I know this is the case but I would like to know the scientific reason behind it. Where [and how] does the light-loss occur?
And is there a formula for brightness loss / magnification?
Be gentle, as I suspect there is a very obvious answer staring me in the face.......although I enjoy the explanations here more than Wiki!
Brightness is reduced as magnification increases.
It's magnification (not the size of the exit pupil as most people will say) that controls the brightness of an image if, as is most often the case in daylight, your eye's entrance pupil is smaller than the exit pupil of the bin.
Why? The eyepiece spreads the same amount of light over a larger angle with higher magnifications (so there is "less to go around" on each spot of the image).
Brightness changes as the inverse square of the magnification (as you magnify by the same amount in two directions.)
Just like a light source gets four times less bright when you go double the viewing distance to the source.