A focus condition detecting system for a cata-dioptric lens wherein it can be judged correctly whether or not detection of a focus condition is possible, and a cata-dioptric lens for use with the focus condition detecting system. The system comprises a focus condition detecting means for detecting a relative displacement between a pair of images of light formed by a pair of optical means from beams of light from a pair of different areas of a plane of an exit pupil of a cata-dioptric lens to detect a focus condition of the lens. Data regarding outer and inner radii of an exit pupil or some other peculiar data are fixedly stored in the lens. The data are successively forwarded to a camera body, and depending upon the data, it is judged whether optically accurate detection of a focus condition is possible from a combination of the optical means and the lens. When the judgment is affirmative, the detecting means is rendered operative. A set of interchangeable lenses which contribute to improvement in function of a body of a camera and also to development of a camera system is also provided.