I went to the eye doc today and had him measure my max pupil - it came out 7mm. (probably was 8mm 20 years ago /ubbthreads/images/graemlins/frown.gif ) Using the simple Formula Obj/Power = exit diameter (ED) I calculated the max power I could put my favorite scopes under low light conditions and not suffer blackout. Plugging in my 7mm: Obj/ED = Power <ul type="square"> [*]56/7 = 8 Power [*]50/7 = 7 Power [*]40/7 = 5.7 Power [/list] Now you can't judge a scope simply by Objective Lens, but you can probably compare a Loopie VX-III 40 v. 50, or a NF 50 v. 56 A fun thing to do is put point your scope near a light (arms length away), and turn the power up and down and see the exit pupil dilate/contract. Is it worth the extra money going from 40 to 50 mm on a loopie or 50 to 56 on a NF? In the case of NF I doubt it as most scopes the question is 40 v 50. The total light gathered is limited by the Obj lens is simply pi*(Obj/2)^2 - so light input goes up as the square of Obj lens - <font color="purple">BUT </font> additional height is simply Obj/2 - so going from 50 to 56mm only requires 3mm more height.