In reality, the average human eye can only use ~5mm of exit pupil. Divide the objective lens diameter by the power, and you will get the effective exit pupil.
So, a set of 8x40's will give you a 5mm exit pupil. A set of 10x50's will give you the same exit pupil, but you have the advantage of the higher power.
This is assuming, of course, that you have modern, fully coated lenses.
Ed
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