Focussing system for cameras

Abstract

First and second photodetector signals, whose relative amplitudes depend upon focus error, are integrated to form respective first and second integral signals whose relative rates of change are dependent upon focus error. Each integral signal is applied to two comparators, one whose threshold level is reached first and the other having a threshold level which would be reached second. When one or the other of the two integral signals reaches its first-reached threshold level, this applies a corresponding signal to the data input of a respective flip-flop, and when this faster-changing integral signal then reaches its second-reached threshold level this clocks both such flip-flops. The time elapsing between the faster-changing integral signal reaching its first-reached and then its second-reached threshold level constitutes a tolerance interval, during which the slower-changing integral signal is given an opportunity to try to reach its respective first-reached threshold level. If the slower-changing integral signal reaches its first-reached threshold level within the tolerance interval, then a corresponding signal is applied to the data input of its associated flip-flop as well before the two flip-flops are clocked, with the result that the states of the two flip-flops are the same as if both integral signals had indentical rates of change of value.