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.
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. 
First of all, nice website. I’m sure you don’t get tired of hearing that.
One area of “research” you find on the internet is focal reducers to use with M42 lens on C-mount or 4/3. My interest is astronomical, mainly parking a black and white low light CCD to observe meteors. A reducer that would take a 50mm f1.4 M42 and reduce it by a factor of two or three would be great. Thus far I’ve only see examples done with focal reducers for telescope, which clearly don’t make sense for a 50mm fl. If you run into someone working on this, please add it to your news.
Do you want photos of old C-mount lenses, say for a gallery? I have a small collection I got before ebay went crazy with 4/3 fever, doubling the prices.
Thanks for the message and the compliments!!
You bring up something I had not considered – in theory it seems a good idea, giving a wider angle and also increasing the light passing through the lens:
for example, it seems to me logical that a 0.5x focal reducer with a 50mm f1.4 lens would give the equivalent of a 25mm f1.2 lens.
As you say I see some people have experimented this. I expect you have already seen this link, but for others who are interested see here:
http://www.personal-view.com/talks/discussion/577/using-focal-reducers-with-gh12-and-old-lenses/p1
http://www.dvxuser.com/V6/archive/index.php/t-170446.html
The sample shots don’t seem wonderful, but maybe better examples are out there!
If you find out anything more, please feel free to post your findings here.
And yes, I am interested in photos of c-mount lenses – I’ll e-mail you.
Thanks
Jason
I think f1.4 would be F0.5 with a 0.5 correction. [0.707 is one stop; 0.5 is two stops.] The problem is focal reducer has to be designed for a given lens focal length, and telescopes will be at least 400mm at a minimum. You don’t even start to telecompress unless the scope is around 640mm or so. So off the shelf reducers are not going to work well.
I’m not sure if that image has coma, or the effect is due to reflection from the extra lens in the path.
This is a good article on focal reducers:
http://www.rc-astro.com/resources/reducer.html
http://www.dvxuser.com/V6/showthread.php?170446-Focal-Reducers-Squeeze-all-your-FOV-and-Light-on-your-GH1-Sensor/page3
Claim is Stanley Kubrick made a 50mm F0.07 using this trick.
For the 4/3 market, you wouldn’t reduce it as much as for the C-mount market. I don’t have a 4/3 camera, but I just can see why these people like C-mount since they don’t cover the sensor.
Digging through the old C-mounts I got off ebay, I found a Century Tele-Athenar 230mm f3.8. Also an old Zeika Nominar 3 inch f1.8. I cringe when you read the work “rare” next to these lens on ebay since they aren’t all that great. Good glass for their day. Everything else I have is more mundane. Mostly Fujinons. Also Ernitec 12.5mm f1.3. I manage to get a Rainbow f1.0. Really not very good. It is hard to make a sharp f1.0 lens. I got it from a local recycler that couldn’t get it to focus. He didn’t notice the close-up lens on the front.
Thanks for the reply.
I think we are actually both wrong about the f stops. 1 stop more than f1.4 is f1.0, 2 stops more is f0.7. The f stop sequence goes like this:
0.7 1.0 1.4 2.0 2.8 4.0 etc.
However, I only allowed 1/2 stop more light passing through the lens. Actually, since you have the same amount of light falling on 1/4 of the image area you should, in theory, get four times as much light, or two stops.
Thus f1.4 would become f0.7 with the x0.5 reducer. I expect it wouldn’t be quite that much though because you must get some light loss due to the extra glass.
Kubrick’s 0.7/50mm lens is a legend that has spurned many claims. Actually it seems it was a Zeiss 0.7/50mm lens made for NASA. See here articles by Ed DiGiulio, who adapted it and John Alcott, Kubrick’s Director of Photography:
http://www.visual-memory.co.uk/sk/ac/len/page1.htm
http://www.visual-memory.co.uk/sk/2001a/bl/page1.htm
Your Century Tele-Athenar sounds good, the Zeika Nominar as well. I have a full range of Fujinons, which I find to be nice lenses.
Many thanks for spending some time to describe the terminlogy towards the newbies!
Visited your blog through AOL. You already know I will be signing up to your feed.
It’s not the size of the boat, it’s the motion in the ocean.
Where is the facebook like button ?
Hi Nana
Thanks for your message – the first on this site!!
The Facebook like button is now there.