Joe Christofori, Camera Assistant, Data Wrangler and Artist from Massachusetts sent this chart in. It’s a good, graphic explanation of why your Canon 7D (EF-S) lenses won’t cover a 5D (EF lenses), and what happens when you put 5D lenses on your 7D.
When you mount an EF lens on a Canon 7D, you’ll notice that the red lens mount index marks line up. When you use an EF-S lens, the white index marks “remind you” that you’re using APS-C format optics.
What happens if you try to mount an EF-S (APS-C) lens onto a full frame camera like the 5D? Notice that the rear of an EF-S lens is protected by a rubber bumper ring. Since EF-S lenses usually protrude into the lens cavity (much like 16mm format PL lenses), they might scratch or break the mirror. The rubber bumper helps protect the mirror.
If Ron Dexter was your teacher and your mastery of bandsaw upon lens skills enabled removal of the rear bumper, you would most likely find what Joe shows us above – the image of a 5D would vignette when using EF-S lenses. Joe’s chart is a much cheaper and faster way of learning this lesson.