JPH0617936B2 - Finder optical system - Google Patents

Description

The present invention relates to a viewfinder of a camera, and more particularly to an albada type viewfinder.

Conventionally, as shown in FIG. 1, this type of viewfinder has an objective lens 1, a reflecting mirror 2 whose surface 2a on the observer side is a semi-transmissive / semi-reflective surface or a partially reflective surface, a portion showing a field frame, etc. The information display body 3 having the reflecting surface 3a and the eyepiece lens 4 are used. Information such as a visual field frame illuminates the partially reflecting surface of the information displaying body 3 by external light, and the reflected light is reflected by a reflecting mirror by an observer. By being reflected to the side, the information display body 3 and the eyepiece lens 4 are transmitted to reach the observer's pupil 5. Alternatively, as shown in FIG. 2, there is also known a method in which the information display body 3 is omitted and a partially reflecting surface indicating a field frame or the like is directly formed on the object-side surface 4a 'of the eyepiece 4'.

However, in such an Albada type finder, the negative objective lens as the inverse Galileo finder is divided into the objective lens 1 and the reflecting mirror 2 each having a negative refractive power, so that if the same refractive power arrangement is adopted. It has a drawback that the outer diameter of the lens is larger than that in the case where the objective lens is composed of one group. In particular, recent lens shutter cameras are required to be compact, and it is not preferable that the outer diameter of the objective lens be large.

An object of the present invention is to provide a compact arbata type finder having a small outer diameter of an objective lens, which overcomes the above drawbacks.

Therefore, the present invention comprises, in order from the object side, the first lens group having negative refractive power, the second lens group having negative refractive power, and the third lens group having positive refractive power, and φ ₁₂
Is the combined refractive power of the first and second lens groups, φ ₃ is the refractive power of the third lens group, R2 is the radius of curvature of the final surface of the first lens group, and D5 is the wall thickness of the third lens group. , (1) 0.145 / φ ₃ <D5 <0.29 / φ ₃ (2) -0.56 / φ ₁₂ <R2 <-0.67 / φ ₁₂ are satisfied.

FIG. 3 is a view showing an embodiment of the present invention (FIG. A) in comparison with a conventional example (FIG. B) having exactly the same refractive power arrangement. The objective lens 1 and the reflecting mirror 2 are exactly the same as in the conventional example, but in this embodiment, the thickness of the eyepiece lens 4 ″ is formed to be thick so that the distance from the rearmost surface of the eyepiece lens 4 ″ to the pupil 5 is shortened. This reduces the outer diameter of the objective lens 1.

Now, in the present embodiment, the thickness of the eyepiece 4 ″ is D5, the distance between the last surface of the eyepiece 4 ″ and the pupil / pupil 5 of the observer is E,
Similarly, the thickness of the eyepiece 4'in the conventional example is D5 ',
E ′ is the distance between the last surface of the eyepiece 4 ′ and the observer's pupil 5.
When the refractive index of the material of the eyepieces 4'and 4 "is n and the radius of curvature of the front surface of the eyepieces is ∞, the following relationship holds.

That is, the distance between the rearmost surface of the eyepiece and the observer's pupil depends on the difference in wall thickness of the eyepiece.

Next, the principle of reducing the outer diameter of the objective lens 1 by shortening the distance E from the rearmost surface of the eyepiece 4 ″ to the pupil 5 by utilizing the thickness of the eyepiece 4 ″ will be described with reference to FIG. To do. In FIG. 4, 6 is a block having a negative refracting power obtained by combining the objective lens 1 and the reflecting mirror 2, 7 is a schematic block diagram of each of the eyepieces 4 ′ and 4 ″, and 8 is a block diagram of the observer. This is the position of the pupil 5. Now, assuming that the pupil position of the configuration shown in Fig. 3 (B) is 8, the thickness of the eyepiece 4 "is set to be the same as in the present embodiment with the same refracting power. If it is made thicker, the principal point of the eyepiece lens 4 ″ moves to the observer's pupil side with respect to the objective lens 1. As a result, from the rearmost surface of the eyepiece lens 4 ″ to the pupil, the refractive power arrangement remains unchanged. This means that the distance has been shortened. Therefore, the pupil position 8 in FIG. 4 moves to 8 ', and the chief ray passing through the center of the pupil 5 also moves from 9 to 9'. As a result, the height of the chief ray in the synthesis block 6 is also reduced from h to h ′, and the outer diameter of the objective lens 1 can be reduced.

On the other hand, if the outer diameter of the objective lens 1 is simply reduced, it is possible to increase the combined refracting power φ _{12 of} the objective lens 1 and the reflecting mirror 2 and the refracting power φ ₃ of the eyepiece lens. There is a limit because it is accompanied by deterioration of aberration.

