JP2000056220A - Variable power finder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a virtual image type zoom finder which is made compact to be suitable for being mounted at a compact camera and especially whose lens diameter is small. SOLUTION: As for a variable power finder that is composed of a first lens group L1 having positive refracting power, a second lens group L2 having negative refracting power and a third lens group L3 having the positive refracting power in order from an object side and by which variable power is executed by moving the second lens group L2 in an optical axis direction; the first lens group L1 has a negative lens on a side closest to the object and has a positive lens on the side closest to a pupil.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable magnification finder, and more particularly, to a virtual image type variable magnification finder mounted on a camera.

[0002]

2. Description of the Related Art As a finder used in a lens shutter type compact camera equipped with a zoom lens as a photographing lens, a small and low-cost zoom finder is required. Various techniques are disclosed for a variable-magnification finder of a real image type and a virtual image type. Generally, the virtual image type has a small number of lenses and has an advantage in cost.

As a virtual image type zoom finder, for example, Japanese Patent Application Laid-Open No. 61-87122 discloses a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a positive refractive power. And a finder that performs zooming by moving the second lens group has been proposed as a finder having a small overall lens length and a small front lens diameter.

[0004]

However, in the finder disclosed in this embodiment, the front lens is particularly larger than the subsequent lens because the first lens group is a positive lens advance type having one positive lens. Further reduction in diameter is desired.

In recent years, the focal length has been increased to 35 mm to 70 mm.
As described above, the widening of the angle of the photographing zoom lens is advanced, and a zoom finder that can cope with a wide angle of view is desired.

An object of the present invention is to provide a virtual image type zoom finder having a compact size, particularly a small lens diameter, suitable for being mounted on a compact camera. It is another object of the present invention to obtain a virtual image type zoom finder having a wide angle of view corresponding to a wide angle of a photographing zoom lens.

[0007]

The above object can be achieved by any of the following. That is, (1) a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a positive refractive power are sequentially moved from the object side, and the second lens group is moved in the optical axis direction. In the variable magnification finder for performing variable magnification, the first lens group has a negative lens closest to the object side and a positive lens closest to the pupil side.

(2) The first lens group includes, in order from the object side, one negative lens and one positive lens.
The variable magnification finder according to (1), wherein the lens group includes one negative lens, and the third lens group includes four lenses including one positive lens.

(3) The third lens group is a positive meniscus lens having a convex surface facing the pupil side. (2)
The zoom finder described in 1.

(4) The on-axis length of the first lens group is L
_L, zoom finder according to and satisfies the following equation (1) when the axial length of the entire viewfinder to L _T.

0.25 ≦ L _L / L _T ≦ 0.75 (1) Note that a desirable condition is 0.35 ≦ L _L / L _T ≦ 0.65.

(5) The variable magnification finder according to (1), wherein the following condition is satisfied.

20 ° ≦ ω _W ≦ 32 ° Expression (2) where ω _W is a half angle of view at the wide-angle end (6) The following condition is satisfied. The variable magnification finder according to (1).

1.45 ≦ n ₁ where n ₁ is d of the negative lens closest to the object side in the first lens group.
(7) The variable magnification finder according to (1), which satisfies the following condition.

0.05 ≦ | φ _1a | ≦ 0.10 (φ _1a <0) Equation (4) where φ _{1a is} the refracting power of the negative lens closest to the object side in the first lens group (unit: 1 / mm) (8) The variable magnification finder according to (1), having a plurality of frames for defining a range of a visual field that changes during zooming.

Here, each conditional expression will be described. The conditional expression [1] defines an appropriate length of the first lens unit. If the length of the first lens unit is out of the lower limit, the power of each of the negative lens and the positive lens constituting the first lens unit becomes strong, and distortion becomes negative. And the aberrations are degraded. Outside the upper limit, the moving space of the second lens unit is reduced, the zoom ratio is reduced, or the power of each lens unit is required to be strong, resulting in deterioration of aberration.

The conditional expression [2] defines a half angle of view at the wide angle end suitable for a zoom finder. If the lower limit is exceeded, the angle of view is narrow, and it is insufficient for a viewfinder of a wide-angle shooting zoom lens to cover the field of view.
If the upper limit is exceeded, the angle of view will be sufficient, but it will be difficult to correct aberrations at the periphery of the field of view, and it will be necessary to increase the diameter of the first lens group in order to secure the amount of light at the periphery. Is impaired.

