A camera system according to the present invention includes an image
pickup device having an imaging surface of an imaging range with a
predetermined aspect ratio, the image circle being set larger than the
imaging range to include the imaging range therein, a mount portion to
which the interchangeable lens is mounted, and an opening portion having
an opening for allowing luminous flux from the interchangeable lens to
pass through, being located at a position spaced from the imaging surface
by a predetermined distance, wherein the size of the opening of the
opening portion is equal to or larger than a square formed by connecting
ends of segments tilted by an angle .theta. with respect to an optical
axis diagonally from four corners of the imaging range and extended to
the opening of the opening portion, and the angle .theta. satisfies: 12
degrees.ltoreq..theta., wherein a distance between the opening of the
opening portion and the imaging surface, herein referred to as flange
back (FB), is: 35 mm.ltoreq.FB.ltoreq.50 mm.