Introduction: C-Mount Lenses for Mirrorless Micro 4/3 and APS-C?

By Prof. Hank Dietz
About: I'm an Electrical and Computer Engineering Professor at the University of Kentucky. I'm probably best known for things I've done involving Linux PC cluster supercomputing; I built the world's first back in Fe…

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....

Step 2: I've Got You Covered?

After physical mounting, the most obvious issue in using C-mount lenses on these cameras is that the cameras have much larger sensors than the lenses were designed for.

Micro 4/3 cameras use a 17.3 by 13mm sensor. That's an over 21mm diagonal, whereas the "Super 16mm" movie format many of the older C-mount lenses were designed for only has a 14.5mm diagonal. Newer C-mount lenses are commonly designed for a "1/2 inch" sensor that has an 8mm diagonal. Corners will usually be dark and image quality is often relatively poor outside the designed diameter even if the corners aren't dark. Swirly bokeh are generally caused by vignetting within the lens, so don't be surprised if you see that too.

Sony NEX cameras use a 23.4 by 15.6mm sensor -- pretty much the same size found in any other APS-C camera, with a diagonal around 28mm.  As a result, most C-mount lenses will vignette horribly, producing a circular image in the middle of the sensor. Note that the image is not a fisheye view, it's just vignetted to a circle.

Ok, the image captured might only be a circle, but we can still crop that to a rectangular image. The question is how many pixels are left? The image circle for the 12 mm Pentax lens used for the intro's self-portrait held just under 5MP out of the Sony NEX-5's 14.2MP sensor. The photo shown in this step was taken with a Tokina zoom that gave about a 6.5MP circle.


Step 3: Be It Resolved?

Another little detail about using C-mount lenses is that lenses are designed to match the expected resolution of the film/sensor. For example, nobody expects "8mm" movie film to resolve 24MP, so there is no point in sacrificing other desirable properties to get that kind of resolution projected onto the film by the lens.

The sad reality is that the cameras that most C-mount lenses were designed for have tiny film/sensors that resolve fewer than a million pixels. In the industry, a common distinction is between the regular lenses and those designed for "megapixel" sensors. Top-of-the-line C-mount lenses rarely target more than 5MP sensor resolution. Stopping these lenses down also hits the diffraction limit quite quickly, further compromising resolution at high f/numbers.

So, for adapted C-mount lenses, are we talking about circular vignetted images that are entirely blurry? Absolutely not! Lens resolution is related to pixel density, and our relatively large sensors don't have anywhere near 5MP in an area as small as these lenses were designed for. APS-C is about 12X the area of "1/2 inch" sensors, so a 12MP APS-C has the same pixel density as a 1MP "1/2 inch" sensor. Thus, the best C-mount lenses are designed to project significantly finer detail than our sensor's relatively large pixels can record... at least for the central portion of the image.

For example, the Tokina lens used on my NEX-5 to capture the photo in the previous step  barely resolves well enough for 640x480 full-frame images on a "1/4 inch" native C-mount firewire camera (Unibrain Fire-i 400 industrial camera ). However, the unprocessed and off-center 640x480 crop shown here looks at least as good -- the NEX-5 has a much better sensor with slightly lower pixel density. As long as the image circle is big enough, resolution of most C-mount lenses is probably ok.

Step 4: A Little List of C-mount Lenses

You can find information about many C-mount lenses on the WWW. There are quite a few different sensor sizes targeted by C-mount lenses; http://en.wikipedia.org/wiki/Image_sensor_format  lists dimensions of many of the more common sensor sizes. However, most lenses will cover (with some image quality issues) an image circle diameter significantly larger than the diagonal of the targeted sensor size... and that diameter is rarely listed anywhere.  One exception is https://spreadsheets.google.com/pub?key=p9kkgjwEQQQ-HJwvNDobeEw , which is a pretty good list.

I've also started a (very) little list here, and contributions are welcome. Each entry is: lens description; approximate image circle diameter(s) measured in mm at infinity focus (where possible); comments. Keep in mind that your copy of any of these lenses could have significantly different properties from mine.

