there has been a lot of discussion and even magazine articles written about using film lenses on digital cameras.
Aside from Manual FOcus vs Auto focus, thich is pretty clear, and whether the lens has an "A" position and aperture controlled by the camera body, there are some other points to consider, which may or may not be relevant to you.
- as already mentioned, newer lenses have ever improving coatings, so the film lenses may not be as good against flare as digital ones.
- Digital lenses do have an additional lens coating on the rear element, to better deal with reflections off the sensor. I personally have not had a big issue with this, but the sensor is much more reflective than film was.
- there have been some reports especially related to wide angle lenses, about excessive vignetting when using film lenses on digital bodies, due to the angle at which light hits the sensor. Digital sensors (and this may be more C-MOS than CCD sensor related, require closer to perpendicular light to hit the sensor. Leica has this issue with their range finder camera, which has the lens much closer to the sensor, aggrivating the problem. Personally I have not seen it, but that only relates to my 43 film lenses, out of the 23million pentax advertised fit the camera
- some film lenses did have shiny parts at the rear of the lens that caused reflections similar to those reported off the rear sensor, I have had this issue as have others in the forum. once identified it can be corrected by flat black brush on paint.
- note that except for pentax's first series of cameras, specifically the *istD, DS and DS2, which supported TTL (through the lens) flash metering, that measured the flash reflected off the sensor for control, as well as P-TTL which measured a preflash using the camera's normal metering system, newer bodies do not support flash metering at all with non A lenses, because the camera needs to know the open aperture of the lens. The preflash is measured knowing the aperture, and predicts the flash duration needed to expose the shot when stopped down to the camera controlled setting, as opposed to actually measuring the flash directly.
- non A lenses show unique and interesting exposure errors as a function of aperture selected, This led to a lot of discussions on the forum over the last 4 years.
on the bright side, there are a lot of excellent film lenses out there, and except for the wide end, actually more variety and options than the current pentax lens line up. Many old lenses are excellent performers when used within their limits (finding these out is part of the fun any way) and there are many exceptional photos posted by forum members using very old lenses. Just look at the Takumar lens club. (Takumars, except the uncoated bayonet lenses of the 1980's went out of production in about 1976)