PURPOSE: To switch surely an optical system to telephotographing by setting up a sub-lens system insertion completion detecting switch in addition to a main lens system movement completion detecting switch in the focal distance switching operation of the optical system.

CONSTITUTION: The sub-lens system 42 is held by an extender 40 pivotally supported by an extender bar 43, and when standard photographing status shown by a dotted line is switched to telephotographing status, the main lens system moves along an optical axis direction and sub-lens system 42 is inserted into the optical axis as shown by a solid line. A circuit detecting said operation has the shown constitution, a switch 31 is turned on when the main lens is reference photographing status and a switch 32 is turned on in case of telephotographing status. Since a switch 33 is not turned on until the sub-lens system 42 is precisely arranged on the photographing optical axis when the telephotographing status is set up, a sequence circuit 34 in the camera is not turned to operable status.