GB1578367A

GB1578367A - Camera incorporating focussing means

Info

Publication number: GB1578367A
Application number: GB1964177A
Authority: GB
Original assignee: Konica Minolta Inc
Current assignee: Konica Minolta Inc
Priority date: 1976-05-15
Filing date: 1977-05-10
Publication date: 1980-11-05
Also published as: JPS52138924A

Description

(54) CAMERA INCORPORATING FOCUSSING MEANS (71) We, KONISHIROKU PHOTO INDUSTRY CO., LTD., of No. 1--10, Nihonbashi Muromachi 3-Chome, Chuo-Ku, Tokyo, Japan, a company duly organized under the laws of Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the apparatus by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a single lens reflex camera having an improved focussing means.

In recent years, a variety of cameras have been proposed which incorporate means for detecting the correctly focussed condition of the camera objective.

One hitherto proposed automatic focussing means utilises photoelectric elements for matching the illumination intensities of two light images. Such a system is disclosed for example in Japanese Patent Publication No. 24,153/64. An improved system of this type is shown in Japanese Patent Laid-open No. 39,543/75.

This focussing means operates by applying the incident light coming from an object to the camera onto a detecting means having at least a pair of photoelements. The arrangement is such that a control circuit issues a signal representative of correct focussing, when the image distributions on the respective photoelements are substantially the same. Drive means are provided for moving the objective lens to the focussing position, in accordance with the signal from the control circuit.

However, since the known system relying upon the double-image matching system employs a specific lens system, in addition to the objective lens, so as to receive the incident light from the object independently of the objective lens, the whole mechanism of the camera is rendered unduly complicated.

In addition, this known focal focussing system requires troublesome lens adjustment, for each replacement of the objective lens of the camera.

The present invention provides a single lens reflex camera incorporating a pentaprism enabling viewing of a light image focussed by the camera objective on a focussing screen, wherein said focussing screen has a transparent area permitting part of the light beam focussed by said camera objective to pass through said screen and to be reflected out of the optical path of the viewfinder by a means located between said focussing screen and said pentaprism, and wherein said camera further includes a pair of photoelectric elements, a pair of image forming lenses so arranged to receive light rays from said reflecting means and to project light images onto said photoelectric elements respectively that said elements receive matched images only when the camera objective is correctly focussed, the length of the optical path between each of said image forming lenses and the respective photoelectric element being greater than that between the focussing screen and the respective image forming lens, and an electric circuit including said photoelectric elements and means for detecting or indicating the condition when said photoelectric elements are equally illuminated.

The invention is illustrated by way of example in the accompanying drawings, in which: Fig. 1 is a diagrammatic perspective view of a photographic camera according to the present invention, Figs, 24 are similar views illustrating the relevant parts of further embodiments of the present invention, Figs. 5, 6 and 7 are ray diagrams illustrating examples of image forming optical systems for incorporation in a camera according to the invention Fig. 8 is a diagram showing an example of means for detecting the focussed condition of a camera objective, and Fig. 9 is a diagram illustrating the operation of the means shown in Fig. 8.

Figs. 1 to 4 and 8 and 9 are merely diagrammatic as regards the distance between the lenses and the photocell; these distances are shown accurately in Figs. 5 to 7.

Referring to the drawings, numerals 1, 2 and 3 designate, respectively, a camera body, a focussing screen and a condenser lens, whereas numeral 4 denotes a pentaprism. Numeral 2a designates a transparent area at the centre of said focussing screen 2.

Numerals 5a, Sb, Sc and 5d represent image forming optical systems of respective embodiments of the invention, while numerals 5a', 5b', 5c' and 5d' represent respective image forming lens surfaces.

A pair of photoelectric converting elements 6 of a rectangular shape with opposing sides constituting electrodes from focal point detecting means. Optical shunting means 7 are adapted to shunt a part of incident light coming from the focussing screen 2. Numeral 7a denotes a half-silvered mirror inclined at an angle of 45 , while numeral 8 denotes a finder eyepiece lens. In Fig. 8, numeral 9 denotes a detecting means. This means includes two elements 10, 11 (which are the same with the photoelectric converting means 6) adapted to become conductive when applied with light and having respective opposing electrode 10a, l0b and l la, l lb. The detecting means further includes variable resistance R1, R2 and an indicating meter M. The elements 10, 11, variable resistance R1,R2 and the meter M are electrically conriected to form a bridge, as shown in Fig. 8.

Numerals 12, 13 denote image forming lenses which correspond with those indicated by numerals 5a', 5b', 5c' and 5d' as shown in Figs. 1 to 4, while numeral 14 designates an objective lens of the camera.

