JPH08136806A - Real image type zoom finder - Google Patents

Abstract

PURPOSE: To decrease the individual moving amount of moving groups by monotonously moving a second lens group to the object side and monotonously moving a third lens group to the pupil side at the time of varying the power from the wide-angle end to the telephoto end. CONSTITUTION: This finder is provided with an objective lens of a positive refractive power, an erect prism part and an eyepiece lens of a positive refractive power enlarging the image by the objective lens. In this case, the objective lens is composed, in order from the object side, of a first negative lens group, a second positive lens group, a third negative lens group and a fourth positive lens group, at the time of varying the power from the wide-angle end to the telephoto end, the second lens group is monotonously moved to the object side and the third lens group is monotonously moved to the pupil side. Further, the first lens group is composed of a single lens whose surface on the pupil side is a concave surface. Either surface of a first single lens is made to be an aspherical surface so that the negative refractive power of the first lens becomes smaller as it goes from the optical axis toward the peripheral part of the lens. Consequently, the moving amount of the lens group for varying the power is reduced and the power variation ratio >=2.2 is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an external real image type finder used in a photographic camera, a video camera or the like, and more particularly to a real image type zoom finder in which a change in finder magnification is 2 times or more.

[0002]

2. Description of the Related Art As a finder composed of an optical system independent of a photographing optical system, a virtual image type finder such as an inverse Galileo type and a real image type finder for forming a real image once in the optical system are known.

Although the virtual image finder has a compact overall length, it requires the use of a half mirror to display a frame, which causes a dark field of view, makes it difficult to see the frame display, and has a large front lens diameter. Therefore, it is not suitable for high zoom ratio.

On the other hand, the real image finder does not need a half mirror for displaying a frame, a bright field of view can be obtained, and the frame can be seen clearly. Further, since the image is once formed by the objective lens, the entrance pupil position is forward and the front lens diameter is smaller than that in the virtual image finder, so that zooming with a relatively high zoom ratio is easy.

For the above reasons, real-image type viewfinders have become the mainstream of viewfinders for cameras equipped with zoom lenses.

One of the factors that determines the size of the real image finder is the size of the objective lens, but as described above, the lens diameter of the objective lens of the real image finder is relatively small,
There is not much contribution to the size of the viewfinder. On the other hand, the size of the real image finder is largely influenced by the total length of the objective lens and the size of the member for moving the lens during zooming.

The objective lens of the real image type zoom finder can be roughly classified into one having a moving group and one having two or more moving groups. In the case of one moving group, there is a change in the visual field during zooming,
When the zoom ratio is increased, the diopter shift becomes too large.

The two or more moving groups are further classified into those in which the lens closest to the object side moves and those in which the lens does not move. A lens that moves the lens closest to the object side is not preferable because dust is likely to enter the viewfinder structurally.

The lens having the most object side lens fixed is further divided into a positive lens group and a negative lens group. The positive first lens group is disclosed in Japanese Patent Application Laid-Open Nos. 2-173714 and 3-
87803, 3-153206, 4-86733 and the like are known.

The first lens group having a negative refractive index is as follows:
JP-A-3-4217, JP-A-4-51108 and JP-A-4-53914 are known.

[0011]

Among these prior arts, all of the positive lens units having a positive first lens group tend to have a large front lens diameter, failing to take full advantage of the real image finder.

Among the negative first lens groups, Japanese Patent Laid-Open No. 4-53
In Japanese Patent No. 914, the negative first lens group is fixed, the positive second lens group is moved to change the magnification, and the positive third lens group is moved to correct the image forming position. There is a drawback that the pincushion distortion of the objective lens becomes large.

Japanese Patent Laid-Open No. 4-51108 also discloses a technique in which the negative first lens unit is fixed and the negative second lens unit and the positive third lens unit are moved to change the magnification, but at the wide-angle end. Negative distortion is -10% at a half angle of view of 27 °, which is rather large.

