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....