 Some TV lenses lack provision to focus or vary the aperture, so may not operate properly with film cameras.Also, some TV lenses may have bits that protrude behind the mount far enough to interfere with the shutter or reflex finder mechanisms of a film camera.Although C-mount lenses have a back focal distance far too short to be used with 35 mm film SLRs or any existing digital SLR, they can be mounted on interchangeable-lens mirrorless digital cameras.