[Nouptownd]

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?

 

[dtekhanson]

You are correct that brightness in a hunting scope is largely a function of the size of the exit pupil. The exit pupil is the size of the beam of light that exits the eyepiece of the scope and enters your eye. The larger the exit pupil, the brighter the image will appear.
Exit Pupil in Rifle Scope and How It Affects Your Shooting - Accurateordnance
In your example, both the Swarovski z6i 2.5-15x56 and the Swarovski z6i 5-30x56 have the same size objective lens (56mm), so they should have the same potential brightness. However, at 15x magnification, the exit pupil on the 2.5-15x56 will be larger (3.7mm) than on the 5-30x56 (1.9mm). This means that the 2.5-15x56 will likely appear brighter at 15x magnification than the 5-30x56.

 

[Doom]

In the question relative to 15xon both scopesthe exit pupil is about the same. Exit pupil= obj dia/ power.
Actual brightness is affected by each lens so the actual brightness my differ.

 

[Charles P]

If you need all that magnification for long range target shooting, brightness is not imperative. I you need that magnification for hunting, you are kidding yourself. I hunt from fixed shooting houses with solid rests. About 14X is all I can manage from my Leupold scopes. Anything more is wasted for me. About 10X is adequate for my needs.