In Japanese Patent Laid-Open No. 3-4217, the positive second lens group is moved to change the magnification, and the negative third lens group corrects the image forming position. However, the moving amount of the second lens group Is large and the third lens group reciprocates greatly, so that a member for driving the lens, such as a cam, becomes large or difficult to move.

The present invention has been made to solve the above problems. In other words, it provides a real-image viewfinder that has a small amount of movement for zooming the lens group, has a zoom ratio of 2.2x or more, and has a half angle of view of 28 ° or more at the wide-angle end, but with little distortion. The purpose is to do.

[0016]

In order to solve the above problems, the present invention comprises an objective lens having a positive refractive power, an erecting prism portion, and an eyepiece lens having a positive refractive power for enlarging an image formed by the objective lens. In the real image type zoom finder, the objective lens is composed of a negative first lens group, a positive second lens group, a negative third lens group, and a positive fourth lens group in order from the object side, and from the wide angle end to the telephoto end. At the time of zooming, the second lens group monotonously moves to the object side, and the third lens group monotonically moves to the pupil side.

Further, the first lens group is composed of a single lens having a concave pupil side surface.

Further, if any surface of the first lens of the single lens is made an aspherical surface such that the negative refracting power of the first lens becomes smaller from the optical axis toward the peripheral portion of the lens, a better result can be obtained. .

In the present invention, the image of the objective lens is
It is arranged between the first reflecting surface and the second reflecting surface of the erecting prism portion, and the first reflecting surface of the erecting prism portion is arranged closer to the pupil than the first surface of the fourth lens group. .

Further, in the above arrangement, if the focal lengths of the first lens group and the third lens group are f ₁ and f ₃ , respectively, the following conditions should be satisfied.

0.5 <f ₃ / f ₁ <3.0 More effectively, the following conditions should be satisfied.

0.8 <f ₃ / f ₁ <1.2 If the focal length of the second lens group is f ₂ and the focal length at the wide-angle end of the objective lens is f _W , the following conditions are satisfied.

1.1 <f ₂ / f _W <1.5 Furthermore, if the focal lengths of the third lens group and the fourth lens group are f ₃ and f ₄ , respectively, in the above configuration, the following conditions should be satisfied.

0.7 <f ₄ / | f ₃ | <1.2 The fourth lens group is composed of a positive single lens and a prism having a convex surface on the object side.

More specifically, the Abbe number of the positive lens included in the second lens group and the fourth lens group is νp, and each of the first lens group and the third lens group is a negative single lens. Then, if the Abbe numbers are νn1 and νn3, respectively, the following conditional expressions are satisfied.

Νp> 50 25 <(νn1 + νn3) / 2 <45 More specifically, each of the second lens group and the third lens group is composed of a single lens, and at least one surface of the second lens group is light-exposed. The aspherical surface has a positive refractive power that decreases from the axis toward the periphery of the lens.

The surface of the first lens group having the smaller radius of curvature is directed toward the pupil side, and the surface of the third lens group having the smaller radius of curvature is directed toward the object side.

When an aspherical surface is used for the first lens group and the second lens group, the aspherical surface deformation amount at the height H from the optical axis is represented by A (H), and the paraxial curvature is shown. When the radius is R, | R · A (H) / H ⁴ | does not decrease monotonically as H increases, and | R · A (H) / H ⁶ | decreases monotonically. .

[0029]

The objective lens is composed of a negative first lens group, a positive second lens group, a negative third lens group, and a positive fourth lens group in this order from the object side, and zooming from the wide-angle end to the telephoto end. On the occasion of
Since the second lens group is monotonically moved to the object side and the third lens group is monotonically moved to the pupil side, the movement of the third lens group can also contribute to zooming, and the movement group The individual movement amount of can be reduced.

Further, since the third lens group does not reciprocate as in the prior art but moves monotonously, the movement of the cam or the like becomes smooth, and even if the size is reduced, the torque for operation is small and the defects can be reduced. .

Since the member required for zooming can be made small in this way, the finder itself can also be made small.

