By Wayne Van Zwoll
Iron sights and fixed low-power scopes have gone the way of carbon paper and telephone cords. I doubt younger hunters would lament the passing of these no-frills “dinosaurs,” but they did the job. And there is something to be said for rugged simplicity.
As scopes have improved, they’ve grown bigger, heavier, more complex—and more expensive. While sporting rifles cost five or six times what they did when I bought my first Scope-Chief, the ascent of scope prices has been even steeper. Some now list for more than $2,000; those under $200 are widely considered “entry level.” To pay more for a scope than for the rifle it serves, most shooters want to know how the optic will help them kill game, drill X-rings, or hit steel plates out yonder. That means retailers who sell scopes must speak an evolving language.
Reflection and Refraction
Every scope worth clamping to a rifle has coated lenses. In the 1930s, a Zeiss engineer found that a lens wash of magnesium fluoride (a colorless crystalline compound with a low refractive index) limited reflection and refraction (the bending of light beams passing from one medium to another of a different refractive index). You can lose up to 4 percent of incident light on every uncoated glass-air surface in a scope. Other rare earths affecting specific wave lengths further trim light loss. Fully .multi-coated optics (every lens, several coatings) yield the brightest images. Another treatment protects end lenses from scratches. For distortion-free aim in rain, hydrophobic coating beads water; hydrophilic coating “slips” it.
Fog-proofing matters as much as lens coatings. In 1947, soon after the debut of its 4X Plainsman, Leupold & Stevens tapped a process used on Merchant Marine vessels to prevent fogging in optics. Two years later, Leupold became the first American firm to replace the air in its scope tubes with nitrogen, and market fog-proof scopes. Argon is now used as well.
High resolution helps you distinguish detail. A healthy human eye can resolve about 1 minute of angle in good light; magnification multiplies that level of resolution. ED (extra-low dispersion) lenses have resolution-enhancing compounds. Fluoride glass contains zirconium fluoride. Fluorite, an optical form of the crystal fluorspar (calcium fluoride), has a low refractive index, ranks low in optical dispersion (separation of wave-lengths or colors), and also boosts resolution.
For clear aim, target and reticle images must be crisp. Rotating a scope’s eyepiece focuses the reticle so it appears sharp in the same apparent plane as the target. The European (aka helical or fast-focus) eyepiece is upstaging ocular housings with lock rings.
Oddly, few hunters adjust either type. Here’s a great tip to help your customer do it properly: Loosen the lock ring, if present, and spin the eyepiece out until the reticle appears soft. Point the rifle at the northern sky. Don’t aim at a target because your eye will try to bring it into focus. You want the eye relaxed, so it registers only the reticle. Now turn the eyepiece in until the reticle is crisp. Shut your eyes, then open them to check. Snug the lock ring. You needn’t re-focus the reticle until your eyes change.
In my youth the only scopes adjustable for target focus were varmint and competition models. An AO (adjustable objective) sleeve up front brought the target into focus and eliminated parallax error—the apparent shift of the reticle as your eye moved off the sight’s optical axis. At the target distance for which parallax is corrected, the crosswire stays put even when your eye moves off-axis. You avoid error at other ranges only when your eye is centered behind the scope. Most AO sleeves and, now, the more convenient parallax/focus dial on the left turret face, appear on high-power scopes.
Ironically, parallax can be most problematic at low power, where your eye has a wide exit pupil in which to move off-axis. Scopes without an AO feature are typically parallax-corrected at 100 or 150 yards.
A scope’s erector assembly, so called because its lenses reverse the upside-down image formed by the front glass, is a tube inside a tube. A cam slot in the erector tube of variable scopes moves lenses closer together or farther apart as you rotate the power ring. Windage and elevation adjustments tilt the erector tube. A 30mm scope may have a larger erector assembly than does a 1-inch (25.4mm) scope–or not. Big lenses yield superior resolution. But an erector tube that’s slender relative to the main tube has a greater adjustment range.
Early reticles, made of hair and spiderweb, broke. Windage and elevation adjustments moved them off-center in the field of view. Now glass-etched reticles (engraved or cut by acid on the lens) are replacing suspended reticles. They’re always centered. The front-mounted, first-plane reticle, standard in Europe, is becoming popular stateside in long-range scopes. Its dimensions remain constant relative to the target throughout a variable’s power range, so it serves as a ranging device at every setting. But in hunting scopes at low power, the reticle is hard to see in cover, and when you crank up magnification for a long poke, it’s thicker, hiding small targets. Rear- or second-plane reticles do not “grow and shrink” with the target.
Some reticles have brand-specific names. “Lee Dot” was an early one. Another, often misused to describe a type, is Duplex. That’s a Leupold label. The generic term for this popular design is “plex”—on which you’ll see various alternative prefixes. Range-finding reticles include ladders that bracket targets on the vertical wire. Range-compensating scopes let you hold center far away. The Leatherwood sniper scope was a pioneer in this field. The mount had a cam calibrated for bullet drop. Moving this cam to the proper position for the range, you aimed in the middle.
Surging interest in long-range shooting has birthed specialty scopes for that purpose. The Burris Eliminator has a laser-ranging device you can program with load data to get a lighted aiming point for a center hold at any reasonable range. In tests, I set this scope for a 150-grain load in my SIG 3000—a .308—then read the range from the laser: 395 yards. An orange dot glowed in the reticle’s bottom wire. Dot on the bull’s-eye, I drilled the target just half a minute from center.
High magnification can cost you low-end utility in “three-times” hunting scopes. That’s not “3-power” but the range of power. Think 4–12x or 6–18x. Now there are five-, six-, even eight-times scopes. For scopes you plan to adjust frequently for range, repeatable adjustments are a must. To check a scope’s adjustments after zeroing, I shoot around the square, 20 clicks at a time: first right, then down, then left, then up to my original setting.
Quarter-minute clicks should yield groups 5 inches apart, the last atop the first. Resettable dials let you index dial “shells” to “0” without an internal change after zeroing. A zero stop sets a travel limit on the dial, for a no-look return to “0.”
Physics & Math Made Easy
Magnification, or power, helps us see detail. I re-call when Weaver sold its K4 4X scope as perfect “for most long-range shooting.” Now hunters carry variables with top ends to 20X. The animals haven’t changed; nor has the optical triangle, which shows how magnification limits eye relief and field of view. Eye relief is the distance from the ocular lens to your eye that delivers a full, shadow-free field. Boosting power can reduce ER (as it does field) and make it more critical–and cause earlier “black-out” as your eye moves toward and away from the lens. Intermediate- and long-eye-relief scopes for carbines and pistols have smaller fields than do riflescopes of standard 3 ½-inch ER.
High power also re-duces the diameter of the light beam reaching your eye, or the exit pupil. You get EP by dividing magnification into objective lens diameter. A variable scope with a 40mm objective has a 5mm EP at 8X, a 4mm EP at 10X. Your eye’s pupil dilates to about 6mm in dusky conditions. In such light, a 4mm EP won’t yield as bright a picture as a 6mm EP would. But an EP bigger than your eye’s pupil doesn’t improve the picture.