing the object side and a negative lens having a strong concave surface facing the object side. The conditions of 1) to (3) are satisfied, and more preferably the condition of (4) 0.15 <| d ₁₂ / f _b | <0.40 (1) 0.50 <| f _b /f|<0.90 (2) 2.0 <| r ₄ / f | (3) 0.40 <| r ₁ / r ₂ | <0.75 (4) where f is the focal length of the entire eyepiece, and f _b is the eyepiece. , The focal length of the negative lens on the eye point side, d ₁₂ is the distance from the object side lens surface of the positive lens to the object side lens surface of the negative lens, and r _i is the radius of curvature of the i-th lens surface from the object side Represents Is satisfied, thereby overcoming the above problems.

[0007]

FIG. 1 is a sectional view of a finder optical system according to a first embodiment of the present invention. The first embodiment is a viewfinder optical system for a single-lens reflex camera, in which a so-called hollow pentaprism with a pentagonal roof-type reflecting mirror combined with a reflecting surface is used as an optical system for forming an erect image. In the figure, reference numeral 1 denotes a focusing screen, 2 pentagon roof-shaped reflecting mirror, L _a positive lens of biconvex shape having a strong convex surface facing the object side, is L _b a negative lens having a strong concave surface on the object side , Lens L
constitute the eyepiece by two _a and L _b.

Next, the above-mentioned conditional expression will be described.

Conditional expression (1) defines the ratio of the focal length of the negative lens to the distance between the object-side lens surface of the positive lens and the object-side lens surface of the negative lens constituting the eyepiece. This is an equation for setting the front principal point position of the lens to an appropriate position. When the distance between the object-side lens surface of the positive lens and the object-side lens surface of the negative lens becomes smaller than the lower limit of conditional expression (1), the front principal point of the eyepiece is located near the eyepiece. As a result, the finder magnification cannot be sufficiently increased while the diopter of the finder optical system is kept substantially constant. Conversely, if the upper limit of conditional expression (1) is exceeded and the distance between the object side lens surface of the positive lens and the object side lens surface of the negative lens is increased, the finder magnification can be increased. The eyepiece itself becomes large, and it becomes impossible to configure a compact viewfinder optical system. It is also difficult to maintain a sufficiently long eye relief.

Conditional expression (2) defines the ratio between the focal length of the negative lens constituting the eyepiece and the focal length of the entire eyepiece. By combining with conditional expression (1), the entire finder optical system is obtained. Is a formula for providing a refractive power arrangement capable of realizing a sufficiently large finder magnification while satisfactorily correcting various aberrations with respect to the magnitude of. If the absolute value of the focal length of the negative lens constituting the eyepiece becomes smaller than the lower limit of conditional expression (2), the finder magnification can be sufficiently increased. Since aberrations generated in each of the negative lenses increase, it becomes difficult to correct various aberrations such as coma.
Conversely, if the absolute value of the focal length of the negative lens becomes larger than the upper limit of conditional expression (2), it becomes relatively easy to correct various aberrations, but the finder optical system as a whole maintains a predetermined size while maintaining a predetermined size. The magnification cannot be increased sufficiently.

In the present invention, by satisfying conditional expressions (1) and (2), a refractive power arrangement capable of sufficiently increasing the finder magnification is realized.

Conditional expression (3) defines the ratio of the focal length of the entire eyepiece to the radius of curvature of the lens surface closest to the eye point of the eyepiece, and is substantially a consideration of a holding member for holding the eyepiece. This is an expression for defining a shape that can make the eye relief sufficiently long. If the radius of curvature of the lens surface closest to the eye point of the eyepiece lens becomes smaller than the lower limit value of conditional expression (3), the substantial eye relief becomes short, which is not desirable.