Further, at the wide-angle end, the positive second lens group is configured so as not to come too close to the positive fourth lens group, and the negative third lens group is arranged between them so that the negative lens group of the first lens group becomes negative. Since the refracting power can be distributed, the extreme retrofocus type refracting power arrangement can be avoided, and the barrel distortion can be reduced.

If the first lens group is a negative single lens, the total length of the objective lens can be reduced and the viewfinder can be made compact.

At this time, if any surface of the first lens is made an aspherical surface so that the negative refracting power of the first lens becomes smaller as it goes from the optical axis to the peripheral portion of the lens, the occurrence of aberration is reduced. be able to.

The zoom finder of the present invention is aimed at a finder with a half angle of view at the wide-angle end of 28 ° or more. Therefore, considering the apparent viewing angle, the finder magnification is 0.3 × or more.
It is desirable to set it to 0.5 times or less. When the finder magnification is in this range, the eyepiece lens and the erecting prism portion can be made compact, and the aberration correction of the eyepiece lens is easy.
Further, when the finder magnification is set in this range, an image of the object by the objective lens is formed between the first reflection surface and the second reflection surface of the erecting prism, and the first reflection surface of the erecting prism portion is If the fourth lens group is arranged on the pupil side of the first surface, the optical path length of the erecting prism on the objective lens side and the optical path length on the eyepiece side of the erecting prism are appropriately distributed, and aberration correction of the objective lens and the eyepiece lens is performed. A well-balanced design can be done without biasing the difficulty. In particular, if the first reflecting surface is folded back in the long side direction of the screen, good results can be obtained.

In the conventional example, the third lens has little refracting power and is mainly used for correcting the image forming position by zooming. However, in the present invention, the third lens group also has a zooming action. In order to make it, it gives sufficient refractive power. The conditional expression is
This is the condition for this. If the upper limit is exceeded, the third lens group does not have sufficient refracting power, the contribution to zooming is reduced, and as a result, the moving amount of the second group becomes large. On the other hand, when the value goes below the lower limit, the refracting power of the third lens group becomes too large, the barrel-shaped distortion aberration at the wide-angle end becomes large, and the pincushion-shaped distortion aberration at the telephoto end becomes large. Changes greatly and is not preferable.

The same applies to the conditional expression, and if this expression is satisfied, a more preferable result can be obtained.

If the upper limit of the conditional expression is exceeded, the amount of movement of the second lens group for zooming will be large, and if it is less than the lower limit, the aberration generated in the second lens group will be large, especially at the telephoto end. Aberration increases, which is not preferable.

If the upper limit of the conditional expression is exceeded, the zooming effect accompanying the movement of the third lens group becomes weak, and the amount of movement of the third lens group becomes large. When the value goes below the lower limit, the back focus of the objective lens becomes too short, and the first reflecting surface of the erecting prism is placed on the pupil side of the first surface of the fourth lens group and in front of the image forming surface of the objective lens. Becomes difficult. Also, the fourth
When the lens group is composed of a positive single lens and a prism having a convex surface on the object side, it is possible to disperse the refracting power on each surface of the fourth lens group and suppress spherical aberration.

Conditional expressions and conditional expressions are related to chromatic aberration. Below the lower limit of the conditional expression and above the upper and lower limits of the conditional expression, chromatic aberration increases, and when the pupil is displaced from the optical axis of the finder, coloring occurs, and its change. The method of will also be noticeable.

Although the upper limit of the conditional expression is not stipulated, this means that all optical materials larger than the lower limit can be used, and the larger the Abbe number νp, the better the chromatic aberration can be corrected. .

Further, each of the second lens group and the third lens group is composed of a single lens, and at least one surface of the single lens of the second lens group has a positive refractive power as it goes from the optical axis to the periphery of the lens. If the aspherical surface is made smaller, the number of lenses is reduced, so that the light transmittance is increased and a bright and transparent viewfinder can be obtained. Also the second
Since the spherical aberration at the telephoto end can be corrected by making the lens group an aspherical surface as described above, there is no adverse effect due to the reduction of the number of lenses.

