C-Mount Lenses for mirrorless micro 4/3 and APS-C?

C-mount lenses were used on many movie film cameras and are still commonly used on video, security, and industrial digital cameras. Designed for a short flange distance and to cover a small image format, C-mount lenses often are physically tiny, let lots of light in, and can be found at very attractive prices.

Are C-mount lenses a viable alternative for use on relatively large-sensor compact mirrorless camera bodies using Olympus/Panasonic micro 4/3 , Samsung NX , and Sony NEX E-mount ? There has been quite a run on C-mount lenses, especially fast ones around 25mm focal length, because many people expect them to be the micro 4/3 equivalent of a "fast fifty" normal lens -- but there are issues. The goal of this Instructable is to help you know what to expect from C-mount lenses before you buy them....

The photo is an uncropped self-portrait (reflection in a mirror) of my Sony NEX-5 with a 12mm f/1.4 C-mount lens attached.

Update: the Pentax Q was announced on June 22, 2011 -- see the new "appendix" in step 6.

Step 1 You'll need an adapter

C-mount lenses  have a 1-inch-diameter thread with 32 threads per inch. They are designed to have a distance of 0.69 inches (17.5mm) between the mounting flange and the film/sensor surface. There also is an issue with the "near thread" diameter of the lens barrel. Lenses vary, but 37.2mm of clear diameter at the flange surface seems to be a common design goal and narrower clear spaces might prevent some lenses from being fully screwed-in. Of course, adapted lenses will generally be manual focus and either completely manual or aperture-priority exposure with stop-down metering.

Of the cameras discussed here, the Sony NEX E-mount has the shortest flange distance at just 18mm (very different from the 44.5mm of Sony's A-mount DSLRs). However, that means the C-mount flange must sit 0.5mm recessed within the throat of the E-mount bayonet to allow the full focus range. Fortunately, the bayonet is more than wide enough, and adapters allowing infinity focus can be had for under $20 on eBay. The photo shows the Sony NEX 18-55mm kit lens and a 12mm C-mount lens in a C-NEX adapter... tiny, isn't it?

The micro 4/3 standard mount, used by both Olympus and Panasonic, has a longer 20mm flange distance and a nearly 8mm narrower throat than the Sony mount, so things are a bit more cramped near the back of a mounted lens. However, infinity-focus C-M4/3 adapters are more common and cost even less than C-NEX adapters.

The Samsung NX mount flange distance is 25.5mm, so the C-mount thread would need to be deep inside the bayonet. The bayonet is theoretically wide enough, but many C-mount lenses have controls (i.e., aperture or focus) very near the mounting thread, and the deep recessing could make these controls inoperable. Adapters are not easy to find.

Lens Variants That Are Problematic To Adapt

CS mount lenses use the same thread as C-mount lenses, but expect flange distance of around 12.5mm. D mount also is similar, but with a narrower throat and 12.3mm flange distance. Thus, CS and D mount lenses would require a deeply recessed adapter to be able to focus to infinity... and I've never seen such an adapter. Most modern small-sensor cameras seem to be CS mount, which can use C-mount lenses using a 5mm extension tube, so be aware that many newer lenses are CS mount even if some other lenses being sold for the same camera are C mount; you have to read the fine print. You can use CS mount lenses on a C-mount body, but only for macro shots. Also, make sure there isn't a 5mm CS adapter tube on the back of a C-mount lens when you're using it.....

The other issue that comes up frequently is auto-iris. Basically, such lenses have a little cable that carries either a DC or Video signal to automatically control the aperture. There usually isn't a manual aperture control on such a lens. The DC interface is not supported by any of the larger-sensor bodies we're talking about in this Instructable. In theory, the lenses controlled by a video signal might be able to be driven by a conventional video output from one of these cameras, but that's not something I have tried nor would I recommend risking it. Motorized focus or motorized zoom lenses have similar issues in needing external power. It is probably best to avoid lenses that have a cable of any kind attached....

Comments

[n6mod]

You say that C-mount to NX adapters are hard to find. Have you found any?

I have a Canon TV-16 25mm f/0.78 that I'm willing to buy and hack an inexpensive a camera for, but it reaches *way* back into the body. I tried a NEX-3 and the shutter is too far forward. Looking at a camera shop, the NX seems to have the shutter very close to the sensor, but I can't find a C adapter at all.

Any pointers would be appreciated.

[ProfHankD]

C adapters are problematic for NX because most lenses will have either their focus and/or aperture ring blocked by any adapter that is recessed enough (8mm behind the flange). Thus, there is no market for "generic" C-to-NX adapters and I have never seen one for sale.

