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Optical Illusions

By Ron Spomer

Optical Illusions
Note the extensive eye relief of Bushnell’s Elite 3200 2-6x handgun scope on a T/C Encore.

Not everything you know about scopes is wrong, but I’ll bet some is. Over the years, hunters have been sharing inaccurate “conventional wisdom” about scopes, manufacturers have been telling little white lies and throwing up smokescreens, and writers (like me) have been dispensing half-baked concepts, too.

It’s hard to understand scopes because most of what makes them function or fail is hidden inside. Half of what they do is in the realm of rocket science.

Did you know that everything you see through a scope is traveling at the speed of light? And did you know you do not actually see deer, elk, coyotes and other targets in your scope? No, all you see are light rays (or waves or particles or photons; science still doesn’t know exactly what light is) bouncing off the objects.

And did you know the images you see in a scope are upside down? Everything we see is projected onto our retinas upside down. Our brains flip it over so it makes sense. This is doubly ironic because a scope initially turns what it “sees” upside down. Manufacturers have to put in special erector lenses to flip the image over again so it can be transmitted to our retinas upside down so our brains can invert it right side up again.

Optical Illusions

And you thought you knew something about scopes.

Okay, maybe you do know something. Quite likely, you know more than I do. But humor me and check out some of the following scope myths and misconceptions. I uncovered them after interviewing optical engineers, designers and PR specialists at Bushnell, Kahles, Leupold, Swarovski, Pentax, Weaver and Burris.

Brightness is the most important criterion in a scope.

Sure, a bright image is useful, but not if the crosshairs don’t stay where you set them or the tube leaks and fogs. I’d rather have a rugged, consistent scope that transmitted 80 percent of the light entering it than a fragile one that transmitted 100 percent of light. Fortunately, most of today’s scopes are durable, consistent and bright, but if you want a rugged, dependable scope with maximum light transmission, buy a top-end model.

A bright scope must have a huge objective lens.

Wrong again. Yes, a large objective is crucial for letting lots of light into any telescope, but more critical is how that light is handled once inside. Without flare control, all the light in the world merely makes a bright glare. In truth, lens coatings do more to create a bright, clearly defined image than do huge objectives. How? Micro-thin coatings of magnesium salts and other elements reduce reflection rates from glass surfaces. Instead of 4 or 5 percent of the light bouncing off each air-to-glass surface, as it does from raw glass, as high as 99.8 percent passes through multicoated lens surfaces. An adequately large objective is still important, but for maximum light transmission in any scope, you need multicoating on every air-to-glass lens surface, inside and out.

Some scopes transmit 99 percent of light.

This is one of those advertising smokescreens. Read carefully, and you’ll see the claims for 99.2, 99.5, even 99.8 percent light transmission are per air-to-glass surface, not per scope. This means each lens surface that touches air, front and back sides included, transmits 99-point-whatever percent light, which is outstanding. But most scopes have from seven to 12 air-to-glass surfaces. Do the math, and you’ll realize total transmission is more like 92 to 98 percent - which is still outstanding, especially when you consider a single pane of ordinary window glass passes about 95 percent of light.

30mm tubes are brighter than 1-inch tubes.

This is another myth that sounds right but isn’t. Something called exit pupil (the circle of light you see in an eyepiece held a foot or more in front of you) determines how much light passes through a scope. Think of it as a window. The bigger the window, the “brighter” the scope. The size of this exit pupil window relates to the pupil in your eye, which dilates about 7mm in young eyes, 5mm in middle-aged eyes. If the exit pupil in a scope is wider than your own, the extra light around the edges never gets into your eye. Wasted.

A scope’s exit pupil diameter is determined by objective lens diameter divided by magnification. A measly little 4x scope with a 32mm objective has an exit pupil of 8mm. A huge 10x by 50mm sports just a 5mm exit pupil. Build a 4x32mm scope in a 30mm tube or 1-inch tube, and its exit pupil remains 8mm. The 10x50 exit pupil remains 5mm. The reason 30mm scopes seem brighter is that most are top-of-the-line models built to the highest standards with the best flare control, multicoatings, lens quality, etc. The same materials and construction in a 1-inch tube would produce equal brightness.

Optical Illusions
Eye relief on this old, inexpensive variable is only 2.3 inches at 9x. Compare it to the next image showing a Leupold VariX-III 2.5-8x with 3.5 inches of eye relief at 8x.

You need a 50mm or 56mm objective for whitetail hunting in low light.

Not necessarily. Shooting hours end a half hour after sunset in most states. Even on cloudy evenings, there is usually enough remaining light to adequately see deer and crosshairs with smaller-objective scopes. Just how much smaller depends on magnification, lens quality, lens coatings, flare control (flat black interiors to reduce stray-light bounce), and contrast, or sharpness. An extremely sharp lens with high contrast that resolves lots of subtle detail and color does more than mere brightness to help you pick a brown deer from a background of brown branches. Low-power scopes with mid-sized objectives can produce huge exit pupils (4x40mm=10mm exit pupil) yet will not enable you to see distant targets simply because magnification is too small. More magnification will help until exit pupil size falls below about 5mm (8x40mm=5mm exit pupil).