Conditional expression (4) defines the ratio of the radius of curvature of the lens surface on the object side of the positive lens constituting the eyepiece lens to the radius of curvature of the lens surface on the eyepoint side. This is an expression for realizing a lens shape capable of further correcting various aberrations when an eyepiece having a refractive power arrangement satisfying (2) and (3) is formed. If the radius of curvature of the object-side lens surface of the positive lens constituting the eyepiece becomes relatively smaller than the lower limit value of conditional expression (4), the spherical aberration mainly generated on the object-side lens surface of the positive lens Becomes large, and it becomes difficult to correct this. In order to satisfactorily correct the spherical aberration that occurs on the object-side lens surface of the positive lens, it is necessary to reduce the radius of curvature of the object-side lens surface of the negative lens so as to cancel the curvature. However, if the radius of curvature of the lens surface on the object side of the negative lens is significantly reduced, astigmatism occurring on this surface increases, making it difficult to correct this.

Conversely, if the radius of curvature of the object side lens surface of the positive lens constituting the eyepiece lens becomes relatively large beyond the upper limit of conditional expression (4), the spherical aberration is corrected to be relatively small. However, coma generated on the lens surface on the eye point side of the positive lens increases,
Under the conditions satisfying the conditional expressions (1), (2) and (3), it becomes extremely difficult to correct this.

As described above, conditional expression (1)
By satisfying (2), (3) and (4), it is possible to obtain a sufficiently large finder magnification and a substantial eye relief while using a finder optical system using an eyepiece having a relatively simple configuration. Is maintained sufficiently long, and various aberrations are successfully corrected.

Further, according to the present invention, it is possible to realize a better finder optical system by making the lens surface of the eyepiece lens aspherical.

That is, by satisfying the above-mentioned conditions, various aberrations of the finder optical system can be corrected almost satisfactorily, and an easy-to-see finder can be realized. However, the lens surface of the eyepiece is made aspheric. As a result, it is possible to particularly excellently correct spherical aberration and astigmatism particularly, and to always realize a good finder optical system even when the observation position is slightly changed.

Therefore, the present invention further provides a finder optical system for observing an image formed by an objective lens with an eyepiece via an erect image forming optical system. A biconvex positive lens with a strong convex surface facing the lens, and a negative lens with a strong concave surface facing the object side are configured as two groups and two lenses, and at least one of the lens surfaces constituting the eyepiece is aspheric. As a result, the above problem has been successfully overcome.

As described above, even when an aspherical surface is used for the eyepiece, in order to overcome the above problems and achieve the object of the present invention, the above-mentioned conditional expressions (1), (2), It is desirable to satisfy (3). In this case, conditional expression (4), which is a conditional expression for favorably correcting various aberrations when a spherical lens is used, is not always required to be satisfied. It becomes possible to have. Of course, an aspheric surface may be provided after satisfying conditional expression (4).

Next, the case where each lens surface of the eyepiece is made aspherical will be sequentially considered.

In general, when the eyepiece having such a configuration is constituted by a spherical lens, spherical aberration is insufficiently corrected,
Due to the influence of the remaining spherical aberration, a diopter shift of the finder image occurs mainly when the observation position is changed, and the visibility is impaired. Therefore, it is now considered mainly to better correct spherical aberration by using an aspheric surface for the eyepiece.

First, it is assumed that only one of the lens surfaces of the eyepiece is aspheric. Since the aspherical surface is mainly intended to satisfactorily correct undercorrected spherical aberration, it is assumed that a lens surface with a positive refractive power will have a refractive power that decreases with distance from the optical axis. It is assumed that a lens surface having a negative refractive power has an aspheric surface whose refractive power increases as the distance from the optical axis increases. Then, when such an aspherical surface is applied to each lens surface of the eyepiece, attention is focused on how various aberrations other than the spherical aberration change.

First, if the lens surface on the object side of the positive lens of the eyepiece is made aspherical and has a shape capable of correcting the spherical aberration generated on this surface to a small extent, the coma generated on this surface and the non- The astigmatisms tend to be undercorrected and overcorrected, respectively, making it difficult to cancel coma and astigmatism generated on the object-side lens surface of the negative lens. That is, in the eyepiece having such a configuration, coma, astigmatism, and distortion are generated between the object-side lens surface of the positive lens and the object-side lens surface of the negative lens, each having relatively strong refractive power. Therefore, even if it is attempted to satisfactorily correct various aberrations by making only one of the surfaces an aspheric surface, it is difficult to correct the aberrations satisfactorily. Therefore, in this case, even if the object side surface of the positive lens is made aspherical, the effect is small and cannot be said to be sufficient. For the same reason, it is not sufficient to make only the lens surface on the object side of the negative lens aspheric.