Tokina 6--15mm f/1.4http://www.tokina-usa.com/pdf/tvr0614.pdf ; 8.5mm--15mm; zoom changes image circle diameter so much that view angle after cropping is barely affected

Pentax/Cosmicar 8.5mm f/1.5 ; 12mm; good quality, fairly wide view angle even after cropping

Pentax/Cosmicar 12mm f/1.4 ; 11mm; a nice and impressively tiny lens

Toyo Optics 12.5--75mm f/1.8 ; ? --15mm; not a small lens and unusably soft at wide setting

Melles Griot microscope objective, 160/0.17 4/0.12 ; 26mm; fixed macro (no focusing) that nearly covers APS-C

The photo with this step shows these lenses in left-to-right order, with the Sony NEX 18-55mm f/3.5-5.6 kit lens on the left edge as a size comparison.  Although all the C-mount lenses have much larger apertures than the kit lens, only the 12.5-75mm zoom is of comparable volume to the kit lens.

Step 5: Conclusions

So, are C-mount lenses worth dealing with for micro 4/3 and Sony NEX E-mount?

You can get a very fast, tiny, used lens and C-mount adapter for well under $100, and possibly under $40.  The images you capture probably will not use the full resolution of your camera. However, you will be able to take advantage of the low-light performance of your camera's large pixels, and many of these C-mount lenses have very large apertures (f/numbers commonly between 1.2 and 1.8). There is also more depth-of-field thanks to the shorter focal lengths, so focus is less critical. In summary, they'll let you discretely take photos even when it's really dark , and that's pretty cool. A well-exposed and sharp 3MP photo (such as the one here, shot with the 12mm at f/1.4) easily beats a dark and blurry 14MP one.

For longer focal lengths, the physical size difference is less, and it is harder to justify use of C-mount lenses. A wide selection of used lenses are cheaply available in a variety of easily adapted full-frame 35mm SLR mounts (M42, Canon FL/FD, Minolta SR/MC/MD, etc.). These SLR lenses generally outperform C-mount lenses with similar cropped viewing angles. This is especially true of f/1.4-f/2.0 ~50mm lenses. Fast shorter focal length SLR lenses are harder to find and usually more expensive, but I have a nice 24mm f/2.0 that cost me only $40. Incidentally, fast rangefinder lenses also typically are better than C-mounts, but are rarely competitive on price.

The only other reason to use a C-mount lens is that you artistically want the defects it introduces . Vignetting is trivially simulated in post-processing, but it can be very difficult to simulate the swirly bokeh and other artifacts caused by using portions of the image circle that are not well corrected. This is why people get into "Lomography."

Step 6: Appendix: the Pentax Q

The Pentax Q interchangeable-lens digital camera has been announced as using a tiny sensor (1/2.3"). It really isn't the kind of large-sensor camera this Instructable is about, but logically it is a digital camera well-suited to C-mount lenses. Here are a few quick comments about that, with the caveat that I have not yet been able to test a Q body myself.

The sensor diagonal is only about 7.7mm and the flange distance a mere 9.2mm. Thus, with an adapter, it should be able to use just about any C-mount or CS-mount lens... and Pentax has made a lot of those. Except for a few lenses designed for 1/3" sensors (6mm diagonal), vignetting should not be an issue. The tiny sensor's ~5.6X crop factor also makes some C-mount lenses become tiny, yet super-fast, telephotos -- a 25mm f/1.4 has the field of view of ~135mm, while still f/1.4. For that matter, my old Canon FL 55mm f/1.2 would act somewhat like a 300mm f/1.2! Also, sensor-shift shake reduction should help steady photos with these "long" lenses.

The question is really how much resolution re-purposed lenses can deliver over such a small sensor area.  My guess is that many will look pretty good for 1080p video, which is only ~2MP.  Very few old lenses will be able to match the 12.4MP still resolution of this camera's sensor, but will people really care? Image quality, including resolution, could be better with many of these lenses than what people accepted from professional DSLRs a decade ago....  Note that Pentax's WWW store classes Q as a "Digital SLR Camera" rather than a "Digital Compact Camera."