an interchangeable lens assembly for a camera including a lens barrel having a diaphragm mechanism mounted therewith , a bayonet coupling mechanism fixed on the lens barrel and adapted to be brought into engagement with a complementary coupling mechanism on the camera for releasably mounting the lens assembly in operative position on the camera , an adapter shell mounted on the lens barrel in rotative relationship relative thereto for establishing predetermined relative positioning between the interchangeable lens assembly and the camera , and a retainer mechanism adapted to be interposed between the adapter shell and the camera for retaining the adapter shell in fixed rotative position relative to the camera during mounting and dismounting of the interchangeable lens assembly . the lens assembly also includes a diaphragm presetting mechanism for transmitting signals representative of a diaphragm aperture between the lens assembly and the camera . since coupling and decoupling of the lens assembly is performed by rotating the lens barrel relative to the camera housing , while the adapter shell remains stationary , the diaphragm presetting mechanism is provided with a device for assuring that an arm extending between the camera and the lens assembly is located in a predetermined angular position relative to a slot in the stationary adapter shell before and after the unit is attached to and detached from the camera housing despite the fact that the range of rotative tightening movement of the tube may be larger or smaller than that of movement of the arm . thus the operator is able to detach the unit with the position of the diaphragm ring left unchanged .