The average human eye pupil only expands to about 5mm. So to keep clear sight thru a scope or binos in low light you have to have a 5mm exit pupil. That's how big the image is thru scope at proper eye relief. So with a 40mm scope,all you have to do is make sure it's turned down a tad so that it's under 8x or so to ensure maximum brightness for that scope. A 50mm isn't a bit brighter until you go above 8x.