SLR manufacturers use different mounts, so you must check the lens you’re buying. But checking the mount doesn’t guarantee compatibility. You also need to ensure it offers the right degree of coverage.
All lenses create a circular image, and the sensor simply records a rectangular portion of this. Some lenses project a large image circle – big enough to cover a full-frame sensor. But when used with a camera with a small sensor, these create a tighter crop – called crop factor.
To create smaller and wider lenses for small-sensor cameras, some lenses create a smaller image circle to begin with, and are thus not compatible with full-frame DSLRs. If they were used on full-frame cameras, the corners of the pictures would be dark.
The infographic below helps illustrate this effect. Simply drag and drop this graphic on to your desktop and you’ll never be confused about crop factor ever again!
Full frame DSLR: do you really need one?