JPH0762735B2 - Focus detection device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a focus detection device suitable for a photographic camera, a video camera, etc., and particularly, the pupil of an objective lens is divided into a plurality of regions, for example, two regions, Focusing of the objective lens is performed by forming two more subject images from the subject images formed on the planned image forming surface by using the light flux passing through each area and determining the relative positional relationship between these two subject images. The present invention relates to a focus detection device that detects a state.

(Prior Art) Conventionally, there is a so-called image shift method as a light-receiving focus detection method that uses a light flux that has passed through an objective lens.

In this image shift method, for example, as proposed in Japanese Patent Laid-Open No. 59-107311, a field lens is arranged in the vicinity of a planned image forming plane where an object image is formed by an objective lens, and a porous mask is provided behind the field lens. And a secondary optical system having one or two secondary lenses, and a light receiving means having a plurality of light receiving element rows behind the secondary optical system.

Then, two more subject images are respectively formed from the subject images formed on the planned image forming plane by using the light fluxes passing through the two regions of the objective lens having different pupils from the field lens and the secondary optical system. It is formed on the surface. The relative positional relationship between the two subject images formed on the light receiving element array surface at this time varies depending on the focused state of the objective lens. For example, the amount of lateral deviation appears in the arrangement direction of the elements on the light receiving element array in accordance with the amount of defocus from the planned image forming surface of the objective lens.

The in-focus state of the objective lens, that is, the amount of defocus is 2
The relative positional relationship between the two subject images, that is, the lateral shift amount is detected by the light receiving means.

Generally, at this time, the relative lateral shift amount δ of the subject images on the two light receiving element arrays and the defocus amount d of the objective lens have a constant function relationship.

As the simplest function, there is a method in which both are considered to be in a proportional relationship and when the constant of proportionality is k, d = k · δ and the defocus amount d is obtained from this.

However, in reality, since a subject image having a constant angle of view is re-imaged on the light receiving element array via the field lens and the secondary optical system, there are various off-axis aberrations and chromatic aberrations of these optical systems. Due to the aberration, the relative positional relationship between the two subject images may be detected differently due to the difference in the distance measuring fields. For example, even if the subject distance is the same, the relative positional relationship is different between the central portion and the peripheral portion of the distance measuring field.

Therefore, in the image shift type focus detection device, it is necessary to satisfactorily correct various aberrations at the time of re-imaging by the field lens and the secondary optical system. If the aberration correction at this time is insufficient, the focus detection accuracy will be insufficient. Will cause to lower.

(Problems to be Solved by the Invention) The present invention aims to improve the focus detection accuracy by maintaining good optical performance during re-imaging by the field lens and the secondary optical system in the image shift type focus detection apparatus. It is an object of the present invention to provide a focus detection device.

In particular, it is an object of the present invention to provide a focus detection device capable of always performing highly accurate focus detection regardless of the size of the distance measuring field.

(Means for Solving Problems) A field lens is arranged in the vicinity of a planned image forming surface of the objective lens, and a secondary optical system is arranged behind the field lens, and a light flux passing through different regions of the pupil of the objective lens is used. A plurality of subject images are further formed from the subject images formed in the vicinity of the image plane,
In a focus detection device for detecting the in-focus state of the objective lens by obtaining the relative positional relationship of the light amount distributions with respect to the plurality of subject images by the light receiving means, the field lens has a curvature of the lens surface on the light receiving means side. A biconvex lens having a radius smaller than the radius of curvature of the other lens surface, and the objective lens side in which the secondary optical system is arranged so as to sandwich the optical axis of the objective lens has a curvature, and the other side is a substantially flat surface. A plurality of plano-convex secondary lenses and a plurality of apertures arranged in front of the plurality of secondary lenses are decentered on the optical axis side of the objective lens with respect to the optical axis of the secondary lenses. It is composed of a mask.

(Example) FIG. 1 is a schematic view of an optical system of an example of the present invention.

