JPS63259526A - Optical system - Google Patents

Abstract

PURPOSE:To execute observation of a satisfactory finder image and a direct photometry by placing a split mirror glass block for a photometry, in the rear of a semi-transparent surface of a prism body so that the optical path length through which a light beam passes before reaching an image surface from the rear surface of an objective lens goes to roughly equal at an image pickup side and a finder side. CONSTITUTION:A crystal low-pass filter 13 and an infrared cut filter 14 are joined to a block of a split mirror prism 9, and in its rear, between the filter and an image pickup element 10, a photometry use split mirror glass block 16 having a half mirror surface 15 is placed. Also, a luminous flux reflected by the half mirror surface 15 is condensed by a condensing lens 17, and detected by a photodetector 18. In such a case, due to existence of the photometry use split mirror glass block 16 placed at the image pickup side, the optical path length goes to roughly equal at the image pickup side and a finder side. In such a way, a satisfactory finder image whose aberration has been corrected can be observed, and by detecting a split light beam, the direct photometry can also be executed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical system for cameras, particularly electronic still cameras.

[Conventional technology]

In the optical system of a conventional single-lens reflex camera using silver halide film, as shown in Fig. 4, the light beam from the objective lens 1 is split into two parts by a split mirror 2 called a quick return mirror, which separates the light from the image side (film 6 side) and the viewfinder side. In addition, the luminous flux to the viewfinder is divided by screen mat 3.
Most of them employ a method in which an image is formed upward, an erect image is formed by a prism 4 toward the pentagonal, and the image is observed with an eyepiece 5.

However, in electronic still cameras, the solid-state image sensor is smaller than film, so the image size is smaller, so the magnification of the eyepiece is greater than that of a conventional single-lens reflex camera, and the camera body is mounted on an electrical board. The problem is that it becomes thick. Increasing the magnification of the eyepiece corresponds to shortening the focal length of the eyepiece, which requires shortening the optical path from the screen mat to the eyepiece, and also requires a thick camera body. In order to cope with this, the position of the eyepiece must be brought to some extent toward the eye relative to the image sensor. Therefore, in electronic still cameras, it is difficult to obtain an efficient single-lens reflex optical system using the same method as in conventional single-lens reflex cameras. Therefore, there is a strong need for an efficient optical system unique to electronic still cameras.

Various optical systems have been proposed for electronic still cameras, but among them, an optical system that uses a half mirror for the viewfinder split mirror is very promising and is attracting attention because it is compatible with high-speed continuous shooting and movie mode. It's one of the things.

Using a half mirror for the finder split mirror has the disadvantage of reducing the amount of light for both the finder system and the imaging system, but since there is no mechanical movement, there is no sound.
It has major advantages such as not requiring an actuator. Furthermore, if the half mirror is constructed from a glass block combined with a prism, the back focus required for a phantom lens in terms of air equivalent length will be extremely short, which has the advantage of greatly contributing to the miniaturization of a phantom lens. There is also.

Japanese Patent Laid-Open No. 61-97633 discloses a simple optical system of an electronic still camera using a half mirror as a finder splitting mirror. As shown in FIG. 5, this includes a split mirror prism 9 having a half mirror surface 7 whose normal line is inclined by about 30 degrees with respect to the optical axis and an object side reflective surface 8.
The transmitted light forms an image on the surface of the image sensor 10, and the reflected light is reflected on the half mirror surface 7, is totally reflected on the object side surface 8 of the prism 9, and is further imaged on the screen mat 3, after which it is transferred to the roof prism 1). It is reflected by the roof surface 12 to form an erect image, which is observed through the eyepiece lens 5. The optical system with this configuration has the advantage that not only the split mirror part is small, but also the eyepiece lens 5 is positioned largely toward the eye side with respect to the image sensor 10, so that it can be used with a thick camera body.

