Re: 40mm, or 50mm scope?
Given equal quality of glass and same magnification the 50mm allows a larger circle of light transmission to the pupil of the eye. Known as "exit pupil".
Exit pupil in mm = diameter of the objective lens divided by the magnification
With a 9x50 scope the exit pupil is 50/9 = 5.6 mm (more light),
compared to a 9x40, 40/9 = 4.4 mm (less light).
There is a practical and functional limit to this advantage. When the exit pupil size exceeds the actual diameter of the pupil the eye cannot use the additional area. So somewhere around an exit pupil of 7 you will achieve the maximum amount of light possible.
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