Originally posted by SOldBear I bet you a 49-28mm step down adapter ring will work nicely for APS-C DSLRs. In the photo below, the lens attached to the camera is Pentax F 28mm F/2.8:
SOldBear, I tried that with an FA28/2.8, and had pretty heavy vignetting. Not sure if it would work on the A 28mm. I use a 52 --> 37 step down ring on a Sigma 24mm, and that seems to work ok, although there might be the slightest bit of darkening in the corners.
For the OP, you could also look for a hood for a 50mm lens from the film days, that should work.