JPS60123827A - Albada finder - Google Patents

Abstract

PURPOSE:To compensate excellently aberration and to reduce ghost reflected light by setting the radius of curvature of a pupil-side surface of a translucent lens which constitutes an Albada finder. CONSTITUTION:The Albada finder is equipped with an objective lens D1 which has negative refracting power, the translucent lens D3 with negative refracting power, and an ocular lens D5 with positive refracting power successively from an object side. Then, 2.0<R4/D<2.8, where D is the gap on the optical axis between the object-side surface of the object lens D1 and the pupil-side surface of the ocular lens D5. Further, the radius R3 of curvature of the object-side surface of the translucent lens D3 is preferably negative while R4<¦R3¦ so that an object is seen well through this lens. Consequently, R3 is negative, the radius R2 of curvature of the pupil-side surface of the objective lens is increased, and the coma aberration of the surface with R2 is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a small Albada type finder in which aberrations are well corrected and ghost reflected light is reduced.

In recent years, with the miniaturization of power camera cameras, the demand for compact finders has increased, and there is a growing interest in Al-Daida type finders.

Generally, in an inverted Galilean type finder, which consists of an objective lens with negative refractive power and an eyepiece lens with positive refractive power arranged in order from the object side, the diopter is set to zero diopter.
opter (geobuter), the angular magnification is r1, the refractive power of the objective lens is ψ11, the refractive power of the eyepiece is 92, and the distance between the principal points between the objective lens and the eyepiece is el, then 1'...・■ ψl γ:=le1ψ2 ......■ holds true.

Now, assuming that the angular magnification rf is 0.45 and the principal point interval e1 is 30, the focal pressure of the objective lens is e1m f 1 (=
1/IJI ), the focal length of the eyepiece fz (=l/ψ
2) or as follows.

fx=-24,55, fz=54.55, and if the angular magnification r is 0.45 and the principal point spacing el is 20, then fr=-16,36 and fl=36.36.

From these results, F! A: So that the angle is multiplied by the ratio r.

In order to shorten the overall length of an inverted Galileo-type finder without changing it, the refractive power of the objective lens and eyepiece must be strengthened, and these refractive powers are uniquely defined by formulas and There is no freedom in correction.

However, in the case of an Al-da type finder in which the objective lens with negative refractive power is divided into two lenses, namely, an objective lens with negative refractive power and a semi-transparent lens with negative refractive power, A degree of freedom is created in the distribution of refractive power between the objective lens and the semi-transmissive lens (this determines the distance between the objective lens and the semi-transmissive lens), and a degree of freedom is created in aberration correction.

In addition, in the Albada type finder, a reflective field-limiting frame (illuminated by external light) that is deposited on the object-side surface of the eyepiece is reflected by the pupil-side surface of the semi-transparent lens, so that the viewfinder can be seen through the eyepiece. , because it has a structure that matches the diopter of the eye in between, when the refractive power of the eyepiece is reduced according to the equations (2) and (2) above, and the distance between the semi-transparent lens and the eyepiece is further given, the difference between the semi-transparent lens and the eyepiece is The radius of curvature of the surface on the pupil side is also uniquely determined (as the distance increases, the radius of curvature becomes smaller; as the distance decreases, the radius of curvature decreases),
There is no degree of freedom in correcting aberrations.

By setting the radius of curvature of the pupil-side surface of the semi-transmissive lens constituting the Albada finder to an appropriate range, the present invention is able to effectively correct aberrations, making it possible to achieve a compact design with less reflected light. This is an attempt to provide an Albada style finder.

In order to achieve the above object, the present invention provides an Albada finder which is equipped with an objective lens having a negative refractive power, a semi-transparent lens having a negative refractive power, and an eyepiece having a positive refractive power on the object side. , the distance between the object-side surface of the objective lens and the pupil-side surface of the eyepiece on the optical axis is 01, and the radius of curvature of the pupil-side surface of the semi-transparent lens is R4. It is characterized by the fact that

When R4/i) is below the above lower limit, the radius of curvature of R4 becomes small, the distance between the semi-transparent lens and the eyepiece becomes narrow, the distance between the objective lens and the semi-transparent lens increases,
The refractive power of the objective lens also becomes too strong, causing extremely strong coma aberration, making the view through the finder extremely poor.

