Abstract: A single lens reflex camera includes an interchangeable objective lens provided with a diaphragm adjusting ring having a drive element and an interchangeable view finder unit housing a light responsive exposure determining means including a follower movable with a varying parameter of the means between a retracted position and a resiliently urged advanced position. A blocking element is normally locked in a position projecting from the unit and preventing the mounting of the finder, and when the follower is released, the blocking element is retractable. The follower is locked in its retracted position and when the finder is mounted the follower is released for movement into operative engagement with the drive element.