Next, if the lens surface on the eye point side of the positive lens of the eyepiece is made aspherical so as to have a shape capable of satisfactorily correcting the spherical aberration occurring on this surface, the coma aberration occurring on this surface at the same time , And astigmatism indicate an undercorrected tendency and an overcorrected tendency, respectively. In the eyepiece having such a configuration, since the lens surface on the eye point side of the positive lens has a relatively low refractive power as compared with the other lens surfaces, a very large aberration correction effect cannot be expected. However, as long as the overall balance of coma aberration and astigmatism is not lost, correction of the intended purpose of spherical aberration can be made to some extent.

Finally, it is conceivable that the lens surface on the eye point side of the negative lens of the eyepiece is made aspherical. However, this lens surface is a surface that exits to the eye point side due to the structure of the camera. In order to keep the relief long enough or to make the shape easy to correct various aberrations satisfactorily, it is effective to use a surface having a strong refractive power as shown in the conditional expression (3). I can not say that. Therefore, in general, when the above-described configuration is adopted, the spherical aberration generated on this surface is originally sufficiently small, and even if this surface is made aspheric, the effect of correcting the spherical aberration is relatively small.

For the reasons described above, when only one surface is aspherical in the eyepiece having the above configuration, the refractive power increases as the lens surface on the eye point side of the positive lens moves away from the optical axis. It is most desirable to use an aspherical surface that weakens

Next, consider the case where a plurality of aspherical surfaces are used in order to more properly correct various aberrations including spherical aberration.

As described above, in the eyepiece having such a configuration, various aberrations generated on the object-side lens surface of the positive lens and the object-side lens surface of the negative lens, which have relatively strong refractive power, are mutually different. Because of the canceling relation, even if only one of these lens surfaces is made aspheric, the effect cannot be expected much. Therefore, both the object-side lens surface of the positive lens and the object-side lens surface of the negative lens are made aspherical, and while various aberrations occurring on these two lens surfaces are well controlled, they occur on the other lens surfaces. As a whole, various aberrations including spherical aberration have been successfully corrected in addition to the various aberrations.

As described above, in the eyepiece having such a configuration, mainly, various aberrations generated on the object side lens surface of the positive lens and various aberrations generated on the object side lens surface of the negative lens substantially occur. Although the relationship cancels out, strictly speaking, due to the difference in the height at which the principal ray passes between the positive lens and the negative lens,
The spherical aberration generated by the positive lens tends to be relatively large, and the coma and astigmatism tend to be relatively small. Therefore, when the object-side surface of the positive lens and the object-side surface of the negative lens are made to be aspherical, their aspherical shapes must be set in consideration of these relationships. Now, assuming that the lens surface on the object side of the positive lens has an aspheric surface whose refractive power becomes weaker as the distance from the optical axis increases, in order to satisfactorily correct the spherical aberration of the entire eyepiece lens, the coma generated on this lens surface is assumed. Aberration and astigmatism are also satisfactorily corrected, and canceling out coma and astigmatism generated on the object-side lens surface of the negative lens tends to be substantially difficult in this case using an aspheric surface. Therefore, in order to correct these aberrations, it is conceivable to increase the refractive power of the lens surface on the eye point side of the positive lens, but in that case, the spherical aberration generated on this surface increases, and Can not achieve the purpose. In addition, distortion is undesirably increased. Then, at the same time, the spherical aberration generated on the object side lens surface of the positive lens is rather increased, and at the same time, the spherical aberration generated on the lens surface generated on the object side lens surface of the negative lens is also increased. It is assumed that the lens surface has an aspheric shape whose refractive power increases as the distance from the optical axis increases. Then, in this case, the coma aberration and astigmatism generated on these lens surfaces also increase, but the ratio of these residual aberration amounts is different between the positive lens and the negative lens in the height at which the principal ray passes. For this reason, it is possible to satisfactorily correct these various aberrations by setting the aspherical shape, which is not constant, satisfactorily. The present invention
Focusing on this point, the lens surface on the object side of the positive lens,
By making the lens surface on the object side of the negative lens an aspheric surface whose refractive power increases as the distance from the optical axis increases, various aberrations have been successfully corrected sufficiently.

