Why are more expensive scopes so much brighter at high power?

[Hugnot]

Optical science guys use the term - "numerical aperture" or NA to describe a lens. A lens with a bigger NA has a bigger angle of acceptance, like a cone with a larger diameter as related to cone height or length. This results in more light being gathered in the pointy end of the cone providing better resolution.

Higher priced scopes have better coatings that reduce light reflection (light bouncing off lens surface) and almost always have larger objective lenses and use higher quality glass. The result is superior resolution when cranked up to max powers.

Thru decades of peering thru various optics I think (opinion of one who struggled thru Physics 201 & 202):

Glass quality & coatings matter. Some 40 mm objective lens scopes are noticeably brighter than other 50mm objective lens scopes - both set at the same power.

Scopes with fat tubes like 30mm & 34mm will have internal lenses having larger NA's.

My Vortex Viper 20-60X85 spotter shows accelerated signs of distress (dull fuzzies) at about 45X magnification, at that point the diameter of the light beam (aperture) going into my eye pupil is reduced resulting in increased light diffraction (light spreading) & image degradation.

Given a choice of scope that would fit into almost all of my shooting - a 16X50 with 30 mm tube, highest quality coating & glass. Probably side focus. Mil Rad reticle & adjust. No reliance on springs for scope adjustments.

I got 3 Sightron STAC 3-16X42's that work just fine for almost every occasion - on sale at Midway for $250 - what a deal! Long range rodent shooting is better with the 4-20X50 Sighton STAC scopes.

 

[chutinlead]

I dove down a slightly tangential rabbit hole not too long ago. A young adult say 20 years old with perfect eyes has a pupil that is able to dilate to roughly 8mm in total darkness in a normal state. That value increases with adrenaline.
So if you look at the exit pupil on a scope, say a 80mm spotting scope, the exit pupil is roughly objective size divided by magnification. So for your roughly 40x sweet spot 80mm/40x=2mm exit pupil.
Obviously old eyes don't dilate as well so a 40yo might max out at 6mm. 60yo at 5mm.

 

[Hugnot]

From Wikipedia:

"The Rayleigh criterion shows that the minimum angular spread that can be resolved by an image forming system is limited by diffraction to the ratio of the wavelength of the waves to the aperture width. For this reason, high resolution imaging systems such as astronomical telescopes, long distance telephoto camera lenses and radio telescopes have large apertures."

Small aperture width, more diffraction & image degradation. Light from a tiny little exit pupil of 2mm or smaller will easily fit into any eye pupil.

Diffraction - bending of light waves caused by the aperture becoming a secondary source (propagating) of light waves - light spreading, less intensity. making an increased dull fuzzy image or sometimes an image consisting of concentric rings.