FREQUENTLY ASKED QUESTIONS

Q. What is a c-mount lens?

A. It is a lens with a screw mount having a diameter of 25.4mm (1 inch) and a thread pitch of 32 TPI. The flange focal distance is 17.52mm. the standard was originally established for use with 16mm cine cameras. Currently they are used mainly on CCTV cameras but have recently found new use with users of the micro 4/3 photography format. See the articles “C-Mount Defined” and “C-Mount Lenses: a brief history” .

Q. What is the difference between a c-mount and a cs-mount lens?

A. They both have the same physical mount (25.4mm diameter, 32TPI), but the cs-mount lens has a shorter flange focal distance of 12.52mm.

Q. Can I use a cs-mount lens on a c-mount camera or vice versa?

A. If you put a cs-mount lens on a c-mount camera (or use it with a c-mount adapter) you will not have infinity focus but you may be able to use it for macro work. It will be as though you were using a 5mm extension tube. Note, however, that most cs-mount lenses project a smaller image circle than many c-mount lenses so you may get vignetting. You can use a c-mount lens on a cs-mount camera by putting a 5mm extension tube between the lens and the camera: it will give infinity focus and will usually work fine in all respects.

Q. Which c-mount lenses can I use on a micro 4/3 camera?

A. With a c-mount to m4/3 adapter you can use many c-mount lenses on a m4/3 camera, but some will not cover the whole sensor while others will not give infinity focus. The 16mm format for which c-mount lenses were originally designed is smaller than the m4/3 format. As a general rule, lenses below 25mm focal length will give severe vignetting while those above will often cover the frame, although there are exceptions to this. Infinity focus depends on the base of the lens being narrow enough to enter fully into the adapter so that the flange surface sits in the right position. See here for more information.

Q. What is an RX lens?

A. An RX lens is a c-mount lens which was produced for use on a Bolex Reflex camera and has been optically optimized for use with the prism that these cameras contain. The flange focal distance is the same as for a standard c-mount lens, but the optical design s slightly different – seethe article “Bolex Reflex (RX) Lenses: The Difference” for more information.

About the Author

jason Jason has been a professional photographer for over 25 years. He has worked in large, medium and 35mm formats in fields ranging from fashion to the travel and leisure industries. In recent years his interests have extended to video and film making as well.