An adapter for connecting an interchangeable lens, such as a lens which is ordinarily intended for use with a single-lens reflex camera, for which the distance from the rear surface on the lens to a focal plane is relatively great, to a camera body, such as a video camera body, for which the distance from the front surface of the lens mount to a focal plane is relatively small. The adapter includes a diaphragm, which is used in place of the diaphragm of the interchangeable lens, and an electric motor for controlling the aperture value provided by the diaphragm. The aperture data supplied by the interchangeable lens is transmitted to the camera body as electrical data, which is preferably produced with a variable resistor or the like. Contacts are provided on the casing of the adapter for transmitting data to the camera body and receiving an operating signal for the motor from the camera body.