If the single lenses of the first lens group and the third lens group are arranged such that the surfaces having the smaller radii of curvature face each other, the occurrence of coma can be reduced.

Further, the aspherical surface deformation amount of the first lens group and the second lens group at the height H from the optical axis is A (H).
, Where R is the paraxial radius of curvature, | R ・ A (H) / H ⁴ | does not decrease monotonically with H, and | R ・ A (H)
By using an aspherical surface such that / H ⁶ | monotonically decreases, spherical aberration and coma can be appropriately corrected, and deterioration of aberration due to decentering can be reduced.

[0045]

EXAMPLES The following will describe the details of the invention by showing numerical examples of the real image type zoom finder of the present invention.

The symbols in each example have the following meanings.

Ω: Half angle of view (maximum incident angle) R: Radius of curvature of refracting surface D: Distance between refracting surfaces nd: Refractive index of lens material νd: Abbe's number of lens material The shape of the attached surface is such that the optical axis is the X-axis and the Y-axis and the Z-axis are orthogonal to each other in a plane perpendicular to the optical axis.
When the axis is taken, it can be expressed by Equation 1.

[0048]

[Equation 1]

Here, R is a paraxial radius of curvature, and K and A _2i are aspherical coefficients.

The aspherical surface deformation amount A (H) is defined by the following equation.

[0051]

[Equation 2]

First embodiment

[0053]

[Table 1]

FIG. 1 shows a sectional view of the first embodiment of the present invention, and also shows a method of moving each lens unit during zooming from the wide-angle end to the telephoto end. 2 and 3 are aberration diagrams at the wide-angle end and the telephoto end of the first embodiment, respectively.

The first reflecting surface is provided between the ninth and tenth surfaces,
This is the erecting prism I, and the other reflecting surfaces are the 11th surface and the 12th surface.
It is provided between the surfaces, and this is called an erecting prism II. Further, the first reflecting surface and the second reflecting surface are folded in the long side direction of the screen, and the third and fourth reflecting surfaces for folding in the short side direction are roof surfaces.

An image formed by the objective lens is formed near the 11th surface, and a field mask is provided on this surface.

Second embodiment

[0058]

[Table 2]

FIG. 4 shows a sectional view of the second embodiment of the present invention at the wide-angle end. 5 and 6 show aberration diagrams at the wide-angle end and the telephoto end, respectively.

In this embodiment, the Abbe number of the first lens is
A chromatic aberration of 30 is improved at the telephoto end.

The arrangement of the reflecting surface of the erecting prism is the same as in the first embodiment.

[0062]

According to the present invention, as shown in the sectional view and the aberration diagram, the amount of movement of the second lens group is 3.5 or less and the amount of movement of the third lens group is 4.5 or less, and a zoom ratio of 2.2 is obtained. It is possible to achieve a zoom finder with an absolute value of distortion of 5% or less, even though the maximum angle of incidence at the wide-angle end is 28 ° or more. Further, the distortion at the telephoto end is also small, and the change due to zooming is small.

Further, other aberrations are well corrected, and it is possible to provide a finder which is compact and has a good appearance.

[Brief description of drawings]

FIG. 1 is a sectional view of a first embodiment according to the present invention and a diagram showing the movement of each lens group during zooming.

FIG. 2 is an aberration diagram at the wide-angle end in the first example.

FIG. 3 is an aberration diagram at the telephoto end according to the first example.

FIG. 4 is a sectional view at a wide-angle end of a second embodiment according to the present invention.

FIG. 5 is an aberration diagram at the wide-angle end according to the second example.

FIG. 6 is an aberration diagram at the telephoto end of the second embodiment.

