What our eyes interpret is the light reflected off a target 1000 yds away after it travels through 1000 yds of air.
If you have an abundance of light, your eye pupil may choke down to 2 mm.
Under that circumstance, With a 40 mm objective and a 10X ,the optics focus the light to a 4 mm dot . The scope can deliver all you can use with your eye constricted to 2 mm.(in theory.)
"Exit Pupil" is not a washer you are peeking through. Its the "dot" of light the optics focus the light that goes through the objective lens. That light gets bent and focused down to a "Dot" Thats exit pupil.
With a 20 X scope and a 40 mm objective on a bright day when your pupil is 2mm, you still have adequate light.(Scope has a 2mm exit pupil)
If its a little cloudy and your eye pupil is 4 mm, he 2mm exit pupil is a bottleneck and you may not have sufficient light reflected off that 1000 yd target to resolve adequately.
An advantage to a variable is that actually the exit pupil is variable. Decreasing the magnification will increase the exit pupil and give your eye more light. You might see the target better at 12 X than at 20 X
With a variable you can adapt to varying light conditions.
A larger objective will collect more light. At 10x, a 50mm objective will have a 5 mm exit pupil and a 40 mm objective will have a 4 mm exit pupil.
If you do the " pi r squared" math its quite a bit more area/light.
When choosing a fixed power, I would balance the objective size with the magnification to coincide with my "best guess" as to what my eye pupil might measure under the light conditions. "Sunny day" target shooting only might allow more ,magnification. My hunting 6X by 42 mm Leupold has a 7 mm exit pupil. Its as good as it gets for a low light scope.
Note on spotting scopes, a 60 mm objective scope will often be fitted with a 15 x or at most 20 x eyepiece. Generally those will "see" the target best. My 77 mm fluorite crystal objective Kowa gains little to nothing over about 45 X in typical light.
For a fixed 20X I might want a 50 mm objective rifle scope . I'd still need decent daylight to use it effectively.
Spec out your reticle . Find out the MOA width of the stadia. A 1/4 MOA stadia wire will cover a 2.5 in aiming point at 1000 yds. A 1/2 MOA will cover 5 in at 1000 yds. That might matter to you.
