"The human eye can only open up to somewhere between 5-7mm depeding on the person. A 4X32 scope will let in 8mm of light. More than the eye can use and is a waste. A 40mm scope set at 4X will let in 10mm of light. Yes it is more, but since the human eye cannot use anymore than 5-7mm it is wasted. A 40mm scope set on 8X will let in 5mm. That is about all the average eye can use and is ideal. The only time a 40mm scope will let in less light than the human eye can use is when set at powers greater than 8X."
The light that falls outside of your eye is wasted, but this ignores 2 very important things that impact your use of the scope. First, if the exit pupil is exactly the same size as your pupil, then you have to line your eye up in exactly the perfect position to see the full field of view. Having extra diameter allows you to move slightly and still see. Second, that extra light gathered by the larger objective lens allows for a higher resolution. I, for one, appreciate the extra resolution and easier use, especially when it's getting darker and harder to see.
The light that falls outside of your eye is wasted, but this ignores 2 very important things that impact your use of the scope. First, if the exit pupil is exactly the same size as your pupil, then you have to line your eye up in exactly the perfect position to see the full field of view. Having extra diameter allows you to move slightly and still see. Second, that extra light gathered by the larger objective lens allows for a higher resolution. I, for one, appreciate the extra resolution and easier use, especially when it's getting darker and harder to see.
