Conversion adapters as the following are on sale in order to attach the single camera lens to the C mount camera.
Flange back of EOS is 44mm
Flange back of C mount is 17.526 mm
Lenses for single lens reflex cameras can be mounted on C mount cameras.
Focus is achieved by offsetting 44 – 17 mm = 27 mm.
However, when attaching a C mount len to a single len reflex camera, the flange back is too close and there is no focus.
(If you do not let the lens into the camera, the focus will not match.)