Unidirectional focussing system

In a photographic-camera focussing system having an objective adjustable in a first direction from a first extreme subject-distance setting to a second extreme subject-distance setting, in combination:

• first and second photodetector means located to receive light from a subject to be photographed;
• comparing means for comparing the light incident on the first and second photodetector means and generating in dependence upon the present subject-distance setting of the objective a first signal when the camera-objective subject-distance setting should be changed in the first direction, a second signal when the camera-objective subject-distance setting should be changed in an opposite second direction, and generating both the first and second signals concurrently when the camera-objective subject-distance setting is within a predetermined tolerance range of acceptable error;
• means constraining the camera objective to be at the first extreme subject-distance setting at the start of a focussing operation, whereby in order to bring the camera objective to the correct subject-distance setting it is necessary that the camera-objective subject-distance setting be changed in the first direction from the first extreme setting towards the second extreme setting thereof;
• and stop-signal-generating means operative in response to said second signal for generating a stop signal commanding that adjustment of the camera-objective subject-distance setting be stopped, whereby as the subject-distance setting is changed in said first direction from said first extreme setting towards said second extreme setting the stop signal is generated in response to generation of said second signal as the subject-distance setting error just enters into the range of acceptable error.