I am thinking of buying a Canon EF50mmf/1.8STM lens, and was wondering if its compatible for a Canon 600d which has a 58mm dia thread?
The thread size is the thin screw threads for the mounting of filters etc to the front of the lens, rather than mounting the lens to the camera.
The lens will fit, (and is probably a good choice), the way to tell is the mount rather than the thread. Canon digital SLRs use either
EF-S mounts. The EF mount works on both crop frame (like the 600D) and full frame sensors. Lenses made for the EF-S mount are designed specifically for crop frame sensors. While most third-party EF-S lenses will physically mount onto a full frame camera with no problem, Canon EF-S lenses have a tab that physically prevents mounting them onto full frame bodies. Additionally, a few Canon EF-S wide angle zoom lenses will interfere with the larger mirror in the full frame body. For example, see: Is there a way to use Canon 18-200 EF-s lens on Canon 6D full frame body?
Note that there is another current line of Canon lenses:
EF-M, which are specifically designed for Canon M cameras (their mirrorless range). EF-M lenses will not fit the
EF-S mount at all.