On the other hand, if R4/b exceeds the above upper limit value, the radius of curvature of R4 becomes large, and the distance between the semi-transparent lens and the eyepiece becomes wide, causing the pupil-side surface of the eyepiece (R6 in the example below) A strong ghost reflection light occurs between the surface (R4 surface) and the surface (R4 surface) on the pupil side of the semi-transmissive lens. . Furthermore, as the radius of curvature of R4 becomes larger, the radius of curvature of the object-side surface of the transflective lens becomes smaller, and strong negative distortion occurs on this surface.

Furthermore, in order to improve the view through the finder, it is desirable that the radius of curvature R3 of the object-side surface of the semi-transmissive lens is negative and that R4<lR31. This is because when R8 has a negative value, the radius of curvature R2 of the pupil-side surface of the objective lens can be increased, and the occurrence of comatic aberration on the surface of R2 can be reduced. Also, if the radius of curvature of Rs is too small, the occurrence of negative distortion on the surface of R3 will be extremely large, which is undesirable. Some specific embodiments of the present invention will be shown below. The optical system of these examples is shown in FIG. 1, where R1 is the radius of curvature of the first lens surface from the object side, DJ is the thickness of the first lens or distance between lens surfaces, and Ni is the radius of curvature of the first lens surface from the object side. The refractive index of the lens for the d-line, νl, is the Abbe number of the first lens.

Example I R1=-200,300Dt "" 1.50R2=
11.561 0z=6.40R3=-252,900
Ds = 1.50R4 = 56.466 0a = 1
2.00Rs -0,0Di = 2', 80 Re = 21.358 Da = 16.00R7 =
0.0 γ=0.45 Example 2 Rt = 200.000 Dt = 1.50R2=
10.720 02=5.80R3=-215,77
0Da = 1.50R4= 51.330 04=1
1.0OR5mo 0-ODs-' 2.80 Re = 19.650 Ds = 16.00R? =
0.0 γ=0.45 4 1=2.27 Ns=1.49171 1=57.4N2=1.
58347 Shi2==373,9Ng=1.5834
7 Shi3=38.9Nl=1.49171 Shi,==5
7.47'h = 1.58347 shi, = 33.9Ns
= 1.58347, = 38.9 Example 3 R summer = -200,000Dt = 1.5R2 = 1
0.869 Dg = 5. OR1; 67.507 D
s = 1.5R4 = 53.380 04 = IQ,
5Rs ”-0,ODs = 2.8 Ra ;-18,080Ds = 16.0Rγ = 0.0 r = 0.45 4 = 2.5L) N1 = 1.49171 Si1: = 57.4) Nz
=1.58347 shi, ==3g, 9) N5=1.5
8347 ν3 depth 38.9] Figures 2, 3, and 4 show Example 1 and Example 2, respectively.
and an aberration diagram at an object distance of 3 m in Example 3, where θ
' is the finder exit angle.

As described above, according to the present invention, it is possible to obtain a compact Albada finder whose aberrations are well corrected and whose overall length is short. Moreover, in the present invention, by setting the radius of curvature of the pupil-side surface (surface R4) of the semi-transparent lens within the range of the mathematical conditions shown in the summary of the invention, the distance between the semi-transparent lens and the eyepiece can be adjusted appropriately. Therefore, the surface reflection that occurs between the pupil-side surfaces (surfaces R4 and R6) of both lenses

[Brief explanation of the drawing]

Figure 1 is a sectional view of the optical system of the Albada type finder of the present invention, Figure M2 is an aberration diagram of the finder of Example 1, and Figure 3 is a diagram of the aberration of the finder of Example 1.
The figure is an aberration diagram of the finder of Example 2, and FIG. 4 is an aberration diagram of the finder of Example 3. Spherical number difference Astigmatism Distortion Linear q Nu difference Astigmatism Distortion 9 Nu difference

Claims (1)

[Scope of Claims] In an Albada type finder constituted by an objective lens with negative refractive power, a semi-transparent lens with negative refractive power, and an eyepiece lens with positive refractive power arranged in order from the object side. , the distance on the optical axis between the object-side surface of the objective lens and the pupil-side surface of the eyepiece is R4, and the curvature-radius of the pupil-side surface of the semi-transparent lens is @1. An Arnoguda type finder that satisfies the following.