I've recently played with a few c-mount lenses. The Pentax 6mm f/1.2, Pentax 8-48mm f/1.0, and a Computar 16mm f/1.4, but these are not "cheap" lenses at all (for c-mount; most retail for $150-250).
First, the "super-fast" apertures of some of these are only there for low-light video surveillance, and the still image quality when wide-open could be generously called "soft-focus." What this means is that everything is soft and hazy, and the bokeh is horrendous. (The 01 Prime has better bokeh than any of these "made-for-cctv" lenses!) Stop them down to f/2.8 and they are pretty good, but still nowhere near the 01 Prime, at f/1.9, in terms of sharpness.
Speaking of sharpness, the focus-peaking and MF magnification on the Q helps out a lot when focusing its native lenses, but the "soft-focus" effect of my wider aperture c-mounts makes it really hard to judge what is in focus, and the peaking just becomes a vague twinkling on some pixels, not an outline! So, I stop these lenses down, whereupon they hold almost no advantage over something like the 02 Zoom (not faster, not sharper, not easier to focus)... which is why I've ordered an 02 Zoom!
Another issue is distortion. All of these lenses have pretty bad barrel distortion, and because they are adapted, the in-camera distortion correction can do nothing about it. Photoshop can correct this to some degree, but the results still look a bit odd. So they're not for architectural or interior photos! If you turn off distortion correction on the Prime 01, you'll also see a pretty significant barrel effect, so this might just be a downside for all short focal-length lenses likes these (note that my 6mm is ~35mm equiv., and the 16mm is ~85mm equiv., and "regular format lenses" at those focal lengths will have little or no distortion).
Next, be aware of the two types of c-mount adapters out there. One is expensive and made from a single piece of metal (usually around $35-40), and the more common one (~$7) is a "two-piece" design with little screws around the outside holding the interior threaded c-mount into the Q-mount adapter. Mine is a cheaper one, and the little screws come loose and the c-mount lens just twirls inside the adapter without unscrewing (or sometimes not properly screwing all the way in)! Plus, the screw heads are tiny-hex, so I now have a nail that I've filed down to specifically tighten these things...
In conclusion, c-mount lenses are "fun" because they can be very small, cheap, and some very fast... but don't expect to do better than Pentax has with the native lenses. Follow
drougge's advice above, and go for the $20 no-name lenses! The worst of my bunch was the most expensive (the Pentax/Cosmicar 8-48mm, @$200 used); relatively big and heavy (think FA 31mm), uselessly soft/hazy at wider apertures, with an enormous zoom range that is really hard to control when hand-held. I'm selling it on craigslist so no one here has to find this out the hard way...
FYI: all of the above is
my own opinion based on limited experience. Other opinions can be equally valid, but if this helped you, press the "like" button.
Last edited by panoguy; 11-28-2012 at 08:02 AM.