June 12th, 2008

Calculating Scope Resolution Capability

High-magnification scopes allow shooters to aim more precisely and to see their targets better at long ranges. Recent years have seen an “optics arms race” in benchrest shooting. It used to be thought that 36-power was plenty for 100- and 200-yard BR matches. Now, top shooters are running 40- or 45-power optics, and some competitors are even using 50X and 60X scopes.

Perils of Too Much Magnification
Extra magnification comes with a price, however. With equal-sized front objective lenses, the view through a 50-power scope will appear darker than the view through a 35-power optic, and the 50X will have a much smaller exit pupil. The exit pupils on some scopes are under 1mm. An exit pupil that tiny makes your head position ultra-critical — move your head very slightly and you’ll lose the circle of light entirely. Small exit pupils are more fatiguing, causing more eye strain. Higher magnification can, in some conditions, also make it more difficult to cope with mirage. Many shooters prefer a zoom scope so they can dial down the magnification when the mirage is extreme.

Advantages of Variable-Power Scopes
So how much magnification do you really need? That comes down to your personal preferences and your budget. Since a good zoom scope, such as the Nightforce BR 12-42×56 or the new Sightron SIII 8-32×56, lets you “back off” your power, there’s really no reason NOT to go with a high-power scope on a target rifle, provided you can make weight.

A Formula for Scope Resolving Ability
From an optics standpoint, how much power do you need to resolve a bullet hole at a given distance? There is a handy online Scope Resolution Calculator that lets you do this. Just type in the bullet diameter and distance, and the calculator will tell you how much magnification you need, at a MINIMUM, to resolve that particular size bullet hole. This calculator assumes perfect viewing conditions and the highest grade lenses. In reality, as viewing conditions get bad, all bets are off. A bullet hole that could be resolved at 30-power in perfect viewing conditions, might not be resolved even at 80-power when conditions are bad. Nonetheless, it’s fun to play with the calculator, created by Twin City Rod & Gun:

CLICK HERE for Bullet Hole Resolution Calculator

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