JPS63280207A - Focus detector - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention (Field of Application of the Invention) The present invention is an improvement of a secondary imaging type focus detection device disposed in a camera, etc., which detects a focus state from the relative positional relationship between two images of an object. It is related to.

(Background of the Invention) Conventionally, one type of camera focus detection device divides the exit pupil of a photographic lens and observes the relative positional displacement of two images formed by the light beams that have passed through each pupil area. There are known methods for determining the state of focus. For example, an aerial image formed on a predetermined focal plane (plane equivalent to a film plane) by two parallel imaging optical systems is guided to two sensor surfaces, and the displacement of the relative position of the two images is detected. The secondary imaging method is
It is disclosed in Japanese Patent Application Laid-Open No. 55-155331 and the like.

FIG. 5 shows an outline of the secondary imaging type focus detection device. A field lens 3 is placed on the same optical axis 2 as the photographing lens 1 whose focus is to be detected. Behind it, two secondary imaging lenses 4a are placed at symmetrical positions with respect to the optical axis 2,
4b is placed.

Furthermore, light receiving element rows 5a and 5b are arranged behind it. Apertures 6a and 6b are provided near the secondary imaging lenses 4a and 4b. The field lens 3 forms an image of the exit pupil of the photographing lens 1 approximately on the pupil plane of the two secondary imaging lenses 4a and 4b. As a result, the beams of light incident on the secondary imaging lenses 4a and 4b are distributed over areas of equal area on the exit pupil plane of the photographing lens 1, which correspond to the secondary imaging lenses 4a and 4b and do not overlap with each other. It will be ejected from.

When the aerial image formed in the vicinity of the field lens 3 is re-imaged by the secondary imaging lenses 4a, 4b onto the surfaces of the light receiving element arrays 5a, 5b, based on the displacement of the aerial image position in the optical axis direction, The two images on the light receiving element arrays 5a and 5b change their positions. FIG. 6 shows this situation. As shown in FIG. 6(A), at the time of focusing, the two images are located at the center of the light receiving element rows 5a and 5b, and as shown in FIG. 6(B). During rear focus, the second image moves away from the optical axis 2, and the sixth image moves away from the optical axis 2.
As shown in Figure (C), when the front is in focus, the two images move in a direction approaching the optical axis 2. The in-focus state of the photographing lens 10 can be detected by photoelectrically converting this image intensity distribution and detecting the displacement (deviation) of the relative positions of the two images through electrical signal processing.

In such a focus detection device, as described above, the object image formed by the photographing lens 1 is re-imaged on the surface of the light receiving element arrays 5a, 5b by the secondary imaging lenses 4a, 4b,
Since the method detects the in-focus state of the photographing lens 1 from the relative positional relationship of images, the secondary imaging lenses 4a, 4
The imaging performance of b has a great influence on the focusing accuracy, and therefore it has been desired to improve the imaging performance.

In addition, as a method to improve the detection accuracy of the device, it is possible to use a fine pitch P of the light receiving element rows 5a and 5b and increase the Nyquist frequency fn (=1/2P). Pitch light receiving element array 5a,
5b, the secondary imaging lens 4a is used accordingly.
, 4b cannot be expected to have such an effect unless the imaging performance of the secondary imaging lenses 4a, 4b is improved.
It has been desired to improve the imaging performance of 4b. Furthermore, in order to improve the sensitivity of the device, the F of the light flux used for focus detection is
The number is made brighter, but the brighter it becomes, the more the imaging performance deteriorates.
From this point of view as well, it has been desired to improve the imaging performance of the secondary imaging lenses 4a and 4b.

Further, when a pair of positive lenses are used as the secondary imaging lenses 4a and 4b, although the overall length of the device can be shortened, there is a problem in imaging performance as described above. On the other hand, if several lenses are used to form a pair of positive lenses, there is an advantage that sufficient imaging performance can be obtained, but the overall length of the device is shortened. The problem was that it was difficult to achieve the goal.

(Object of the Invention) An object of the present invention is to provide a focus detection device that can solve the above-mentioned problems and achieve both shortening of the overall length of the device and improvement of the imaging performance of the secondary imaging lens. That's true.

(Features of the Invention) In order to achieve the above object, the present invention makes at least one surface of a secondary imaging lens consisting of a pair of positive lenses aspherical, and the shape of the aspherical surface is adjusted relative to the paraxial radius of curvature. The positive refractive power becomes weaker toward the periphery (so that the aspherical coefficient B is set to O<
(Ni −Ni ′)

(Embodiment of the Invention) FIG. 1 is a side view showing a main part of the present invention, that is, an embodiment of a secondary imaging optical system, and the same parts as in FIG. 4 are designated by the same reference numerals. The difference from FIG. 4 is that the secondary imaging lenses 101a and 101 are made of acrylic, for example.
b, and its shape in FIG. 1 (the curved surface on the side into which light enters, and the surface from which light exits is referred to as the second surface) is an aspherical surface. More specifically, the aspherical shape is defined by X as the traveling direction of the light beam (optical axis direction), Y in the direction perpendicular to X, and R5 as the paraxial radius of curvature. A, B, C, and D represent the aspheric coefficients.

When E, it is expressed as +CY'+DY"+EY", and the refractive index of the medium before and after the aspherical surface is Ni
, Ni', it is formed so as to satisfy 0<(Ni - Ni')XB, A=0. In other words, this means that the aspherical coefficient B is set to O< (Ni - Ni '
)B and A are configured to satisfy the condition A=O(7).

