Entrance pupil

Both lenses are at f/5.6; the longer 250mm focal length of the right-hand lens implies a larger entrance pupil image by Voxphoto (Image rights)

The Entrance pupil of a lens is the diameter of the clear opening which intercepts and focuses light, as seen from the outside. It varies in size along to the aperture opening; but it does not exactly equal the aperture's physical diameter. This is because the aperture's apparent diameter is distorted by the refractive lens elements in front of it.

The F/ratio of a particular aperture is the focal length of the lens divided by the entrance pupil at that aperture.