Today there is an enormous range of affordable M42 or Pentax screw mount adapters available. Here is a Nikon Ai example under $10.Using an M42 adapter you can adapt any ol' screw mount lens to fit many 35mm, digital SLR or even a micro four thirds cameras.
Such adapters are an excellent way to breathe some life back into older lenses.
However try before you buy,
as not all M42 adapters are created the same!
Illustrated is my favorite Fujinon 16mm fish eye lens mounted on a Canon 35mm SLR camera using two different M42 to EOS adapters.
The screw thread on the adapter on the right stops 45° before the lens aligns with the vertical. (Click on the picture for a closer look)
This is not an issue with a crop factor camera, but the lens becomes unusable on a full frame camera. The lens hood intrudes into the picture frame :(
I guess I'll relegate that to adapter to the bottom drawer.