Symbol L represents a bundle of light coming from the object into the detecting means.

In Fig. 9, reference W denotes an object and 0 denotes a position of said object W.

A, B, C represent positions of the adjustable object lens 14, respectively. Af, B,, C, represent positions of focussed images corresponding to said positions A, B, C of the object lens 14 respectively. Am, Bm, Cm represent images of said object W formed by the image forming lenses 12, 13, respectively. Distances between one end (upper end in Fig. 9) of said elements 10, l l and said images Am, Bm, Cm are (leo, mO), (l" m1) and ('2, m2), respectively. The focal point detecting apparatus of the present invention will now be explained with reference to an embodiment shown in Fig.

9.

The images Am of the object W are formed by the lenses 12, 13 when the objective lens 14 is positioned on A and the focussed image of the object W is formed on the position A,. In this state, a film is arranged on a position equivalent optically to the position A, and the elements 10, 11 are arranged on positions where the images Am are formed at the centres of said elements 10, II, respectively, namely, on positions where the distance 19 is equal to 1/2. The resistances of respective elements 10, 11 which are determined by the positions of the images are equal with each other only when the centre portion of the image is formed on a position where the distance 1o is equal to 1/2, because the positions of the images on the elements 10, 11 are varied symmetrically by moving the objective lens 14. When the resistances of the elements 10, 11 are equal, the difference between the electric signals of the elements 10, 11 becomes zero or nearly zero and a signal representative of correct focussing is provided. If the objective lens 14 is positioned on B or C, the image of the object W is formed on the position B, or C, and the images Bmor Cm are formed by the lenses 12, 13 on the positions with each other symmetrical with respect to the optical axis. In this state, a large difference signal showing such a state that the image of the object is not focussed is generated, because the distance 11 becomes larger than the distance m1 with respect to the image Bm and m2 becomes larger than 12 with respect to the image Cm, respectively. The signals generated by the elements 10, 11 are detected by the meter M in the bridge circuit shown in Fig. 8.

Since the images on the elements 10, 11 are of identical size, the resistances residing in the elements 10, l l can be regarded as identical. Thus, the signal represented by the meter M materially depends on the change of resistance attributable to the difference in the length of the electrode of the focal point detecting element (the length from the upper end of the element.

An illuminating device may be associated with the point of the meter M to act at the time when the image is focussed, so as to provide the focussing display in accordance with the movement of the objective lens back and forth.

Alternatively, driving means adapted to stop the operation when a correct focussing has been attained is provided, so that the focussing point of the objective lens may be automatically detected through the driving means.

The image on the focal point detecting means 6 is preferably a focussed one.

However, in view of the function of the elements, the image may be slightly out of focus.

At the same time, since the image forming lenses 12, 13 have much smaller focal lengths as compared with normally used camera objective lenses, the resultant focal point is almost identical to that of an image forming lens having a small focal length, so that it is guaranteed that no substantial loss of focus of the image on the detecting means during the focal point detecting operation.

In the embodiment shown in Fig. I, the condenser lens 3 plays also the function of the optical shunting means 7, being provided with a half mirror 7a at the centre thereof. One side surface of the condenser lens 3 is inclined at 450 to form a reflecting mirror 3a. Part of the incident light shunted through the transparent area 2a by the condenser lens is converted by the convex lens 5a' and forms the image on the focal point detecting means 6, after having passed through a relatively long path of light of the image forming optical system formed laterally of the pentaprism.

In the embodiment shown in Fig. 2, optical shunting means 7 is provided between the condenser lens 3 and the pentaprism 4. The optical shunting means 7 has also at its intermediate portion a half mirror inclined at about 45". The image forming optical system Sb is provided in front of the pentaprism, while the focal point detecting means 6 are disposed on the head portion of the pentaprism.

Referring to the embodiment of Fig. 3, the condenser lens 3 is adapted to play the role of the optical shunting means, as is the case of the embodiment of Fig. 1.

The image forming optical system Sc is an unitary body consisting of convex lens surfaces Sc' at both ends of the bottom thereof, a pentagonal perforation provided at the centre thereof, and inclined surfaces for deflecting the incident light at the desired angle and provided at both ends of the top thereof.

Since the image forming optical means are constructed unitarily, the symmetry of the two images is ensured to a higher extent.

The focal point detecting means 6 are disposed atop of the system 5c.

