C-Mount Lenses, what is to know?

Alexis Vanier · May 22, 2006

Hey there,

I'm surfing and prowling around, looking for my first 16mm cam (spotted the NPR quite a bit). In the process I've came across a couple very inexpensively priced c-mount lenses at VisualProducts. I understand c-mount is threaded and is not the strongest mount. Is there something in particular I should know about c lenses and mounts? What worries me is actually camera vibration loosening the lens from the port and focus slowly crawling out of alignement shot after shot, and such. Also, using physically long telephotos and zooms on c mount could damage the lens, isn't it? Unless c-mount has some kind of locking mechanism. Otherwise, Switar and Schneider are pretty nice as far as I know.

So, really, my question is more like : what's the deal with c mount?

Charles MacDonald · May 23, 2006

I understand c-mount is threaded and is not the strongest mount. Is there something in particular I should know about c lenses and mounts? What worries me is actually camera vibration loosening the lens from the port and focus slowly crawling out of alignement shot after shot, and such. Also, using physically long telephotos and zooms on c mount could damage the lens, isn't it? Unless c-mount has some kind of locking mechanism

If the designer is on the ball, they will alow for the strenth of the mount in deciding what lenses to make. I would be a bit nervous in say putting a 1000 MM telephoto on the camera without some extra mount. BUT I have a six Inch Eglet that fits fine, - I would need a bigger tripod to use it!

My 16mm Camera is a FILMO, it comes with a turet with 3 c-mount lenses. IT is sold as the camera that Uncle sam sent to war. One site claimed that half the TV news footage shot in the american adventure in Veitnam was done with A Filmo... I don't have any idea if that is true, but I have never had a lens come loose with my wanting it to do so. If it us loose, you would notice fairly quick as the lens would turn in the mount rather than actually focus. a quick tighten would be all you would need. There is no lock, but like any thread mount, (think PENTAX M-42) once tightened, the lens tends to stay until loosened.

IN the case of a motorcycle mount or something like that you may want to add checking for tightness, but if the mount is smooth enough that you can take a sharp shot, it probaly does not vibrate enough to loosen the lens.

Here is another pciture-- Do the LugNuts on the tires on your car frequently come loose?

I am not sating that the Bigger lock in mounts are not NICE, they are!

Your best bet of course is to Pick a Camera that you can use, and then find lenes that will fit it. Changing the mount on a lens will cost a lot more than the price of some nice C-mount lenes. (if it is posible in a given case)

Alexis Vanier · May 23, 2006

Yeah, I heard of the M-42 since my still 35 kit is an old K-1000. Yet, I've never even seen one such lens. I've avtually never seen a single screw on lens of my life.

May 23, 2006

I like C-mounts myself, but one big drawback is that they make swapping one lens for another a real chore. With just about any other mount changing lenses can be accomplished quickly, and without worries about whether the lens is seated at the correct depth. (Cameras with a turret compensate for this problem somewhat, but then you've got the turret mechanism adding another layer of complexity to the equation.) C-mount lenses are also (mostly) small, meaning that changing focus or aperture in the middle of a shot is really tough. Finally, with a small C-mount lens, you can actually twist the lens out its mount while trying to focus, if you're not careful!

Robert Hughes · May 24, 2006

One important optical issue with C mount lenses is to know if your camera requires an "RX" lens. Bolex H16 reflex cameras use special C mount lenses with an "RX" designation to show the optics are corrected for the H16 reflex viewing system prism. RX lenses focus differently than standard lenses and will exhibit aspheric distortion on non-RX cameras (and vice versa; non-RX lenses also have aspheric distortion on Bolex RX cameras). The distortion problem is usually noticable only on shorter focal length lenses (less than 50mm) and at wide apertures.

Alexis Vanier · May 25, 2006

Finally, with a small C-mount lens, you can actually twist the lens out its mount while trying to focus, if you're not careful!

That ought to make the use of a follow focus unit relatively impossible.

Any way to fight that? Like welding a rubber mouthed wisegrip claw thingy under the lens port to jam the lens in its position? Anybody's seen such a gadget?