noelex, you totally misread me. I never mentioned the pupil, as in the pupil of the eye, as being a factor in this. What I said was the the EXIT PUPIL of the binocs is all the same, and THAT is stated to be a limiting factor by all the binoc makers. That is, the field of light coming out of the binocs is constrained to a circle 4.2mm in size at the point where is focuses on the retina. (At least, that's how some of them define it.)
The same comments apply to the exit pupil.( not surprisingly as this should coincide with the eyes pupil). All the image goes through all parts of the exit pupil. If we chop off part of the exit pupil we do not loose part of the image, the image just becomes less bright.
The exit pupil is not on the retina. It has nothing to do with the retina. It is is the same position as the eyes pupil (technically slightly incorect, but close enough without confusing the issue)
The exit pupil can be the same size with a very different FOV. We can reduce the exit pupil without effecting the FOV.
If we chop of some of the light in front or behind the exit pupil we will loose some image. This is why its important the position of the binoculars exit pupil ( the eye relief) at the the same position as eyes pupil, otherwise if the eyes pupil is smaller than the light circle parts of the view will be obscured so the FOV will be reduced.
This is why people using glasses need binoculars with a long eye relief. The glasses move the eyes pupil further back and the exit pupil of the binoculars must be also be moved back or portions of the image will be cut off.
The eye relief is the position of the exit pupil of the binoculars and this should match the position of the eyes pupil.