The conditional expression [3] defines a refractive index suitable as a base material of the negative lens (L1a) closest to the object side in the first lens group, and is indicated by a value with respect to the d-line. If the lower limit is exceeded, in order to obtain the same power, the surface of the lens must be strengthened. In the case of the negative lens closest to the object side in the first lens unit having a negative refractive power, the edge thickness increases and the workability at the time of production increases. As the lens surface becomes worse and the lens surface becomes tighter, the aberration is likely to deteriorate. Preferably, 1.5 <n.

The conditional expression [4] is a condition suitable for a finder having a wide angle of view. If the lower limit of the conditional expression [4] is exceeded, the power of the negative lens (L1a) closest to the object side in the first lens unit is weakened, and the angle of view is reduced. Lens. If the upper limit is exceeded, a wide angle of view can be obtained. Aberration tends to deteriorate.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A finder according to the present invention will be described with reference to FIG. FIG. 1 is a schematic diagram for explaining a refractive power arrangement of a finder according to the present invention. In FIG. 1, the first lens unit L1 is represented as a combined system of a negative lens L1a having a negative refractive power and a positive lens L1b having a positive refractive power.

The entire optical system is almost an afocal system,
The finder magnification is changed from the low magnification end to the high magnification end by moving the second lens unit L2.

SL and SL 'indicate the object distance and the image distance of the second lens unit L2 at the low magnification end, respectively.
H, SH 'indicate those at the high magnification end. In order for the diopter difference between the low-magnification end and the high-magnification end to be small, the distance between the object images of the second lens group needs to be substantially equal at the low-magnification end and the high-magnification end, and Equation 1 is derived. .

[0023]

(Equation 1)

If the focal lengths of the first lens unit and the third lens unit are f ₁ and f ₃ , respectively, and the finder magnifications at the low magnification end and the high magnification end are m _L and m _H , respectively, m _L and m _H. Becomes “Equation 2”.

[0025]

(Equation 2)

Therefore, "Equation 3" is derived.

[0027]

(Equation 3)

By the way, in the viewfinder of a compact camera, the geometric mean of the magnification is given by the following "Equation 4".

[0029]

(Equation 4)

Since it is generally smaller than 1, 0 <f
The ₁ <f _3. Here, as shown in FIG.
If one negative lens L1a and one positive lens L1b are used, the rear focal point is located rearward even if the focal length of the first lens group is short. Accordingly, as shown in FIG. 1, the moving space of the second lens unit L2 between the first lens unit L1 and the third lens unit L3 is changed to the rear focal point of the first lens unit L1 and the front side of the third lens unit L3. It can be positioned almost halfway between the focal point and a compact finder without wasted space. In addition, since the first lens unit L1 is ahead of the negative lens, a finder having a small lens outer diameter can be obtained.

Further, if the third lens unit L3 is a positive meniscus lens having a convex surface directed to the eye point side, the principal point of the third lens unit L3 depends on the eye point side, so that the degree of retrofocus of the first lens unit is reduced. And is advantageous in aberration correction.

As described above, by the movement of the second lens group,
Although the magnification of the finder magnification can be achieved, particularly for a wide angle of view, it can be achieved by increasing the power of the negative lens (L1a) closest to the object side in the first lens group. That is, in the variable magnification finder having the four-lens configuration of the present invention, the lens of the negative lens L1a also has a role of a wide converter for the three lenses on the eye point side. According to the present invention, a zoom finder having a wide angle of view can be obtained by appropriately setting the refractive index of the material of the negative lens L1a and the power of the negative lens L1a.

In the finder according to the present invention, when the outline of the field of view is determined by a rectangular lens frame or the like, for example, the field of view is limited by the object side lens frame of the first lens group at the wide angle end, and the first lens is set at the telephoto end. It may be good to limit the field of view with the lens frame on the image side of the group. As described above, it is effective to determine the visual field range by a plurality of frames so that the predetermined visual field range is also determined by zooming.

[0034]

Next, examples of the present invention will be described. The symbols used are as follows.

