- Exit pupil The point from which all rays of light appear to emerge from a lens.
- Entrance pupil The point from which rays of light appear to emerge from a lens when it is reversed (turned around).
For compound (multi-element) lenses, the precise location of the pupil is best found by looking at the manufacturers detailed specification (if available), but for practical purposes, it is where the iris appears to be when you look into the lens.
Note that for an asymmetrical lens, the pupil appears to be in a different place depending on which side of the lens you examine, for which reason we talk of `entrance' and `exit' pupils. The entrance pupil is the one you can see from outside the camera.