Telephoto lenses are expensive and if you have one intended for an older SLR camera, you may consider using it with with your DSLR. My experience is that this is doable but there are a few critical issues that you have to deal with.
Contemporary DSLR cameras are rather unfriendly to older lenses for different reasons such as the lens-sensor distance , the mounting mechanism or even the camera software.
It is important to mount the old lens on the camera at the precise distance without damaging the internal parts..
In this instructable I'll show you how this can be done by a specific example using common tools and materials.
I picked this used and damaged telephoto from a street market. It was a zoom 80-200mm F/4.5, ~400gr weight, with the JCPenney (an American multistore) brand on it and the sign "made in Japan" . Even at its own time it would be inferior compared to those made by Yashica, Nikon or Vivitar with similar specifications.
It was practically separated in two and full of dust, but the external lenses did not seem to have any scratches. Initially I intended to remove the lenses for other uses but looking at it more carefully I realized that the mechanical problems could be fixed.
The challenge was to repair it and mount it on my Olympus E-420.