Originally the C-mount was no affair concerning still cameras. The one inch screw-mount was one for cine lenses, used on 8mm and 16mm cine cameras, surveillance cameras and TV-cameras. Old sophisticated cine cameras had a revolver with several one inch threads to hold a whole set of C-mount lenses. After the invention of the zoom lens the most cine cameras lost this revolver feature.
Cine cameras need fast lenses because of shooting 18 or more images per second. That's the reason for a completely different culture behind the large variety of cine lenses. Thus f1.2, f1.4, f1.5, f1.8 and so on are typical standard lens speeds of these items, whilst f1.7, f1.8 and f2.8 are the most common standard lens speeds of still cameras. Even cine zoom lenses have speeds beginning with f1.2, f2, f2.4, while most still camera zoom lenses start with lens speed f3.5 .
|Schneider-Kreuznach 1:1.5 25mm Xenon on Micro 4/3 adapter|
image by mr. wood (Image rights)
Since Olympus and Panasonic had introduced the first cameras for the Micro Four Thirds autofocus lenses there are still cameras in the market with a flange focus distance almost like that of the C-mount lenses. That was the first chance for photo amateurs to think about using C-mount cine and CCTV lenses on still cameras. Soon the appropriate adapters were available, even cheap ones made in China. When Sony introduced the concurrent Sony NEX camera system the adapter makers offered C-mount adapters for the NEX too.
The original reason for that c-mount lens hype of still camera users is that especially sophisticated cine cameras were often equipped with superb optics made by great old lens makers like Taylor-Hobson, Angénieux, Cooke, Elgeet, Berthiot, Kowa, Canon and Schneider-Kreuznach. Pentax Cosmicar lenses are partially even available as new items, as well as simpler so-called "CCTV" lenses for surveillance cameras. Customers must care not to buy the cheaper CS-mount lenses instead. These are only usable for a newer generation of surveillance cameras, and definitely not on the C-mount adapters for still cameras.
The quality of C-mount lenses differs much. There are sophisticated lenses, once used on luxurious Beaulieu cine cameras, and there are simple lenses without aperture selector. Thus the lenses with limited settings are good for experimental photography while well-preserved cine lenses of higher quality will be full-scale photography lenses but with cine characteristics when used on a still camera. Nowadays there are much more C-mount adapters available than good old C-mount cine lenses. That's because of the CCTV lens having been discovered as new kind of camera toy. Many people like to experiment with these simple lenses on their digital cameras like others do with pinhole "lenses" even on sophisticated SLRs.