JPS6243611A - Optical system for focus detection - Google Patents

Abstract

PURPOSE:To obtain an optical system for focus detection suitable to an electronic still camera by arranging closely a field lens for forming the secondary object image on a photodetecting element and a focus plate where the primary image is formed. CONSTITUTION:Light from the photographic lens 1 after being reflected by a movable mirror 2 forms a finder image on the focus plate 9 and the image is observed through prisms 10 and 11 and an ocular 12. Part of the light is guided out of the reflecting surface S2 of the prism 10 to form the secondary object images on photodetecting elements 28 and 29 through an image re-formation system 23 having a field lens 21, image-reforming lenses 24, 25, thereby detecting the focus of the lens 1. A lens 21 is arranged on the reflecting surface S2, so the distance to the focus plate 9 as an expected image forming plane is shortened to display the function of the field lens sufficiently, so that the optical system is suitable to a single-lens reflex camera.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a focus detection optical system suitable for a photographic camera, a video camera 2, etc. A plurality of secondary objects S from the light flux passing through
This invention relates to a focus detection optical system that detects the focal position of a photographing lens from the relative positional relationship of a plurality of secondary object images.

(Prior Art) Conventionally, there is a so-called image shift method as a relatively high-precision light-receiving focus detection method.

This image shift method is proposed in Japanese Patent Application Laid-Open No. 55-155331, in which two re-imaging lenses are installed behind the expected imaging plane on which the primary object image is formed by the taking lens. It is common to arrange an imaging system and a light receiving means having two rows of light receiving elements, and also to arrange a so-called field lens near the intended imaging plane. The field lens is 2
Eyes of two re-imaging lenses? JK has a function of forming an image near the pupil of the shadow lens.This allows for the clan of the light flux from the photographing lens that enters the re-imaging system when the photographic lens is divided into two regions. I'm doing less. The re-imaging system uses the light flux that has passed through the two pupil areas of the photographing lens to
Two secondary object images are formed on the light receiving means surface from the secondary object image. The relative positional relationship of the two secondary object images on the light receiving means surface changes variously depending on the focusing state of the photographic lens.

Therefore, the focus of the photographing lens is detected by detecting the relative positional relationship between the two secondary objects at this time using the light receiving means.

There is also a system in which the same effect can be obtained by using two prisms having opposite wedge angles and one re-imaging lens instead of the two re-imaging lenses. In either case, in focus detection using the image shift method, a field lens placed near the intended image formation plane plays a major optical role. As a field lens, it does not interfere with the imaging action of the re-imaging system, and moreover, when the pupil of the re-imaging system is imaged onto the back of the photographing lens, it passes from the pupil of the re-imaging system to the pupil of the photographing lens. It is best to place it in a position where the imaging magnification of the lens will not become too high. For this reason, in many cases it is placed near the intended imaging plane.

For example, when adapting an image shift type focus detection optical system to the optical system of a single-lens reflex camera, 4! Kaisho 58-1065
As disclosed in No. 11, one method is to place the camera at the bottom of the camera in series. This method has the advantage of allowing the field lens to be placed near the intended image plane. However, part of the rotating mirror is a semi-transparent surface, and the light beam that passes through this semi-transparent surface is reflected to the bottom of the camera by a small mirror attached to the rotating mirror, so when the nine-turning mirror is flipped up, the small mirror is It must be busy so as not to obstruct the photographic light flux. For this reason, if the reflecting surface of the small mirror is made small, it becomes difficult to utilize the light beam effectively, and if the small mirror is folded, it tends to become mechanically complicated. In other words, the configuration for routing the light flux to the bottom of the camera depends on the size, arrangement,
It is disadvantageous in terms of mechanical complexity.

On the other hand, it is also conceivable to provide a light flux extraction port near the pentaprism instead of the bottom of the camera, and to arrange a focus detection optical system near the light flux extraction port.

However, in this arrangement, one method is to arrange the focus detection optical system at the position of the donut surface or front reflection surface of the pentaprism due to spatial constraints. As a result, the field lens moves far away from the focusing plate, which is the intended image formation plane.
However, if the p1 field lens is placed too close to the re-imaging system, the scratch resistance magnification will be high, and an end point will occur where the number of clans of light beams to the re-imaging system will increase.