Ω: half angle of view d: axial top surface distance n _d : refractive index for d-line ν _d : Abbe number The shape of the aspherical surface (the surface marked with *) is defined by setting the vertex of the surface as the origin as the origin along the optical axis. In a rectangular coordinate system with the X axis, the vertex curvature is C, the conic constant is K, and the aspheric coefficient is A _i (i = 4,
6, 8) are represented by "Equation 5".

[0036]

(Equation 5)

Example 1 FIG. 2 shows a lens optical axis section of Example 1, and FIG. 3 shows a lens aberration diagram of Example 1 for an object at a distance of 3 m. Tables 1 and 2 show lens data.

[0038]

[Table 1]

[0039]

[Table 2]

Example 2 FIG. 4 shows a lens optical axis section of Example 2 and FIG. 5 shows a lens aberration diagram of Example 2 for an object at a distance of 3 m. Tables 3 and 4 show lens data.

[0041]

[Table 3]

[0042]

[Table 4]

(Embodiment 3) FIG. 6 shows a lens optical axis section of Embodiment 3 and FIG. 7 shows a lens aberration diagram of Embodiment 3 for an object at a distance of 3 m. Tables 5 and 6 show lens data.

[0044]

[Table 5]

[0045]

[Table 6]

In the above embodiment, the first lens unit L1
And the third lens unit L3 are fixed, but the first lens unit L1 or the third lens unit L3
The lens unit L3 may be moved.

In the embodiment, the material of the negative lens (L1a) closest to the object side in the first lens group is acrylic. However, polycarbonate having a high refractive index may be used, or the negative lens (L1a) may be replaced by two negative lenses. Is also effective for correcting aberrations and improving mass productivity.

Table 7 shows the values of the conditional expressions in each embodiment.

[0049]

[Table 7]

As shown in Table 7, each conditional expression is satisfied.

[0051]

The following effects are obtained by the above configuration. A virtual image type zoom finder not only having a short overall finder length but also having a small lens diameter and suitable for mounting on a compact camera can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram for explaining a refractive power arrangement of a finder according to the present invention.

FIG. 2 is a sectional view of a lens optical axis according to a first embodiment.

FIG. 3 is a lens aberration diagram of the first embodiment.

FIG. 4 is a sectional view of a lens optical axis according to a second embodiment.

FIG. 5 is a lens aberration diagram of the second embodiment.

FIG. 6 is a sectional view of a lens optical axis according to a third embodiment.

FIG. 7 is a lens aberration diagram of the third embodiment.

[Explanation of symbols]

 L1 First lens group L1a Negative lens closest to the object side of first lens group L2 Second lens group L3 Third lens group

Claims (8)

[Claims] 1. A first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a positive refractive power are sequentially moved from the object side, and the second lens group is moved in the optical axis direction. In the variable magnification finder for performing variable magnification, the first lens group has a negative lens closest to the object side and a positive lens closest to the pupil side. 2. The first lens group includes one negative lens and one positive lens in order from the object side, the second lens group includes one negative lens, and the third lens group includes 1
2. The variable magnification finder according to claim 1, wherein the variable magnification finder has a four-element configuration including four positive lenses. 3. The variable magnification finder according to claim 2, wherein the third lens group is a positive meniscus lens having a convex surface facing the pupil side. 4. An axial length of the first lens group is L _L ,
Zoom finder according to claim 1, characterized in that the following expression is satisfied when the axial length of the entire viewfinder and L _T. 0.25 ≦ L _L / L _T ≦ 0.75 5. The variable magnification finder according to claim 1, wherein the following condition is satisfied. 20 ° ≦ ω _W ≦ 32 ° where ω _W : half angle of view at wide angle end 6. The variable magnification finder according to claim 1, wherein the following condition is satisfied. 1.45 ≦ n ₁ where n ₁ is d of the negative lens closest to the object side in the first lens group.
Refractive index for line 7. The variable magnification finder according to claim 1, wherein the following condition is satisfied. 0.05 ≦ | φ _1a | ≦ 0.10 (φ _1a <0) where φ _{1a is} the refractive power of the negative lens closest to the object side in the first lens group (unit: 1 / mm) 8. The variable magnification finder according to claim 1, further comprising a plurality of frames for defining a range of a visual field that changes during zooming.