It varies with the make of the camera. For instance, Canon EF lenses for EOS film cameras are usable with EOS DSLRs. However, the reverse is not true.

I have a Nikon SLF (exact model I am currently not sure of) and a Tamron lens.

Depends on which lens you have and whether it is an autofocus lens. If it doesn't have a chip in it, you will have to expose manually. And even if it's an autofocus lens, if it doesn't have an autofocus motor it won't focus with the less expensive Nikon digital bodies.

Tamron lenses are fairly inexpensive. I wouldn't base purchasing a camera based on a single old inexpensive lens. Most of the cameras are available as kits with bundled zoom lenses which will probably be better image quality.

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I have a Nikon SLF (exact model I am currently not sure of) and a Tamron lens.

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It varies with the make of the camera. For instance, Canon EF lenses for EOS film cameras are usable with EOS DSLRs. However, the reverse is not true.

Hang on, Bill, Canon EF lenses can be used with both SLRs and DSLRs, it's the Canon EFS lenses that can be used only with DSLRs (but not SLRs)

For example, I have the USM EF17-40mm L4 lens, and it is very happy sitting on an old EOS50 -no problem.

Yep, that was my point. I just forgot about the "S" at the end of the "EF"....