Exit pupil question

[Conksarmy]

Shopping for a high end hunting scope... I understand that beyond glass, brightness is largely a function of the size of the exit pupil. Maybe that isn't true, help me understand...

Assuming that I understand correctly, will a say Swarovski z6i 2.5 -15 x 56 at 15x be brighter than a Swarovski z6i 5-30 x 56 @ 15x or is there some thing that causes it to be less bright/clear?

If weight is comparable and price similar wouldn't the idea be to go 5-30 so if needed you would have the extra zoom in a target or observation situation?

 

[Frog4aday]

Brightness is a function of light transmission thru the lenses. To maximize that each lens surface needs an anti-reflectant coating (like magnesium flouride and such.) Multilple coatings per lens surface are even better.

Exit pupil is a function of objective diameter & magnification. Taking your example of a 56mm obj. @ 15x, both scopes will have the same exit pupil at that setting (3.73mm). Assuming same scope model (z6i) lens coatings will be the same, so brightness will be the same.

I like a scope to have @ least a 3mm exit pupil at the highest magnification just to make it easier to find and see a full image thru the scope. Taking a scope to 30x w/56mm objective, the exit pupil will only be 1.87mm, which will be a bit too small for my taste, but once you get your eye in the right place, you'll still be able to see enough to find your target & mires to make a hit. It'll just be less forgiving of any movement of your eye or the gun.

 

[Conksarmy]

Thank you, that was exactly what I was looking for.