@scottbb Canon EF-S lenses will not physically mount on Canon FF cameras. Many Third party lenses that only project an APS-C sized image circle will mount on EF mount FF cameras, but the image circle does not cover the entire sensor. With some third party APS-C zoom lenses, the image circle grows as the lens is zoomed to longer focal lengths and will cover the entire sensor with part of the zoom range.

@MichaelC That's pretty much what I understood. But I thought there was a "special case", or perhaps a certain mod, where a subclass of (or maybe just one) EF-S lenses could be made to mount on EF bodies (perhaps with a modification, or under certain specific instances). Generally, I understnd that EF-S lenses don't work on EF bodies, but I seem to recall a (or perhaps a few) couple special circumstances (not that those circumstances are recommended, but I thought they existed).

But to be clear, I don't know Canon. I'm going by faint recollection here. And if my recollection is wrong, that's entirely on my end.