There are alternatives if you are sure that sitting 8mm back in the NX mount PLUS "*way* back" beyond that will still clear both the mount flange and the sensor (which I doubt) and don't mind a little mount hacking. There are NX reverse mounting rings available for about $12. Measure the diameter of the section of your lens that would be at the level of the flange when the lens is held at the right depth; call that Xmm. If you buy an NX reverse ring that is slightly wider than Xmm, there's a good chance you could hack that into a workable mount by using glue to fix the lens (permanently) to the adapter. Even if that meant the focus ring was glued to the adapter, focus might still be possible by turning the lens relative to the adapter. Not pretty... but might work.

If you don't mind getting another body (and I can't imagine this lens covering an APS-C sensor anyway), the easy answer is to go with a micro4/3, Nikon 1, or Pentax Q body and C-mount adapter... if you're sure they'll clear. ;)

[n6mod]

I'm not sure what you mean by clearing the sensor... any lens will clear the sensor.

Clearing the shutter is a different matter entirely, and is the problem with this lens. At infinity, there is just a little over 5mm between the rear element and the image plane.

You're right that the lens doesn't cover APS-C, but that's not a huge concern to me, I'll happily crop the usable portion of the image out of the file.

I have a ton of m4/3 gear and the issue here is that the portion that protrudes beyond the mount is larger in diameter than the "mirror box" on the camera.

I'm going to go measure the shutter location on my E-P2 again, though... with E-P1's going so cheap right now, it might be worth grabbing one and doing some more invasive hacking.

Nikon 1 is interesting, but I think suffers from the same issue with the area around the sensor.

I'm sure the Q would clear, but that sensor is such a tiny portion of the image on from this lens that I think the image quality would be pretty disappointing. It would be a 135mm equivalent as well... a bit long. :D

[n6mod]

A bit more info. The E-P1 shutter is about 10mm below the mount, which puts it about 9mm above the sensor. So that won't work.

Thinking harder about Nikon 1, especially since the shutter is optional. I'll have to see if I can get my hands on one to take some measurements. And I'll probably wait for them to start showing up on the used market. I'm fine with $200-300 for this project, $500-$700 is a bit rich. :)

[ProfHankD]

When I said clearing the sensor, I meant and associated stuff, i.e., the filter stack, shutter, and any other associated obstructions. Sony is rumored to have a NEX model coming that doesn't use a mechanical shutter -- and no doubt there will be mirrorless cameras like that because the shutter is a bit of an electromechanical nightmare (and significant cost).

Just 5mm between lens rear and image plane is really tight... I bet coverage is tiny too. It is likely that your lens was designed for 1/3" sensors. The Pentax Q sensor is 1/2.3" -- 28% larger diagonal than the 6mm of a 1/3" sensor, or about 7.7mm. Of course, there's a huge difference between "high-quality coverage" and "it doesn't vignette to black." ;) Anyway, the Pentax Q has a 9.2mm flange distance, so I'm not so sure you could get infinity focus even on that.

There are C-mount cameras that can accommodate a 5mm rear focus, but most, including both I have sitting in my office, are designed for tethered operation (security/machine vision). Your best bet for a portable camera would probably be to hack a camera that had a "dumb" lens, e.g., like the $15 one I hacked in http://aggregate.org/DIT/PEEPFISH/ . Unfortunately, IQ on those cheap cameras is terrible, they are usually 1.2MP or less, the sensors are tiny, there are few if any controls for exposure, etc.

[avatar_i]

O.K., you got me again!
I have several old, FAST, variable long C-mount lenses that are left over from my Paillard-Bolex 16mm days.
I have tried for years to try to put one on an SLR body. Now it looks like I need to buy yeat another camera just to be able to use another obsolete lens for what it was not designed to be useed for.
HURRAY!

[ProfHankD]

Since you have the lenses now, I'd suggest checking their image circle diameters by viewing their infinity-focused image on a sheet of paper. That should give you a better idea of how they'll work with whatever body you would buy to mount them on....

[avatar_i]

Oh, that's not the problem-
The problem is that I have too many things I am using for what they were not intended to be used for to begin with, and now I need to figure out another way to get another thing to do something else it was not intended to do on somethig it was never intended to be mounted on.
Simple, no? Uh, no.
But- fun... maybe?
Hmmm. I confuse myself sometimes.

[ProfHankD]

In May 2011, I hosted the "C-mount Scenes" challenge at dpreview, asking folks to enter images that demonstrate that C-mount lenses can make impressive images using micro 4/3 or NEX bodies. There was only 1 entry and voting didn't give it a very high score....