As described above, in the eyepiece in which the object-side lens surface of the positive lens and the object-side lens surface of the negative lens are aspherical, more preferably, the curvature of the lens surface on the eye point side of the positive lens It is preferable to set the ratio of the radius r ₂ to the focal length of the entire eyepiece lens in a range satisfying the following expression.

1.0 <j | r ₂ /f|<10.0 (5) If the lower limit of conditional expression (5) is exceeded and the radius of curvature of the lens surface on the eye point side of the positive lens becomes smaller, If it becomes difficult to correct the distortion generated on the lens surface, and conversely, if the radius of curvature of the lens surface on the eye point side of the positive lens exceeds the upper limit value of the conditional expression (5), it is advantageous in correcting the distortion. However, it is necessary to sharpen the curvature of the lens surface on the object side of the positive lens, and it is necessary to increase the thickness of the lens sufficiently to maintain a predetermined lens outer diameter.
It is not preferable in processing the lens. Further, as a result, the space efficiency is poor, which is not suitable for downsizing the device itself. As described above, in the present embodiment, the object-side lens surface of the positive lens of the eyepiece and the object-side lens surface of the negative lens are made to be aspherical surfaces whose refractive power increases as the distance from the optical axis increases. By satisfying the expression (5), a better finder optical system is realized.

Hereinafter, numerical examples according to the present invention will be described. The aspheric surfaces used in the numerical examples are all shown in FIG.

[0033]

[Outside 1] And the respective aspheric coefficients K, B, and C are also described below. In the above equation, r represents a paraxial radius of curvature.

In the following numerical examples, the distance from the focus plate to the first surface of the eyepiece is 80 mm,
The distance (eye relief) from the fourth surface of the eyepiece to the pupil of the observer is 18 mm.

In the finder optical system, a field ratio of 9
The diopter is about 0%, the diopter is about 1 diopter, and the finder magnification when the focal length of the objective lens is about 52 mm is about 0.72 times, which is sufficiently high.

FIG. 3 is a sectional view of the eyepiece of each numerical example.
10 to 10 and aberration diagrams are shown in FIGS. All have successfully corrected various aberrations.

[0037]

[Outside 2]

[0038]

[Outside 3]

[0039]

[Outside 4]

[0040]

[Outside 5]

[0041]

[Outside 6]

[0042]

[Outside 7]

[0043]

[Outside 8]

[0044]

[Outside 9]

Of the above numerical embodiments, numerical embodiments 1 to 3 correspond to claim 1, and numerical embodiments 4 to 8 correspond to claim 2. More specifically, Numerical Examples 4 to
Numeral 5 corresponds to claim 3, and numerical embodiments 6 to 8 correspond to claims 4 and 5.

In Numerical Examples 1, 2, 4, and 6, the material of the eyepiece is made of only acrylic resin.
In Numerical Examples 3, 5, 7, and 8, the material of the eyepiece is an acrylic resin and a polycarbonate resin. The latter configuration has an advantage that chromatic aberration of magnification can be favorably corrected compared to the former configuration.

As described in the above description, the spherical aberrations are better corrected mainly in Numerical Examples 4 and 5 than in Numerical Examples 1 to 3, and in Numerical Examples 6 to 8, The distortion has been successfully corrected.

In the above embodiment, the material of the eyepiece lens is acrylic resin or polycarbonate resin, and examples of using the same together are shown. Of course, optical glass may be used. As for the aspherical surface, an example in which only one surface is used and an example in which two surfaces are used are shown. However, a case in which three or more surfaces are used is naturally considered, and a certain effect can be expected.

[0049]

As described above, according to the present invention,
Despite being a finder optical system using an eyepiece with a relatively simple configuration, while keeping the finder magnification sufficiently large, and also keeping the substantial eye relief long enough,
There is an effect that an easy-to-see finder optical system in which various aberrations are well corrected can be realized.

[Brief description of the drawings]

FIG. 1 is a sectional view of a finder optical diagram according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram of an aspherical shape.