In the embodiment of Fig. 4, the condenser lens 3 also performs the function of the optical shunting means as is the case of the embodiments of Figs. 1 and 3. The distance between the image forming optical system 5d and the focal point detecting means 6 is made smaller, by shortening the optical length of the light path of the system 5d, thereby to render the whole apparatus compact Figs? 5, 6 and 7 show exploded views of optical systems illustrating how the distance (b in Fig. 5) between the convex lens section of the image forming optical system and the focal point detecting means is made larger than that (a in Fig. 5) between the lens section and the focussing screen, to ensure a higher accuracy of the focal point detection.

WHAT WE CLAIM IS: 1. A single lens reflex camera incorporating a pentaprism enabling viewing of a light image focussed by the camera objective on a focussing screen, wherein said focussing screen has a transparent area permitting part of the light beam focussed by said camera objective to pass through said screen and to be reflected out of the optical path of the viewfinder by a means located between said focussing screen and said pentaprism, and wherein said camera further includes a pair of photoelectric elements, a pair of image forming lenses so arranged to receive light rays from said reflecting means and to project light images onto said photoelectric elements respectively that said elements receive matched images only when the camera objective is correctly focussed, the length of the optical path between each of said image forming lenses and the respective photoelectric element being greater than that between the focussing screen and the respective image forming lens, and an electric circuit including said photoelectric elements and means for detecting or indicating the condition when said photoelectric elements are equally illuminated.

2. An apparatus as claimed in Claim 1, wherein said image forming lenses and said photoelectric elements are disposed in front of said pentaprism.

3. An apparatus as claimed in Claim 1, wherein said image forming lenses and said photoelectric elements are disposed at the side of said pentaprism.

4. Apparatus as claimed in any one of Claims 1--3, wherein said reflecting means is incorporated in a condenser lens disposed between the focussing screen and the pentaprism.

5. A single lens reflex camera as claimed in Claim 1, substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. camera objective lenses, the resultant focal point is almost identical to that of an image forming lens having a small focal length, so that it is guaranteed that no substantial loss of focus of the image on the detecting means during the focal point detecting operation. In the embodiment shown in Fig. I, the condenser lens 3 plays also the function of the optical shunting means 7, being provided with a half mirror 7a at the centre thereof. One side surface of the condenser lens 3 is inclined at 450 to form a reflecting mirror 3a. Part of the incident light shunted through the transparent area 2a by the condenser lens is converted by the convex lens 5a' and forms the image on the focal point detecting means 6, after having passed through a relatively long path of light of the image forming optical system formed laterally of the pentaprism. In the embodiment shown in Fig. 2, optical shunting means 7 is provided between the condenser lens 3 and the pentaprism 4. The optical shunting means 7 has also at its intermediate portion a half mirror inclined at about 45". The image forming optical system Sb is provided in front of the pentaprism, while the focal point detecting means 6 are disposed on the head portion of the pentaprism. Referring to the embodiment of Fig. 3, the condenser lens 3 is adapted to play the role of the optical shunting means, as is the case of the embodiment of Fig. 1. The image forming optical system Sc is an unitary body consisting of convex lens surfaces Sc' at both ends of the bottom thereof, a pentagonal perforation provided at the centre thereof, and inclined surfaces for deflecting the incident light at the desired angle and provided at both ends of the top thereof. Since the image forming optical means are constructed unitarily, the symmetry of the two images is ensured to a higher extent. The focal point detecting means 6 are disposed atop of the system 5c. In the embodiment of Fig. 4, the condenser lens 3 also performs the function of the optical shunting means as is the case of the embodiments of Figs. 1 and 3. The distance between the image forming optical system 5d and the focal point detecting means 6 is made smaller, by shortening the optical length of the light path of the system 5d, thereby to render the whole apparatus compact Figs? 5, 6 and 7 show exploded views of optical systems illustrating how the distance (b in Fig. 5) between the convex lens section of the image forming optical system and the focal point detecting means is made larger than that (a in Fig. 5) between the lens section and the focussing screen, to ensure a higher accuracy of the focal point detection. WHAT WE CLAIM IS:

1. A single lens reflex camera incorporating a pentaprism enabling viewing of a light image focussed by the camera objective on a focussing screen, wherein said focussing screen has a transparent area permitting part of the light beam focussed by said camera objective to pass through said screen and to be reflected out of the optical path of the viewfinder by a means located between said focussing screen and said pentaprism, and wherein said camera further includes a pair of photoelectric elements, a pair of image forming lenses so arranged to receive light rays from said reflecting means and to project light images onto said photoelectric elements respectively that said elements receive matched images only when the camera objective is correctly focussed, the length of the optical path between each of said image forming lenses and the respective photoelectric element being greater than that between the focussing screen and the respective image forming lens, and an electric circuit including said photoelectric elements and means for detecting or indicating the condition when said photoelectric elements are equally illuminated.