Claims (13)

[Claims] 1. In a real image type zoom finder provided with an objective lens having a positive refractive power, an erecting prism portion, and an eyepiece lens having a positive refractive power for enlarging an image formed by the objective lens, the objective lens is negative in order from the object side. First lens group, second positive lens
The third lens group includes a lens group, a negative third lens group, and a positive fourth lens group, and the second lens group monotonously moves toward the object side during zooming from the wide-angle end to the telephoto end, and the third lens group is a pupil. A real-image zoom finder that moves monotonically to the side. 2. The real image type zoom finder according to claim 1, wherein the first lens group is a single lens having a concave pupil side surface. 3. The real image type zoom finder according to claim 2, wherein the negative refracting power of the first lens becomes smaller as any one surface of the first lens of the single lens goes from the optical axis toward the peripheral portion of the lens. A real image type zoom finder characterized by having such an aspherical surface. 4. The real image type zoom finder according to claim 1, wherein the image of the objective lens is between a first reflecting surface and a second reflecting surface of the erecting prism portion, and A real image type zoom finder, wherein one reflecting surface is on the pupil side of the first surface of the fourth lens group. 5. The real image type zoom finder according to claim 1, 2, 3 or 4, wherein the focal lengths of the first lens group and the third lens group are f ₁ and f ₃ , respectively, the following condition is satisfied: A real image type zoom finder that is characterized by 0.5 <f ₃ / f ₁ <3.0 6. In the real image type zoom finder according to claim 1, 2, 3 or 4, if the focal lengths of the first lens group and the third lens group are f ₁ and f ₃ , respectively, the following condition is satisfied. A real image type zoom finder that is characterized by 0.8 <f ₃ / f ₁ <1.2 7. The real image type zoom finder according to claim 1, 2, 3, 4, 5 or 6, wherein the focal length of the second lens group is f ₂ and the focal length at the wide angle end of the objective lens is f _W. Then, a real image type zoom finder characterized by satisfying the following conditions. _{1.1 <f 2 / f W <} 1.5 8. The real image type zoom finder according to claim 1, 2, 3, 4, 5, 6 or 7, wherein the focal lengths of the third lens group and the fourth lens group are f ₃ and f ₄ , respectively. A real image type zoom finder characterized by satisfying the following conditions. 0.7 <f ₄ / | f ₃ | <1.2 9. The real image type zoom finder according to claim 1, 2, 3, 4, 5, 6, 7 or 8, wherein the fourth lens group comprises a positive single lens and a prism having a convex surface on the object side. A real image type zoom finder characterized by being configured. 10. Claims 1, 2, 3, 4, 5, 6, 7,
In the real image type zoom finder described in 8 or 9, the Abbe number of the positive lens included in the second lens group and the fourth lens group is νp, and the first lens group and the third lens group are negative single lenses, respectively. A real-image zoom finder characterized by satisfying the following conditional expressions when the Abbe numbers are νn1 and νn3, respectively. νp> 50 25 <(νn1 + νn3) / 2 <45 11. Claims 1, 2, 3, 4, 5, 6, 7,
In the real image type zoom finder described in 8, 9, or 10,
Each of the second lens group and the third lens group is composed of a single lens, and at least one surface of the second lens group is an aspherical surface whose positive refracting power becomes smaller from the optical axis toward the periphery of the lens. A real-image zoom finder characterized by this. 12. The method of claim 1, 2, 3, 4, 5, 6, 7,
In the real image type zoom finder described in 8, 9, 10 or 11, each of the first lens group and the third lens group is composed of a single lens, and the surface of the first lens group having a smaller radius of curvature is on the pupil side. A real image type zoom finder in which the surface of the third lens group having the smaller radius of curvature is directed toward the object side. 13. The method according to claim 1, 2, 3, 4, 5, 6, 7,
In the real image type zoom finder described in 8, 9, 10, 11 or 12, the aspherical surfaces of the first lens group and the second lens group are
The amount of aspherical deformation at the height H from the optical axis is represented by A (H),
Let R be the paraxial radius of curvature.
A real image type zoom finder having an aspherical surface such that (H) / H ⁴ | does not monotonically decrease and | RA (H) / H ⁶ | monotonically decreases.