|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 according 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.