(Problems to be Solved by the Invention) When the present invention places a one-shift type focus detection optical system behind the focus plate of a single-lens reflex camera, the field lens is placed near the focus plate of the photographic lens, and the field lens is used as a field lens. An object of the present invention is to provide a focus detection optical system that is particularly suitable for electronic still cameras and can fully demonstrate the functions of the present invention.

(Means for solving the problem) A prism is arranged behind the expected image formation plane where the first object image is formed by the photographing lens, and the prism has an entrance surface S on which the light beam from the first object is incident. Adjacent to the entrance plane SIK,
It has a first reflecting surface S2 having a surface S' that transmits light in a part that reflects the light beam incident from this surface, and an exit surface that emits the light beams that have been sequentially reflected thereafter, and transmits the light. A re-imaging system and a light receiving means are arranged behind the surface Si,
The re-imaging system forms a plurality of secondary object images on the light receiving means using the light beam from the first object image that has passed through the light transmitting surface 37, and utilizes an output signal from the light receiving means. When performing focus detection, a field lens is provided near the surface Si through which the light passes.

Other features of the invention are described in the Examples.

(Implementation row) FIG. 1 is a schematic diagram of an optical system of an embodiment when the present invention is applied to an electronic still camera, and FIG. 2 is a perspective view of a portion of FIG. 1. In the figure, 1 is a photographing lens, 2 is a movable mirror that is retracted from the photographing optical path during shooting, 3 is an optical low-pass filter, 4 is an image soter unit, 5 is a protective glass for the photographing object, 6 is a photographing surface, 7 is a package of one image.

A part of the luminous flux from the movable mirror 2 ((9) is reflected and becomes the finder luminous flux, and passes through the optical path correction plate 8 (on the receiving focus plate 9) to equalize the spherical aberration of the lt>1 system. An elephant is formed in the finder.This finder image is formed by the first prism 1, which is part of the finder system.
An erect normal image is formed through a second prism 11 having a roof surface and a roof surface, and is viewed through an eyepiece 12 and observed at a lens 11113.

In this embodiment j, the position of the focusing plate 9 corresponds to the expected image forming plane of the photographing lens l, and the finder image on the focusing plate 9 corresponds to the primary object image. The light beam is a prism for extracting a light beam, which is disposed approximately at the center of the first reflective surface S2 of the first prism lO, in close contact with, for example, the semi-transparent surface 14, which is a surface that transmits the next light. A light beam for focus detection is extracted from the finder light beam incident on the prism 10. Reference numeral 21 denotes a field lens disposed close to the semi-transparent surface 14, which is constructed integrally with the prism in this embodiment, but may be constructed separately.

n is a prism for reflecting the light beam from the field lens 21 in the direction n of the re-imaging system having two re-imaging lenses and 25; 29 is a prism for reflecting the light toward the light receiving means.

In this implementation, as shown in FIG. 2, two secondary object images are formed on the light receiving element array 29 from the primary object image formed near the focusing plate 9 by the refocusing system n. ing. The focus of the photographing lens 1 is detected by detecting the relative positional relationship between the two secondary object images using the output signal from the light receiving element array 29.

In this implementation row, the position of the field lens 21 and the focusing plate 9 is determined by arranging the field lens 4 of the focus detection optical system near the first reflecting surface S2 of the triangular prism IO placed above the focusing plate 9. By shortening the distance to the intended image formation plane, the lens fully functions as a field lens. In the barrel-mounted arrangement, it is preferable to configure the focusing surface above the focusing plate 9 so that the focusing surface is as close to the field lens 21 as possible.

FIG. 3 is an explanatory diagram when only the focus detection optical system shown in FIG. 1 is taken out and developed.

