We used to say that a T5 (or any Canon camera that has an APS-C size sensor -- which is what the T5 has) can use ANY Canon EOS lens. However, a couple of years ago, Canon briefly introduced a mirrorless EOS camera and made exactly two lenses for it which has the "EF-M" designation.
SO... other than those two EF-M lenses, the T5 can use ANY Canon EOS lens in the lineup.
The 6D is a different story. It has a "full frame" sensor which is physically larger. It can use any lens that can project an image circle large enough to fill a full-frame sensor. The "EF-S" lenses are designed specifically for cameras with APS-C sensors (a smaller sensor) and those lenses will NOT project an image into the camera large enough to fill the full-frame sensor of a 6D (in fact, the lens mount is designed so you cannot even attach an EF-S lens to a 6D body). And of course it also cannot use those two mirrorless "EF-M" lenses.
In summary:
T5 can use every Canon EOS lens except: EF-M
6D can use every Canon EOS lens except: EF-M & EF-S
And yes, any Canon lens that would fit on and work with a 6D will also fit on and work on a T5, but the reverse is not necessarily true because any EF-S lens that would work on the T5 will not work on the 6D.
As for 3rd party lenses... those lens makers "reverse engineer" the interface to make compatible lenses. Canon does not publish their specs. That means the 3rd party lens maker generally tests the lens until they think they've got everything working. Just occasionally... they don't quite get it right and you find a lens that worked on one camera but doesn't work on another. But usually it's because the generation is different (e.g. a 3rd party lens made to be compatible with all known camera bodies 7 years ago (and worked on everything on the market 7 years ago) might have compatibility problems with cameras made in the last couple of years.
While this 3rd party scenario is not very common... just occasionally it does happen.
Tim Campbell
5D III, 5D IV, 60Da