Therefore, the numerical conditions (1) and (2) are necessary. The expression (1) is a condition for limiting the thickness of the eyepiece lens for reducing the outer diameter of the objective lens 1 as described above. If the lower limit is exceeded, the outer diameter of the objective lens 1 becomes large, and if it exceeds the upper limit. The wall thickness is too large for the outer diameter, making it difficult to manufacture. Equation (2) limits the radius of curvature of the rearmost surface of the objective lens 1. If the lower limit is exceeded, the negative refracting power becomes excessive and negative astigmatism and distortion occur. If the upper limit is exceeded, If the refractive power is too small, positive astigmatism occurs, which is not preferable for image performance.

Another feature of the present invention is that the information display body 3 of the conventional example as shown in FIG. 1 can be omitted by providing the partial reflection surface 4a ″ showing the field frame or the like on the front side of the eyepiece lens 4 ″. It is in. Of course, the information display body 3 can be omitted in the conventional example shown in FIG. 2, but in this case, the radius of curvature of the rear surface of the reflecting mirror 2 must be large in order to appropriately obtain the diopter of information. However, the radius of curvature of the rear surface of the objective lens 1 becomes small, and negative astigmatism and distortion are likely to occur. In the case of the present invention, such a defect does not occur because the information can be arranged at the position optically equivalent to that in FIG. Further, it goes without saying that the radius of curvature does not have to be infinite on the front surface side of the eyepiece lens 4 ″ as long as the partial reflection surface 4a ″ can be provided.

Next, numerical examples of the present invention will be shown.

In the numerical example, R _i is the i-th lens surface radius of curvature in order from the object side, D _i is the i-th lens center thickness or air gap in order from the object side, and N _i and ν _i are respectively in order from the object side. It is the refractive index and the Abbe number of the i-th glass.

Aberration diagrams corresponding to each of Numerical Examples 1 to 3 are shown in FIG.
It shows in order in FIG. In the aberration diagrams, the vertical axis of spherical aberration is the height of the exit pupil, the vertical axis of astigmatism and distortion is the viewfinder viewing angle, the horizontal axis of spherical aberration and astigmatism is diopter, and the horizontal axis of distortion is %. Further, ΔS indicates a sagittal image plane, and ΔM indicates a meridional image plane.

Example 1 R1 = 100.080 D1 = 2.00 N1 = 1.50137 ν1 = 56.4 R2 = 9.187 D2 = 4.70 R3 = 0.0 D3 = 1.50 N2 = 1.50137 ν2 = 56.4 R4 = 28.404 D4 = 7.78 R5 = 0.0 D5 = 8.20 N3 = 1.50137 ν356. 4 R6 = -16.048 R2 = -0.659 / φ ₁₂ D5 = 0.256 / φ ₃ Example 2 R1 = 150.000 D1 = 2.20 N1 = 1.49171 ν1 = 57.4 R2 = 12,000 D2 = 7.05 R3 = 500.000 D3 = 1.70 N2 = 1.49171 ν2 = 57.4 R4 = 56.317 D4 = 14.14 R5 = 250.000 D5 = 7.00 N3 = 1.49171 ν3 = 57.4 R6 = −25.822 R2 = −0.571 / φ ₁₂ D5 = 0.146 / φ _{3 As} described above, according to the present invention, the eyepiece lens By making the inner thickness appropriate, it is possible to provide a compact fiber optical system with a small objective lens diameter.

[Brief description of drawings]

1 and 2 are sectional views of a conventional example, FIG. 3 is a sectional view showing a state in which an embodiment of the present invention and a conventional example are compared, FIG. 4 is a diagram showing the principle of the present invention, and FIG. ~ Fig. 6 is an aberration diagram of each numerical example. 1 ... Objective lens, 2 ... Reflector, 4 ... Eyepiece.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hideo Yokota Inventor Hideo Yokota, 770 Shimonoge, Takatsu-ku, Kawasaki City, Kanagawa Canon Inc. Tamagawa Plant (56) Reference JP-A-58-149017 (JP, A) JP-A-50 -27539 (JP, A) Actually developed Sho-49-48438 (JP, U)

Claims (1)

[Claims] 1. A first lens element having a negative refractive power in order from the object side.
A lens group, a second lens group also having a negative refractive power, and a third lens group having a positive refractive power and a large thickness,
₁₂ is the combined refractive power of the first lens group and the second lens group, φ ₃ is the refractive power of the third lens group, and R2 is the first refractive power.
When the radius of curvature of the last surface of the lens group and D5 are the wall thickness of the third lens group, (1) 0.145 / φ ₃ <D5 <0, 29 / φ ₃ (2) −0, 56 / φ ₁₂ <R2 A finder optical system characterized by satisfying the condition of <-0, 67 / φ ₁₂ .