I don't really know the canon range, but I think its an issue with auto focus/aperture: Older lenses doesn't have internal focus motors, and were driven by the body. Newer lenses typically all have focus motors, so the bodies (especially lower spec/budget bodies) no londer have focus motors. If you then put an older lense on one of these newer bodies, they don't play nice. You can try putting the body in manual focus, it may solve the problem. The second issue is diaphragm control - newer lenses have electronic control (with no aperture ring), so the body doesn't have a control pin. Older lenses are controlled mechanically by the body. Check canon's website for conpatibility tables.
