I have some Olypus lenses that were used with my OM20 camera that I would like to use on my Pentax K-r camera. Is this possible, and what would I need to do it?
Early this year, I saw this on Pentaxforum where someone made a replacement mount ring for his Pentax DSLR to allow mounting of other mount lenses like Nikon, Olympus (your case), Contax/Yashica and even Konica. Quite a feat I say and he actually fabricated some for sale. Below is the original post:
A Google search for "pentax olympus adapter" turns up "Leitax", a provider of replacement mount that you have to screw onto the olympus lens.
You will also need to turn the Pentax camera into a mode where it works with the missing aperture lever that regular Pentax lens provide for measuring. This mode is called stop-down metering. You can read more about this on this pentaxforums page.
Fotodiox offers an adapter (http://www.fotodioxpro.com/selective-35mm-olympus-zuiko-lens-to-pentax-k-camera-lens-mount-adapter.html). I have not tried this particular adapter myself, but I use another adapter from Fotodiox that works fine for me. The Olympus to PK adapter contains corrective optics that might influence the quality.
It is possible, but not ideal. You cannot use a simple ring adapter, because the registration distance for Olympus OM mount is smaller than that of Pentax K. And as you can't simple jam the lens back into the camera body the required distance, if you use a simple ring adapter, and have the lens sitting too far forward, it's like using a macro extension tube--you lose the ability to focus to infinity.
The only adapters that can compensate for this distance difference will either have a glass element to act like a short teleconverter (which will decrease the max. aperture, increase the focal length, and, if it's cheap, probably add softness), or you need to shave off the difference from the lens mount somehow (e.g., a Leitax mount replacement kit).
Understand, too, that you will not have autofocus, you will not have communication between the camera body and the lens. You will not be able to control the lens's aperture from the camera body, so you have to use stop-down metering, you can only shoot in M and A modes, and lens EXIF information will be missing (focal length, lens, aperture, etc.) There is also the fact that modern dSLR focus screens are not ideal for manual focusing, as they no longer have the prism color or split-circle aids we were used to from film SLRs, and dSLR crop body viewfinders tend to be smaller and dimmer than the viewfinders in film SLRs (the mirror is no longer 100% effective, since some of the light has to be diverted to the AF sensor array).