Conversely, the aspherical coefficient B is O> (Ni −Ni')B,
If A is set to A=O, the imaging performance will be significantly deteriorated and the focus detection accuracy will be adversely affected. This will be explained using spot diagrams shown in FIGS. 2(A) to 2(C).

FIG. 2(A) shows the spot diagram at chest height when the first surfaces of the secondary imaging lenses 101a and 101b are aspherical surfaces with an aspherical coefficient B of O<(Ni −Ni')B. FIG. 2(B) shows a spot diagram at chest height when the first surfaces of the secondary imaging lenses 101a and 101b are not aspherical. (C) is the secondary imaging lens 101
a.

The aspherical coefficient B of the first surface of 101b is O> (Ni −N
i') It shows a spot diagram at each chest height when the aspherical surface is B. By comparing these figures, the aspherical coefficient B can be calculated as O< (Ni
-Ni')B has a spot length that is approximately 1/2 that of one that does not use an aspherical surface, and the spot length of the one with an aspherical coefficient B of O> (Ni -Ni')B. It can be seen that each has been shortened to about 1/3.

Next, Numerical Example 1 and Numerical Example 2 of the secondary imaging lenses 101a and 101b are shown on pages 10 and 11. In the numerical examples below, R1 and R2 are the radii of curvature of the first and second surfaces of the field lens 102, and R3
, R, are secondary imaging lenses 101 ai , 101
The radius of curvature of the first and second surfaces of bl, D is the lens thickness or air gap, Nd is the refractive index between lenses, υd
is the Atsube number, θ is the secondary imaging lens 101 al ,
101 The inclination angle of the second surface of bl, β is the secondary imaging lens 1
01al.

The distances between the apex of 101 bl and the optical axis are shown respectively.

Furthermore, Fig. 3 shows the positional relationship of the secondary imaging optical system in (Numerical Example 1) on page 10, and Fig. 4 shows the positional relationship of the secondary imaging optical system in (Numerical Example 1) on page 10. This shows the positional relationship of the imaging optical system.

(Numerical Example 1) *marked is aspherical surface θ = 86.2 @ β = 0.695 Aspherical coefficient A = 0 Aspherical coefficient B = -1, OXIO"" Aspherical coefficient C = O Aspherical coefficient D = 0 Aspherical coefficient E=0 (Numerical Example 2) *marked is aspherical surface θ=85.8" 4=0.695 Aspherical coefficient A=0 Aspherical coefficient B=-5, I Xl0-" Aspherical coefficient C=O Aspherical coefficient D=0 Aspherical coefficient E=0 According to this embodiment, as described above, the secondary imaging lens 101
a and 101b are formed by a pair of positive lenses, at least one of which is aspherical, making it a compact focal point with good imaging performance! It becomes possible to provide a detection device. Furthermore, by adopting such a shape, the secondary imaging lens can be formed not only by several lenses, but also by a conventional pair of positive lenses, as can be seen from the comparison between Figures 3 and 4. The overall length of the device can be made much shorter than that formed by the above method.

(Modification) In this example, in order to facilitate the manufacture of the aspherical lens, acrylic was used as the material.
The material is not limited to this, and may be made of glass or plastic material.

(Effects of the Invention) As explained above, according to the present invention, at least one surface of the secondary imaging lens consisting of a pair of positive lenses is made an aspherical surface, and the aspherical surface is arranged around the paraxial radius of curvature. The aspherical coefficient B is set to O< (Ni
-Ni')B, since the aspherical coefficient A is formed to satisfy the condition of A=0, it is possible to both shorten the overall length of the device and improve the imaging performance of the secondary imaging lens. becomes.

[Brief explanation of drawings]

Figure 1 is a side view showing one embodiment of the present invention, Figure 2 (A)
~ (C) is a spot diagram in the case where the surface is aspherical in different directions and when it is not aspherical, respectively;
FIG. 3 is a side view showing the respective positional relationships in the case of Numerical Example 1 of the present invention, FIG. 4 is a side view showing the respective positional relationships of the conventional device of the parts corresponding to FIG. 3, and FIG. 6 is a side view for explaining the positional relationship of optical systems in a general secondary imaging type focus detection device, and FIG. 6 is a diagram for explaining a focus detection method in the device shown in FIG. 101 a, 10 l b... Secondary imaging lens, 102... Field lens.

Claims (1)

[Claims] (1) A secondary imaging lens consisting of a pair of positive lenses that re-images the objective image formed by the photographing lens onto the light receiving element surface, and the exit pupil of the photographing lens is the entrance pupil of the secondary imaging lens. and a field lens for forming an image on the light-receiving element surface, the focus detection device detects the focus state of the objective lens based on the relative positional relationship between the two images re-imaged on the light-receiving element surface, as the secondary imaging lens; At least one side is an aspherical surface, and the aspherical surface has an optical axis direction of X, a direction orthogonal to it of Y, a paraxial radius of curvature of R, and an aspherical coefficient of A, B, C, D.
When E, X={(Y^2/R)/1+√[1-(Y/R)^2]
}+AY^2+BY^4+CY^6+DY^8+EY^
1^0, and the refractive index of the medium before and after the aspherical surface is Ni
, Ni', 0<(Ni-Ni')×B, A=0.