GB2065902A - Focusing arrangement for reflex cameras - Google Patents

Abstract

An optical arrangement for the automatic or semi-automatic critical focusing of reflex cameras, comprising a pentagonal prism and a pair of measuring wedges which are arranged in the focal plane of the viewfinder ray path and which lie side by side with oppositely directed inclinations, characterised in that behind the image-erecting reflecting surface 4 of the pentagonal prism 3 there are provided two optical image- forming systems 1; 2, with the aid of which it is possible to form images of image planes that lie forward of and behind the actual focal plane. At correct focus, there are formed on photoreceivers E1 and E2 images with an even lack of definition, leading to identical signals. <IMAGE>

Description

SPECIFICATION Optical arrangement for reflex carneras The invention relates to an optical arrangement for the automatic or semi-automatic critical focusing of reflex cameras, comprising a pentagonal prism and two measuring wedges which are arranged in the viewfinder ray path in the focal plane and which lie side by side with oppositely directed inclinations.

For the focusing of photographic reflex cameras, it is known to arrange in the ray path of the lens, namely in a focal plane produced by optical means and equivalent to the film plane or viewfinder image plane, two receiver areas with several photovoltaic cells which adjoin one another. The splitting of the image on the two areas is effected with two measuring wedges which are arranged so as to be opposite to each other. For critical focusing, the brightnesses of photovoltaic cells which are associated with one another on opposite sides are compared. If the brightnesses are identical, then the lens is sharply focused. Focusing is fully or semi-automatically effected in response to an extreme value signal.

Another known device comprises an intermediate image-forming lens with a photometric double wedge which splits the image ray path into two aperture zones and feeds it to separately arranged photoreceivers, which are subdivided in a strip-like manner, a brightness comparison of the strips being effected on the photoreceivers in pairs. Here, too, focusing is effected according to an extreme value if there is identity on the strip pairs.

In both cases, there are used optical arrangements in which the path of rays is guided for critical focusing by the reflex mirror via an auxiliary mirror, which is hinged to this latter, in the downward direction towards the camera base.

In a further development of the first-mentioned known principle, the image-erecting surface of the pentagonal prism is designed so as to be partially transmitting, there being effected on the receiver areas a direct image formation from the wedges arranged in the viewfinder image plane.

In the case of the known arrangements described here, the image plane is imaged in sharp focus on the receivers via two opposed aperture zones of the lens by suitable optical means. As receivers there are required arrangements which are subdivided in a strip-like manner and several closely adjoined photovoltaic cells on receiver areas respectively. Receivers of this kind are as a rule expensive special constructions, which furthermore necessitate a relatively high expenditure of circuitry, Moreover, partially transmitting surfaces are provided in the shooting or viewfinder ray path for reflecting the ray path out for cricitcal focusing, which cause the viewfinder brightness to be reduce.

The present invention aims at avoiding the mentioned disadvantages, namely the use of complicated special receivers and the reduction in the viewfinder brightness by partially transmitting coatings.

The present invention preferably provides an optical arrangement for automatic or semiautomatic critical focusing which operates according to the reflecting-out principle.

According to the present invention there are provided, behind the image-erecting reflecting surface of the pentagonal prism, two optical image-forming systems, with the aid of which images of image planes lying in front of and behind the actual focal plane can be formed.

Expedientiy, the image formation is effected on two receivers which are provided outside the reflecting zone of the reflecting surface and which are preferably active to the right and left of the reflecting surface. An essential feature of the invention consists furthermore in that the viewfinder split-image being formed in the measuring wedges is imaged in a balanced condition with an even lack of definition. In a further development of the invention, there are provided as the receivers photoconductive cells which are arranged in a bridge circuit and in which the non-linear characteristic is utilised for the automatic critical focusing.

The invention will hereinafter be explained in more detail with reference to an exemplified embodiment.

