In its long history, Canon has made two completely different kinds of SLRs, F (FD) and EOS. All the autofocus cameras, film and digital, belong to the EOS line, also referred to as AF (autofocus) or EF (electronic focus). The older FD-series manual-focus lenses do not fit EOS cameras at all, not even with an adapter.
Canon also makes a few EF-S lenses ("electronic focus, smaller"?) that fit only the newer DSLRs. They have an EOS-type mount that protrudes slightly farther into the camera body.
If all you need is a T-ring, the situation is simple. Any T-ring marked "Canon EOS," "Canon AF," or "Canon EF" is the right kind for an EOS camera. The other kind, Canon F, FD, or FL, will not fit on the DSLR at all.
Beware of Canon lenses made by Sigma and other third-party manufacturers in the early days of the EOS system. Sigma and some competitors reverse-engineered the EOS aperture system and got it slightly wrong, so the older lenses will not stop down to the selected aperture on a modern DSLR. Instead, you get the error message Err 99. At one time, Sigma could "rechip" older lenses, replacing internal microchips to make them work correctly, but this service is no longer offered. The lenses can be bargains if you are content always to use them wide open.
With Nikon, there is only one kind of SLR lens mount - introduced on the Nikon F in 1959 - but there are many variations.
All Nikon T-rings fit all Nikon SLRs and DSLRs. But when the lens has to communicate its aperture or other information to the camera, the situation is complicated. The old "pre-AI" Nikon F lens mount used a prong on the outside of the aperture ring to connect to the light meter. It was followed by Nikon AI (which may or may not still have the prong), and then Nikon AF for autofocus. Each of these exists in many variations (AI-S, AF-S, AF-D, etc.).
To find out which lenses fit your DSLR, and with what level of functionality, consult your camera's instruction manual. Pre-AI lenses do not, in general, fit modern Nikon cameras at all.
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