The real value of huge objectives (coupled with fully multicoated lenses) is realized when they are matched to higher magnifications. A 10x50mm scope has a 5mm exit pupil - nearly all the light you can take in. A 15x50 shrinks exit pupil to 4.3mm, sacrificing useable light. The optimal low-light scope is arguably a 10x50 or 12x56 built to the highest standards with fully multicoated lenses. Be aware that it will be heavy, bulky and mounted high to clear the barrel - essentially a sniper scope poorly suited for quick action in the woods or extensive hiking.

Steel tubes are stronger than aluminum.

At first blush, this seems obvious, but then why don’t they make arrows and airliners out of steel? Let’s remember, aluminum is stronger per unit of weight than steel. You can make a steel scope stronger than an aluminum one, but it’ll weigh considerably more. The vast majority of scopes and the finest scopes in the world are built with aluminum tubes and they hold up beautifully, so what’s the fuss?

The higher the magnification, the more accurately you’ll shoot.

Are you kidding? Have you ever tried holding a 30x scope on a target offhand? Every magnified shake, shimmy and wiggle will drive you nuts. You’ll think you’re nowhere near the bull, so you’ll try to snap off a shot when you see it drift by. Screw the power down to a more manageable 6x, and it’ll look as if the crosshair hovers pretty close to the bull most of the time. You relax and squeeze off a shot. Try it.

If you’re shooting from a solid rest, higher magnification can help, of course, particularly on tiny, long-range targets. As they say, you can’t hit what you can’t see. This is why big zooms are most popular on varmint rifles. But even here, excessive power can hurt. It magnifies heat waves, causing your target to turn fuzzy, shimmy and dance. Turn down the power, and the target sharpens.

High-power variables are the most versatile for big game hunting.

Not when you have to lug that big 6.5-24x telescope up a few mountains. A big scope weighs more, slowing you down in every way. It catches on brush. It unbalances the rifle and makes it slow to mount. You’re slow to find the crosshairs because the tube is so high to clear the objective bell that your cheek no longer fits against the buttstock, which rises up to slam your face when you fire.

Opportunities for using a scope at high power from a solid rest are few and far between, and high magnifications reduce field of view drastically. It’s often very difficult to find a target at high power. For every critter you see across an open meadow at 400 yards, you’ll jump 10 under 200. Chances are you’ll have forgotten to turn the power down from 24x, and you won’t be able to find the deer in the scope.

Then there’s the parallax adjustment on high-power scopes. You’ve got to remember to set that for distance, too.

It starts to get confusing, so most shooters set their big variables at 6x, parallax at about 150 yards, figuring they’ll crank things up at the moment of truth. But the moment comes, and only after they finish field-dressing their trophy do they realize they shot it at 6x.

Darrel Holland, master gunsmith and extreme-range shooting instructor, says 10x is more than enough magnification for taking any big game out to 700 yards, and darn few hunters can shoot accurately to 400. Save the big power for targets and varmints.

A fixed 4x is bright, light and all the power you need for big game.

Maybe this was true 50 years ago when Jack O’Connor recommended 4x. Today’s more accurate rifles, range-finders and superior bullets make long-range shooting effective for those who practice. And an 8x or 10x view can help you pinpoint a buck’s shoulder through a small alley. Consistency, durability, compact size, reasonable weight and repeatability make mid-range variables the best all-round scopes for most big game hunting.

A wide field of view is critical for finding game.

Subscribe Today!Extra-wide-angle views, usually advertised to give you “the big picture” for spotting game, might be useful when glassing with a binocular, but a scope is for aiming. You don’t need to see half the county on either side of your target to shoot it. If you can’t find the deer in the scope when you raise it, you’re not doing it correctly. Don’t look away from the deer and focus on the scope as you bring the rifle up. Keep looking at the deer and raise the rifle automatically until the scope aligns with your eye, and you’re suddenly seeing the deer magnified. This is why it’s important to mount scopes low so you don’t have to lift your head to see through them.

“Eye relief” is something you get from a pharmacy.

No, eye relief is a critical measurement in a scope, the distance from the eyepiece to your brow when you see a full image. It should be a minimum of 3 inches in light-kicking rifles, 4 in really big bangers, to prevent scope and brow from meeting violently during recoil. Eye relief is sacrificed when field of view is increased in a scope. It also decreases in many scopes when power is zoomed higher. Check this before you buy. More eye relief is better than less, especially if you tend to “creep” the stock.

Unless we tear scopes apart, get degrees in physics and mechanical engineering and perhaps metallurgy, we’ll never fully understand scopes. Fortunately, we only need to know some basics to select and use them effectively.

Reprinted from the July 2005 issue of Buckmasters GunHunter Magazine

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