Patent GB-1325182-A (GB)
Title: Camera exposure control system
IPC: G03B7/16
Filed: 1970-11-06
Abstract: 1325182 Camera exposure control systems CANON K K 6 Nov 1970 [6 Nov 1969 27 Dec 1969 (2) and 21 Sept 1970] 53000/70 Heading G2A A camera exposure control system operates for a flash exposure so as to satisfy a first relation among guide number, film sensitivity, F/number and object distance and at the same time operates so as to satisfy a second relation among object brightness, film sensitivity, square of F/number and shutter speed to control an exposure control device for exposure to ambient illumination. In a first embodiment, Fig.2, a focusing ring 2 and a diaphragm ring 4 are selectively engageable by a connector pin 3 engaging one of a plurality of holes 2' according to flash guide number and film sensitivity. A contact member 8 fixed to the diaphragm ring slides over resistor R1 connected in series with a photolvoltaic element P1, power source E o  and ampere meter M1. A rotatable shutter setting ring 10 has a sawtoothed step-wise portion 10b engaging a meter pointer 19, a limiter pin 20 engaging a projection 10c of the shutter ring to limit the shutter speed to less than a pre-set value, e.g. 1/30<SP>th</SP> sec. Over exposure of the object in the above embodiment due to illumination by ambient light is prevented in a further embodiment, Fig.4, in which a photo-sensitive element P2 receives light from the central portion of the scene, whilst element P3 receives light from the scene as a whole, a detector circuit B producing a signal corresponding to the brightness difference between the main object and the remainder. The current fed to meter M2 is adjusted by this signal, and also by adjustment of resistors R2, R3, R4 for respectively film sensitivity, object distance and F/number. Meter M2 controls taking lens aperture 23 and also on aperture 22 in front of a photo-sensitive element P4 which controls an exposure time circuit A. Other embodiments are described having only two or even one  photo-sensitive element, and with a progressively opening shutter with a constant speed. The system may be used with the shutter shown in Fig. 16-1. Depression of release lever a1 causes a pin a2 thereon to move lock lever a3 away from engagement with a shutter stop ring a4 which rotates at a constant speed with the aid of the braking action of the governor a6 and a7 to thus produce progressive opening of the exposure aperture. Shortly after the opening movement of ring a4, magnet Mg 2 is de-energized to allow diaphragm blade ring b1 to open quickly. A projection 45 contacts a flash switch Sf to fire a flash discharge tube, and thereafter rings b1 and a4 continue opening under the braking action of governor a6, a7. Magnet Mg 3 is energized at the end of the exposure interval to free a leverd2 driven by a strong spring 44 to close the shutter rapidly without the braking action of governor a6, a7. Projection 45 operation a diaphragm 32 located in front of photo-sensitive element P4. In a further embodiment, Fig. 17 (not shown) the shutter blades are opened only by an amount determined by a stepped feeler engaging a meter pointer. The shutter opening and closing action may be controlled by a single magnet. Fig. 18 (not shown). In a further embodiment, Figs. 19-21 (not shown) the system is designed to permit flash photography in daylight, a changeover switch from AUTO to FLASH being provided and the diaphragm being manually varied at a set object distance until a meter pointer is aligned with a pre-set mark, non- alignment being indicated by a superimposed objection of the meter pointer.