This is the apparent opening that we see as humans when we look towards lens from the object side.
Light of _each_ object point in the field of view of the lens reaches _each_ point on the front lens of the objective, but only the light directed to the entrance pupil of the lens reaches the matching image point.
Rays from the object point directed to the center of the entrance pupil are called “chief ray”.
The example above is over emphasized. Here a more realistic example with a front iris, which also acts as a entrance pupil:
equal area lenses are a type of fisheye lenses
Equal area lenses use a image mapping function of the type
such lenses maintain areas. Objects that have the same area have images of the same area.
[table caption=”typical angles: ” width=”500″ colwidth=”50|50|50|50″ colalign=”center|center|center|center”]