The Physics
Healthy young eyes dilate to about 7mm in total darkness. An EP of 6mm, even 5, is all you need on most hunts. It’s easy to get in binoculars and rifle scopes but the high magnification of spotting scopes throttles brightness. A 20×60 spotting scope has a 3mm EP, big enough for glassing on sunny days, but limiting in poor light. An 80mm lens at 20x brings EP to 4mm — but of course adds weight and bulk to your pack.
Big front glass yields higher resolution than do smaller lenses of equal quality. In other words, it reveals more detail, crisply. Resolution helps you ensure a ram’s curl clears the bridge of his nose or the hooks on a pronghorn buck are long enough to offset weak cutters.
Save in very poor light, dialing power above 20x behind 80mm glass increases resolution without costing you brightness — but there’s a limit to usable resolution.
Your unaided eye can distinguish about a minute of angle (60 seconds of arc, or about an inch at 100 yards). Many moons ago a fellow named Rayleigh came up with a constant — 114.3 — which, divided by objective lens diameter in millimeters, yields actual resolution in seconds of angle. Divide 114.3 by 65mm (lens diameter on mid-size scopes) and you get a resolution value of 1.76 seconds. To find the highest power at which this resolution is useful, divide 60 seconds by 1.76. Result: 34x.
More power makes images bigger but not clearer. Larger front glass — say, 80mm — boosts both resolution and highest useful power: 114.3/80mm = 1.43 seconds. Then, 60 seconds/1.43 = 42x.
For example, to identify an obscure detail on a computer image, you may have bumped power above the “point of diminishing return” in resolution. Sometimes bigger is better even if blur increases apace. In the field, however, the little benefit you get from blurry enlargement is offset by magnified mirage and image movement, and reduced exit pupil.
