The difference between a 40 mm and a 50 mm scope at dawn or dusk is about 1X of scope range..

In other words if you could use a 40mm scope at 6X at dusk, then you could use a 50mm scope at 7X.

You also have to understand that in any decent light, there is no difference. This will only make a difference in very low light situations, such as the first or last 15 - 20 minutes of the shooting day.

Hope this explains it for you.

MET