Automatic exposure control device for single lens reflex camera

Claim 1

An automatic exposure mechanism for a single lens reflex camera of the type having stop blades for controlling the stop position of the camera, a brightness measuring element, film and shutter speed setting mechanisms, and a shutter release button for initiating picture taking operation, said automatic exposure mechanism comprising: (a) an electromagnet, (b) electronic circuit means for controlling said electromagnet in response to brightness of a scene to be photographed, and film speed and shutter speed settings, to alter the energization condition of said electromagnet at a time following depression of the camera shutter release button dependent upon the circuit -- determined stop position of said camera's stop blades, said circuit performing a timing operation which is linearly proportional to the number of stop positions required for proper exposure, (c) stop determining means biased to move in a predetermined direction when free to move, said means being linked to said stop blades whereby the position of said stop determining means determines the position of said stop blades, (d) an engaging lever positioned in a normal position to prevent free movement of said stop determining means, said engaging lever being linked to said shutter release button to move away from its normal position to no longer prevent free movement of said stop determining means, (e) magnetically controlled means responsive to said electromagnet for moving from a first position to a second position in response to said change of energization of said electromagnet, said first position permitting free movement of said stop determining means and said second position preventing free movement of said stop determining means, whereby the depression of said shutter button starts said circuit timing operation and permits free movement of said stop determining means and at the end of said time said stop determining means ceases movement and said stop blades move to a position determined by said stop determining means, wherein said electronic circuit means comprises: (i) photosensor means responsive to the scene brightness, (ii) first log compression means connected in series with said photosensor means, (iii) variable resistance means adjustable according to shutter speed and film speed, (iv) second log compression means connected in series with said variable resistance means, (v) timing means, including a capacitor and a constant current generator connected to said capacitor, connected to the junction of said variable resistance means and said second log compression means, and (vi) switching means connected to the junction of said photosensor means and said first log compression means and also connected to said timing means for generating a switching signal when the output of said timing means exceeds the output at said junction of said photosensor means and said first log compression means, the time of switching t being a function of the APEX stop size A v according to the following formula: t = K 3 (K 0 + A v ), where K 0 and K 3 are constants of proportionality.