In the figure, 1 is an objective lens, L is an optical axis of the objective lens 1, 2 is a planned image forming surface of the objective lens 1, 3 is a field lens, and they are arranged so as to coincide with an optical axis L near the planned image forming surface 2. ing. Reference numeral 4 denotes a secondary optical system having an image forming action, which has substantially the same shape.
It consists of two secondary lenses 5 and 6 and a mask 7. Of these, the front of the secondary lenses 5 and 6 on the objective lens 1 side is a spherical surface, and the rear is a flat surface. The secondary lenses 5 and 6 are made of, for example, a plastic material, and the side portions of the lenses are cut so that both lenses are joined at their side surfaces or integrally formed, and they are symmetrically arranged with the optical axis L interposed therebetween. The mask 7 has two openings 7-1 and 7-2, and the two openings are arranged so as to correspond to the lens surfaces 5a and 6a of the two secondary lenses 5 and 6, respectively. At this time, the opening center 7-1 of the openings 7-1 and 7-2
a, 7-2a are decentered so that they are located on the optical axis L side of the objective lens 1 with respect to the optical axes 5-1 and 6-1 of the secondary lenses 5 and 6, respectively. Reference numeral 8 denotes a light receiving means, which is composed of two light receiving element rows 9 and 10 made of, for example, a CCD or the like. 11 is a calculation means.

In this embodiment, the field lens 3 is configured so that the openings 7-1 and 7-2 of the mask 7 are focused on the pupil of the objective lens 1. As a result, the pupil of the objective lens 1 is optically
It is divided into two areas 1a and 1b. Then, from the subject image formed in the vicinity of the planned imaging plane 2 by the objective lens 1, the field lens 3 and the secondary optical system 4 use the light fluxes passing through the regions 1a and 1b having different pupils of the objective lens 1 to correspond to each other. A second subject image is formed on the surfaces of the light receiving element rows 9 and 10.
Then, the in-focus state of the objective lens 1 is detected by obtaining the relative positional relationship between the subject images formed on the two light-receiving element rows 9 and 10 by the light-receiving element 8 and the calculating means 11.

Particularly in this embodiment, the lens shape of the field lens 3 is composed of a biconvex lens in which the radius of curvature of the lens surface 3b on the side of the light receiving means is smaller than the radius of curvature of the lens surface 3a on the side of the objective lens 1 as described above, and the secondary By arranging the lens shapes of the lenses 5 and 6 and the opening center of the mask 7 so as to be closer to the optical axis L of the objective lens than the optical axes of the secondary lenses 5 and 6, the light receiving element rows 9 and 10 The optical performance of the subject image formed on the upper side is maintained well.

That is, along the arcs of the lens action surfaces 5a and 6a of the secondary lenses 5 and 6, the optical performances of the two object images on the light receiving element surfaces are obtained by obtaining the same prism action as when the secondary lenses are tilted. Is maintained satisfactorily, thereby improving the focus detection accuracy.

For example, when the optical performances of the field lens and the secondary optical system are not well maintained as in the present embodiment, the relative positional relationship between the two subject images on the light receiving element array varies depending on the distance measuring field of view. This causes a decrease in focus detection accuracy.

FIG. 2 is an explanatory diagram showing an optical path of an optical system in a conventional image shift type focus detection device.

In FIG. 2, a point on the optical axis L of the subject 0 is 01, and a point off the optical axis L from the point 01 is 02. Then, after the light flux from the point 01 is imaged in the vicinity of the planned imaging surface 21 of the objective lens 20,
The points at which the field lens 22 and the secondary lenses 23, 24 re-image on the surfaces of the light receiving element arrays 25, 26 are designated as P1 and Q1, respectively.

Further, the points where the light flux from the off-axis point 02 is imaged in the vicinity of the planned imaging surface 21 and then re-imaged on the surface of the light receiving element array 25, 26 by the field lens 22 and the secondary lenses 23, 24 are P2, Q2. Z2 point
If the distance between P1 and point P2 is Z1 and the distance between point Q1 and point Q2 is Z2, these correspond to the line segment connecting point 01 and point 02 of subject 0, and both must be equal.

However, in reality, the field lens 21 and the secondary lens
The distance Z1 and the distance Z2 are not equal due to the influence of off-axis aberrations 23 and 24.

A similar result occurs due to the influence of the chromatic aberration of the secondary lens.

FIG. 3 shows that due to the chromatic aberration of the secondary lens, a difference Z3 occurs in the subject image formed on the surface of the light receiving element rows 25 and 26 due to the difference in wavelength, that is, the difference Z3 occurs depending on the hue of the subject.
It is explanatory drawing in case the discrimination | determination of the focus state of 20 differs.

