A single lens reflex camera in which the diaphragm aperture is stopped-down to a preset position before measuring the image light for use in the automatic exposure control mechanism of the camera. A mirror, which is provided in the camera along its optical path, is movable between a viewing position and a retracted photographing position. The camera has a focal plane shutter with two curtains. Upon exposure, the curtains travel in sequence in the same direction in which the mirror is retracted. Initiation of the movement of the first curtain begins before the mirror is fully retracted, thereby enabling the time lag between a shutter release operation and exposure to be reduced.