The situation may be different for Pentax, but I found that--for Olympus 4/3 DSLR--that one can use the film SLR lenses (for which Olympus offers an adapter) but Olympus also has a table that shows the f-range and lens range where the old lenses will work on the DSLR body. I found that after correcting for the digital lens factor (2:1 for my camera) that the useable performance of my better quality old lenses was no better than the inexpensive line of digital lenses. The old lenses aren't recommended for use with the lenses stopped wide open. When doubling the effective lens length, my expensive large-aperture, wide angle lenses are just regular length on the new camera and can't be used at full aperture.
As a result, the only one of my lenses that ended up worth keeping was a 400mm telephoto. It's now an 800mm lens, but I do give up autofocus (it's a manual lens). At least the Olympus's image stabilization in the camera body provides IS with this lens as well. Helpful with that long a lens!