Apparatus for detecting open f number in zoom lens camera

摘要

An apparatus for detecting an open F number in a zoom lens camera including a cam ring which rotates to vary the focal length of a zoom lens, a reflection plate which is secured to the outer periphery of the cam ring and which has light reflecting portions and non-reflecting portions alternately arranged, as viewed along the circumferential direction of the cam ring, a photoreflector which emits light onto the reflection plate and which detects light reflected by the reflection plate, and an arithmetic device which calculates an open F number of the zoom lens using the output of the photoreflector.