[Problem that the invention seeks to solve]

However, in the optical system with the above configuration, when trying to form an image of the divided light beam on the viewfinder side outside the prism, the length of the glass path through which the light passes from the rear surface of the objective lens 1 to the imaging position is In this configuration, the ratio of the glass path on the imaging side and the finder side is approximately 2:5, which means that
fi) size image sensor, the difference is 15
It will be about the size of a crane. In normal lens design, aberrations are corrected in consideration of the optical path length of the imaging system, so in this case, the image in the finder system will be disturbed by aberrations caused by the difference in optical path length.

On the other hand, on the imaging side, a solid-state imaging device 10 such as a CCD is also used.
In front of the camera, there is usually an optical low-pass filter made of a crystal plate, etc. to prevent moiré, an infrared cut filter, and a COD.
A flat glass such as a protective glass is placed. This reduces the difference in glass path length between the imaging side and the finder side, but when using a 2/3 inch size image sensor, the total thickness of the optical low-pass filter etc. is approximately 8 m.
As the saying goes, the difference in glass path length between the imaging side and the finder side is about 7 liters. Considering a large-diameter objective lens with a diameter of approximately Fl. Therefore, it is difficult to observe a good finder image with the difference in glass path length of seven dragons.

In addition, there is a strong demand for electronic still cameras to use so-called direct photometry, which uses reflected light from the imaging surface, in the photometry system. However, in the case of solid-state image sensors such as COD, unlike silver halide film, the light that hits the imaging surface is strongly required. Since the light does not scatter much and is mostly reflected specularly, it is difficult to perform direct photometry in which the photometric element is placed off the optical axis, as in silver-halide cameras. Therefore, in order to perform direct photometry with an optical system using a solid-state imaging device, it is necessary to arrange a prism or pellicle mirror including a half mirror surface in the middle of the imaging optical system.

SUMMARY OF THE INVENTION In view of the above problems, it is an object of the present invention to provide an optical system as described above that enables good viewfinder image observation and direct photometry.

[Means and effects for solving the problems] The optical system according to the present invention includes an objective lens, a semi-transparent surface that transmits a part of the light flux passing through the objective lens and reflects the remaining part, and a semi-transparent surface that a prism body having a reflective surface that re-reflects the reflected light; a finder system that observes an image formed by the light reflected by the semi-transparent surface and the reflective surface of the prism body; In the optical system, a split mirror glass block for photometry is arranged behind the semi-transparent surface of the prism body in the lift system, and a split mirror glass block for photometry is arranged behind the semi-transparent surface of the prism body in the lift system. By making the length of the glass path through which light passes before reaching the image plane approximately equal on the imaging side and the finder side, it is possible to observe a good finder image with well-corrected aberrations, similar to the object image formed on the imaging side. In addition, direct photometry can also be performed by dividing the light beam with a photometry splitting mirror placed on the imaging side and detecting the split light.

Preferably, the thickness of the glass block of the split mirror for photometry is set so as to satisfy the following conditions.

1Δ1<− where Δ is the difference in the air-equivalent glass path length between the imaging side and the finder side from the rear surface of the objective lens to the image plane, and h is the diagonal length of the effective imaging surface of the imaging element.

If this condition is exceeded, aberrations in the finder image increase, making it difficult to observe the finder image well.

Furthermore, although the photometric optical system has been considered so far, it is also possible to configure an autofocus optical system with a similar configuration by replacing the light receiving element with a distance measuring element.

〔Example〕

Hereinafter, the present invention will be described in detail based on the illustrated embodiment, with the same reference numerals assigned to the same members as in the above-mentioned conventional example.

FIG. 1 shows a first embodiment. In this embodiment, a crystal low-pass filter 13 and an infrared cant filter 14 are joined to a block of a split mirror prism 9, and behind the crystal low-pass filter 13 and an infrared cant filter 14 are connected to an image sensor 10. A split mirror glass block 16 for photometry having a half mirror surface 15 is arranged. A condenser lens 17 is placed on the optical axis divided by the photometric split mirror glass block 16. A light receiving element 18 is arranged, and the light beam reflected by the half mirror surface 15 is condensed by a condensing lens 17 and detected by the light receiving element 18. In this embodiment, the diameter of the light beam is 2Δ3 inches (approximately 16.9 inches).
+n) size solid-state image sensor, and the thickness of the split mirror glass block 16 for photometry is 7.
m, the difference between the imaging side and the finder side in the air-equivalent glass path length through which light passes from the rear surface of the objective lens l to the image plane 1
Δ1 is 0.4 tm.