FIG. 3 is a sectional view of an eyepiece corresponding to Numerical Example 1.

FIG. 4 is a sectional view of an eyepiece corresponding to Numerical Example 2.

FIG. 5 is a sectional view of an eyepiece corresponding to Numerical Example 3;

FIG. 6 is a sectional view of an eyepiece corresponding to Numerical Example 4;

FIG. 7 is a sectional view of an eyepiece corresponding to Numerical Example 5;

FIG. 8 is a sectional view of an eyepiece corresponding to Numerical Example 6;

FIG. 9 is a sectional view of an eyepiece corresponding to Numerical Example 7.

FIG. 10 is a sectional view of an eyepiece corresponding to Numerical Example 8;

FIG. 11 is a diagram illustrating various aberrations of Numerical Example 1.

FIG. 12 is a diagram showing various aberrations of Numerical Example 2.

FIG. 13 is a diagram showing various aberrations of Numerical Example 3.

FIG. 14 is a diagram showing various aberrations of Numerical Example 4.

FIG. 15 is a diagram showing various aberrations of Numerical Example 5.

FIG. 16 is a diagram showing various aberrations of Numerical Example 6.

FIG. 17 is a diagram showing various aberrations of Numerical Example 7.

FIG. 18 is a diagram illustrating various aberrations of Numerical Example 8;

[Explanation of symbols]

Negative lens constituting the positive lens L _b eyepiece which constitutes one focusing plate 2 pentagonal roof-shaped reflecting mirror L _a eyepieces

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁶ , DB name) G02B 9/00-17/08 G02B 21/02-21/04 G02B 25/00-25/04

Claims (5)

(57) [Claims] In a finder optical system for observing an image formed by an objective lens with an eyepiece through an erect image forming optical system, a strong convex surface is directed to the eyepiece in order from the object side. A finder optical system comprising a biconvex positive lens, a negative lens having a strong concave surface facing the object side behind the positive lens, and satisfying the following conditions. 0.15 <| d12 / fb | <0.40 0.50 <| fb / f | <0.90 2.0 <| r4 / f | 0.40 <| r1 / r2 | <0.75 f is the focal length of the entire eyepiece, fb is the focal length of the negative lens on the eyepoint side of the eyepiece, d12 is the distance from the object side lens surface of the positive lens to the object side lens surface of the negative lens, r1 and r2 Is the radius of curvature of the object side lens surface and the eye point side lens surface of the object positive lens, and r4 is the radius of curvature of the eye point side lens surface of the negative lens. 2. A viewfinder optical system for observing an image formed by an objective lens with an eyepiece via an erect image forming optical system, wherein the eyepiece has a strong convex surface directed to the object side in order from the object side. A positive lens having a biconvex shape, and a negative lens having a strong concave surface facing the object side behind the positive lens. At least one surface of the eyepiece is made aspherical and the following conditions are satisfied. Viewfinder optical system. 0.15 <| d12 / fb | <0.40 0.50 <| fb / f | <0.90 2.0 <| r4 / f | where f is the focal length of the entire eyepiece, and fb is the eyepiece. The focal length of the negative lens on the eye point side, d12 is the distance from the object side lens surface of the positive lens to the object side lens surface of the negative lens, and r4 is the paraxial radius of curvature of the eye point side lens surface of the negative lens. Shall be represented. 3. The eyepiece lens according to claim 1, wherein the eye surface of the positive lens disposed on the object side has an aspheric surface having a shape such that the positive refractive power becomes weaker as the distance from the optical axis increases. 2 finder optical system. 4. The eyepiece lens is disposed on the object side of the positive lens disposed on the object side and on the eye point side of the positive lens disposed on the object side.
4. The finder optical system according to claim 2, wherein the lens surface on the object side of the negative lens is formed as an aspherical surface having positive and negative refractive powers increasing with distance from the optical axis. 5. The finder optical system according to claim 4, wherein the eyepiece further satisfies the following condition. 1.0 <| r2 / f | <10.0 where f represents the focal length of the entire eyepiece, and r2 represents the paraxial radius of curvature of the object-side lens surface.