I'll throw in one caveat that hasn't been mentioned.
IF you own 3rd party lenses... it turns out Canon doesn't publish the specs for the EOS electronics. These 3rd parties have to reverse-engineer the interface in order to build a compatible lens. They tend to test these lenses with whatever Canon bodies are currently on the market at that time.
Just occasionally... Canon will come out with a new EOS body and the 3rd party lens maker (and it's customers, unfortunately) will discover that the lens is no longer compatible with the "new" body. Canon did nothing wrong... they're still following the same specs... but it means the 3rd party's reverse-eningeering job didn't cover all cases and the new camera is taking advantage of some feature that the 3rd party never tested. Some 3rd party lenses an be sent back to their manufacturers to have their electronics reprogrammed or updated for compatible (but not always... sometimes you're just out of luck.)
If you have all Canon glass... you wont have this issue.
As for Canon compatibility...
Canon lenses come in the following flavors:
EF lenses: these lenses will work on EVERY Canon EOS body... regardless of when it was made... film or digital... doesn't matter. They work.
EF-S lenses: these lenses were designed for use on EOS bodies which have APS-C size sensors. The APS-C sensor is a little smaller than the "full frame" sensor (that's a sensor which is roughly the same size as a single frame of 35mm film). Since an APS-C sensor is physically smaller, the EF-S lens is designed to project a smaller image circle into the camera body. This allows the lenses to be smaller and lighter and usually also less expensive... but the trade-off is that they cannot be used on any camera body that has a "full frame" sensor.
EF-M lenses: Canon introduced (and seems to have already abandoned... at least in North America markets) a "mirrorless" camera in the EOS system. This camera was called the EOS-M. You can't buy it anymore. But in any case, the came with just a couple of lenses designed JUST for this particular camera body (but the camera could also use other EOS lenses.) The EF-M lenses can ONLY be used on the EOS-M body.
TS-E lenses: These are specialty lenses which can shift image perspective as well as tilt the focal plane. You can think of them like "EF" lenses in that they can be used on ANY Canon EOS body.
MP-E lens: This is a specialty "macro photo" lens. Canon makes a number of macro lenses. A macro lens allows particularly close focusing distances and most "true" macro lenses will allow you to focus at a close enough distance to provide 1x scale -- meaning the image on the sensor is as large as the subject is in "real life". But with a normal macro lens you can also focus all the way out to infinity. The MP-E 65mm is special... it's "closest" focusing distance is so incredibly close that it can capture subjects at 5x size (instead of the 1x size of other macro lenses). But the trade-off is that the lens does not focus all the way out to "infinity". No other camera company makes a lens like it. The MP-E lens can be used on ANY Canon EOS body.
Tim Campbell
5D III, 5D IV, 60Da