Therefore, due to the presence of the photometric split mirror glass block 16 placed on the imaging side, the glass path length is approximately equal on the shadow side and the finder side, so that the aberrations are well corrected as well as the object image formed on the imaging side. A good viewfinder image can be observed.

Further, since the light beam is divided by the half mirror surface 15 of the split mirror glass block 16 for photometry, direct photometry is also possible by detecting the divided light.

FIG. 2 shows a second embodiment. The basic configuration of this embodiment is the same as that of the first embodiment, but the difference is that the split mirror glass block 16 for photometry uses the same method as the split mirror prism 9. It has a half-mirror surface 15 whose line is inclined at about 30 degrees to the optical axis, and the light split by the half-mirror surface 15 is focused onto a light-receiving element 18 by an optical cone 19. Condensing light by the optical cone 19 has the effect of increasing the effective area of the light receiving element 18 compared to condensing light by the lens 17, and is particularly effective in the case of average photometry.

FIG. 3 shows a third embodiment. This embodiment is an example in which a distance measuring element 20 is used instead of the light receiving element 17. Crystal low-pass filter 13 and infrared cut filter 14
is bonded directly onto the protective glass of the image sensor 10,
The divided mirror glass block 22 for distance measurement having a half mirror surface 21 is further bonded thereon, and this embodiment is also designed using a 2/3 inch size solid-state image sensor, and is used for distance measurement. The thickness of the split mirror glass block 22 is 8 wa, and lΔ1 is 0.

In this case, the image sensor 10 is connected from the half mirror surface 21.
By making the glass path length from the half mirror surface 21 to the photodetector 20 for distance measurement almost equal, the image for distance measurement will also have less aberration, making distance measurement more accurate. can be done.

〔Effect of the invention〕

As described above, the optical system of the present invention has the practically important advantage of enabling good viewfinder image observation and direct photometry. Furthermore, it is also possible to configure an autofocus optical system by replacing the photometric light receiving element with a distance measuring element.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of the first embodiment of the optical system according to the present invention, FIGS. 2 and 3 are diagrams showing the configuration of the second and third embodiments, respectively, and FIGS. FIG. 7 is a diagram showing the configuration of a conventional example and another conventional example, respectively. 1...Objective lens, 3...Screen mat,
5... Eyepiece lens, 7, 15. 21... Half mirror surface, 8... Object side reflective surface, 9... Split mirror prism, 10... Solid-state image sensor, 1 )...
- Roof prism, 12... Roof surface, 13... Crystal low-pass filter, 14... Infrared cut filter, 16... Divided mirror glass block for photometry,
17... Condensing lens, 18... Light receiving element, 19
...Optical cone, 20...Distance measurement element, 22...Distance measurement split mirror glass block.゛Ko Kinmugi iP1 Figure 2

Claims (2)

[Claims] (1) an objective lens, a prism body having a semi-transparent surface that transmits a part of the light flux passing through the objective lens and reflects the remaining part, and a reflective surface that reflects the reflected light from the semi-transparent surface again;
A finder system for observing an image formed by light reflected by a semi-transparent surface and a reflective surface of the prism body, and an imaging system for capturing an image formed by light transmitted through the semi-transparent surface of the prism body. In the optical system, a split mirror glass block for photometry or distance measurement is arranged behind the semi-transparent surface of the prism body in the imaging system, and a glass block through which light passes from the rear surface of the objective lens to the image plane is arranged. An optical system characterized by having substantially the same path length on the imaging side and the finder side. (2) The optical system according to claim (1), characterized in that an optical cone is used as a condensing element in a photometric optical system formed on a reflected optical path by a split mirror glass block.