In the figure, the same elements as those in FIG. 2 are designated by the same reference numerals. In FIG. 3, the solid line light beam 31 is, for example, a blue light beam,
The dotted light flux 32 is, for example, a red light flux.

As described above, if various aberrations remain in the field lens and the secondary lens, even if the object distance is the same, the focusing state of the objective lens is different between the center portion and the peripheral portion of the distance measuring field, and If signal processing is performed by a later calculating means in such a state, it may cause deterioration of focus detection accuracy.

On the other hand, in this embodiment, by specifying the configurations of the field lens and the secondary optical system as described above, the distance Z1 and the distance Z2 regarding the object image on the light receiving element surface are made substantially equal, that is, Z1-Z2. The difference between the two is reduced so that the same in-focus state can be obtained in the central portion and the peripheral portion of the distance measuring field.

At the same time, the difference Z3 in the image forming position due to the difference in the wavelength of the chromatic aberration of the field lens and the secondary lens is reduced.

This improves the focus detection accuracy.

Next, numerical examples of the field lens and the secondary optical system in this embodiment will be shown. R1 and R2 are the radius of curvature of the lens surface of the field lens, R3 and R4 are the radius of curvature of the lens surface of the secondary lens, D
1, D3 is the lens thickness of the field lens and the secondary lens, D2 is the distance between the field lens and the secondary lens, N1 and N2 are the refractive indices of the lens material, and ν1 and ν2 are the Abbe numbers of the lens material.

Numerical example R1 = 42 D1 = 1.5 N1 = 1.49171 ν1 = 57.4 R2 = -9.5 D2 = 16.46 R3 = 2.32 D3 = 4.9 N2 = 1.49171 ν2 = 57.4 R4 = ∞ The distance between the optical axes of the two secondary lenses is 1.263, The distance between the centers of the two openings of the mask 18 is 1.19.

With the above configuration, the difference between the intervals Z1 and Z2 on the light receiving element array surface is set to Z1−Z2 = −0.2 μm (corresponding to a height of 2 mm on the planned image forming surface), and the difference due to chromatic aberration is | Z3 | <0.1 μm ( (difference between the d-line and the g-line) is small, and good optical performance is obtained as a whole.

(Advantages of the Invention) According to the present invention, when the field lens and the secondary optical system are configured as described above in the image shift type focus detection, the optical performance of the subject image on the array surface of the light receiving elements can be favorably maintained, Without depending on the size of the subject in the distance measuring field,
Further, it is possible to achieve a focus detection device capable of always performing highly accurate focus detection regardless of the hue of the subject.

[Brief description of drawings]

FIG. 1 is a schematic view of an optical system according to an embodiment of the present invention, FIG.
FIG. 3 is a schematic diagram of an optical system of a conventional image shift type focus detection device. In the figure, 1 is an objective lens, 2 is a planned image plane, and 3 is
Is a field lens, 4 is a secondary optical system, 5 and 6 are secondary lenses, 7 is a mask, 7 is a light receiving means, 8 is a light receiving element array, 9 and 10 are light receiving element arrays, 11 is a computing means, and L is an objective lens 1. It is the optical axis.

Claims (1)

[Claims] 1. A field lens is arranged in the vicinity of a planned image forming surface of an objective lens, and a secondary optical system is arranged behind the field lens, and a light flux passing through different areas of the pupil of the objective lens is used in the vicinity of the planned image forming surface. Focus detection for detecting the in-focus state of the objective lens by further forming a plurality of subject images from the formed subject images and obtaining the relative positional relationship of the light amount distributions of the plurality of subject images by the light receiving means. In the apparatus, the field lens is composed of a biconvex lens in which the radius of curvature of the lens surface on the side of the light receiving means is smaller than the radius of curvature of the other lens surface, and the secondary optical system is arranged so as to sandwich the optical axis of the objective lens. The objective lens side has a curvature and the other side is a plano-convex secondary lens having a substantially flat surface, and the center of the aperture arranged in front of the plurality of secondary lenses is the optical axis of the secondary lens. Focus detecting apparatus characterized by being configured and a mask having a plurality of openings eccentric to the optical axis side of the remote the objective lens.