In this figure, the same elements as those shown in FIG. 1 are given the same reference numerals. In the figure, the distance between the S□ surface and the 82nd surface of the first prism is significantly shorter than the optical path length assuming that the light beam is taken out from the front reflection surface of the pentaprism, so the field lens 21 is close enough to the surface. The field lens 21 disposed near the planned imaging plane 91 connects the two re-imaging lenses 24 and 25 of the re-imaging system 23 to the exit pupil 102 of the photographic lens 1.
The images are focused on two areas 110 and 111, respectively.

The re-imaging system 23 re-images two secondary object images from the primary object image formed near the intended image-forming surface 91 onto the two light-receiving element arrays 28 and 29 of the light-receiving means 27. . Then, depending on the distance from the imaging position of the primary object image to the planned imaging plane 91, that is, depending on the focus state, the two @secondary object images move along the light-receiving element array, 29, as indicated by the arrow, with the focusing position a as a reference. move in the direction. Therefore, the relative positional relationship between the two secondary object images at this time is detected by the light receiving means n, and the photographic lens 1
Focus detection is performed.

FIG. 4 is a perspective view of the first prism 10 and the second prism 11, which are part of the finder system shown in FIG. The finder light flux will be explained with reference to FIGS. 1 and 4.

The light ray E on the finder optical axis enters the incident surface 81 of the first prism 10 approximately perpendicularly, is reflected by the first reflective surface S2 on which a reflective film is deposited, and is reflected by the second reflective surface S2, which is approximately on the same plane as the incident surface S□. After being totally reflected by the reflecting surface S', the light is emitted from the first prism 10 through the exit surface S3. Thereafter, the light ray E enters the second prism 11 through the entrance surface S4 and passes through the two reflective surfaces S5 and 2 on which the reflective film is deposited. After being reflected by the roof surface formed by S5' and further reflected by a fourth reflecting surface Sl which is substantially coplanar with the incident surface S4, it is emitted substantially perpendicularly to the exit surface S6. The light ray E emitted from the exit surface S6 is incident on the eyepiece lens 12.

In this embodiment, two light-receiving element arrays of the light-receiving means n,
When there is a possibility that the secondary object images OI& formed on the surface 29 overlap each other and become noise, for example, as shown in FIG.
Alternatively, as shown in FIG. 6, if a groove 61 is provided in the center of the prism section and the inside thereof is treated with black, it is possible to prevent the two secondary object images from overlapping.

(Effects of the Invention) According to the present invention, by configuring a part of the prism of the finder optical system in a specific shape and bringing the focusing plate and the field lens close to each other, the function as a field lens can be fully demonstrated, and the focus detection optical system can be Placing the system behind the focusing plate eliminates the need for a movable mirror with a complex mechanism, making it possible to achieve a focus detection optical system suitable for a single-lens reflex camera.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram of an optical system of an embodiment when the present invention is applied to an electronic still camera, Fig. 2 is a perspective view of a portion of Fig. WX1, and Fig. 3 is an expanded view of the main part of Fig. 1. FIG. 4 is an explanatory diagram of a part of FIG. 1, and FIGS. 5 and 6 are explanatory diagrams of other implementations of the part of FIG. 1, respectively. In the figure, 1 is a photographing lens, 9 is a focusing plate, 91 is a planned imaging plane, 1O is a first prism, 1 is a second prism, 14 is a semi-transparent surface, 21 is a field lens, 23 is a re-M imaging system, I
is a light receiving means.

Claims (1)

[Claims] A prism is arranged behind the expected image formation plane where the first object image is formed by the photographing lens, and the prism is adjacent to the entrance surface S_1 through which the light beam from the first object image is incident, and is adjacent to the entrance surface S_1. The first reflecting surface S_2 has a surface S_2' that partially transmits the light and reflects the incident light beam, and the exit surface S_2 sequentially emits the reflected light beams, and the surface S_2 transmits the light. A re-imaging system and a light receiving means are arranged behind the surface S_2 which transmits the light.
A plurality of secondary object images are formed on the light receiving means from the light beam from the first object image that has passed through the light receiving means, and when focus detection is performed using an output signal from the light receiving means, the light is transmitted. A focus detection optical system characterized in that a field lens is provided near the surface S_2'.