In the drawings:- FIGURE 1 shows the characteristic lines of the photoconductive cells E, and E2, FIGURE 2a) shows a side view of the pentagonal prism and the reflecting-out arrangement, and b) a front view of the pentagonal prism with the arrangement of the image-forming systems and the receivers.

As shown in Fig. 2, two photoreceivers El, E2 are separately arranged to the right and left of the reflecting surface 4 behind the image-erecting surface of the pentagonal prism 3, as viewed in the light direction. The ray paths coming from the individual measuring wedges of the measuring wedge pair 5 are directed to the optical imageforming systems 1, 2 which are arranged forward of the photoreceivers E, and E2 respectively. The image-forming systems are arranged in such a way that image planes which lie in front of and behind the wedge point of intersection, that is to say the exact viewing image plane, are imaged in sharp focus. By this means, there are formed, at a coincidence setting, on the photoreceivers E" E2 images with an even lack of definition, which lead to identical signals.If photoconductive cells are used as the photoreceivers in a bridge circuit, there come about upon critical focusing identical values of resistance so that the automatic control loop is rendered currentless. If the photoreceivers E1, E2 are used in a parallel circuit, then there come about identical signal currents as the criterion for the completed lens adjustment.

Critical focusing is thus terminated when the imaged split images are imaged with a uniform lack of definition and there is no difference in their signals.

If, for example, there is formed, as shown in Fig.

1, a sharp image of the image plane, which is forward of the exact viewing image plane by -As', on the photoreceiver E1, then there flows a photoelectric current 11. At this setting, there is formed on the photoreceiver E2 an entirely unsharp image of the image plane, which is behind the exact viewing image plane by +As'. The photoelectric current 12 flows herein. With the photoelectric current difference Al = I, -- there is effected, with an appropriate amplification, the control operation for the automatic critical focusing. At an exact setting of the focal plane in s0', there are formed on the photoreceivers E1 and E2 equally unsharp images of the image planes which are displaced by +As'.In this case, the photoelectric current difference Al equals 0, that is to say 11 = 12 applies to the photoelectric currents.

With a manual adjustment of the lens for semiautomatic critical focusing, this photoelectric current difference can be used for a luminous display of the setting state.

Due to the constructionally laid down positional difference of the image planes from the viewfinder image plane, there comes about a clearly defined directional dependence for the automatic lens adjustment. The optical reflecting-out principle according to the invention has the advantage that there do not arise any light losses and impairments in the viewfinder image due to geometrical or physical beam splitting.

Furthermore, the automatic critical focusing can be watched and checked directly in the viewfinder image. Since the image to be judged is transmitted from the viewfinder image plane to the photoreceivers and is fed to an electric evaluating unit, the method does not depend on the focal length and aperture of the lens used.

Claims (6)

1. An optical arrangement for the automatic or semi-automatic critical focusing of reflex cameras, comprising a pentagonal prism and a pair of measuring wedges which are arranged in the viewfinder path in the focal plane and whose measuring wedges lie side by side with oppositely directed inclinations, characterised in that behind the image-erecting reflecting surface of the pentagonal prism there are provided two optical image-forming systems, with the aid of which it is possible to form images of image planes that lie forward of and behind the actual focal plane.

2. An optical arrangement as claimed in Claim 1, characterised in that the optical image-forming systems have far advanced principal planes.

3. An optical arrangement as claimed in Claim 2, characterised in that the images are formed on two photoreceivers which are arranged outside the reflecting zone of the reflecting surface and which are preferably fastened to the right and left of the reflecting surface.

4. An optical arrangement as claimed in Claim 2, characterised in that the viewing split image being formed in the measuring wedge pair is imaged in a balanced state with a uniform lack of definition on the photoreceivers.

5. An optical arrangement as claimed in Claim 4, characterised in that as the photoreceivers there are provided in a bridge circuit photoconductive cells, whose non-linear characteristic is utilised for automatic critical focusing.

6. An optical arrangement for the automatic or semi-automatic critical focusing of reflex cameras substantially as described herein with reference to the accompanying drawings.