A Primer On Long-Range Optics

Long-range optics can be somewhat complicated in nature and in their operation, but there are a few key things to remember to simplify the process.

What you need to consider when shopping long-range optics:

• Magnification suffers if it doesn't come with clarity.
• A 40x eyepiece is a maximum for a field spotting scope.
• The larger the ocular lens, the larger the exit pupil and the more light is transmitted.
• Lens coatings reduce light loss due to reflection and refraction.
• Variable power spotting scopes tend to be more useful.
• Given it shrinks the field of view, too much magnification in a rifle scope can be a handicap.
• Too much magnification increases reticle quivers.
• It also increases how much mirage you see.

A spotting scope moves things. It pulls things tiny with distance right into your hand. But the “Big Eye” is cumbersome too. On a hunt, you must weigh the benefits of detailed looks at distance against the various costs of making them.

Spotting scopes date way back. In 1608, Dutch spectacle maker Hans Lippershay lined up a pair of lenses on a distant weathercock. A huge chicken appeared, as if by magic! A year later, Galileo Galilei built his first telescope. With it and later versions, he discovered four of the moons orbiting Jupiter and distinguished individual stars in the Milky Way.

Galileo’s first telescopes had a convex objective lens and a concave ocular lens, so the focal point of the front lens lay behind the rear lens. Johannes Kepler changed Galileo’s design to put the image inside the tube. The image was upside down there, but, since these men were viewing stars, upside down didn’t matter! Later, engineers would devise an erector system to right the image. Early ’scopes delivered a fuzzy image, a result of “spherical aberration,” or the failure of light rays to meet at a common point along the lens axis. With no way to refine lens curvature, makers of early telescopes just increased focal length. Some versions had lenses several hundred feet apart! Tubes were both impractical and unnecessary. Compound lenses would eventually correct spherical and chromatic (color) aberrations.

By the time target shooters used spotting scopes to see bullet holes at 600 yards, and hunters took them afield to find bighorn rams, magnification had become an industry. Bull’s-eyes and animals at very long range popped, sharp-edged and super-sized, from a tube no bigger than a rolled-up newspaper. Four hundred years of progress in optics have put close images in a rucksack, and the last 50 years have delivered refinements early astronomers could not have imagined. Still, the Big Eye’s function is the same as it was in the seventeenth century: to help us see better.

Bigger Is Better, But…

Magnification is one assist. But bigger images aren’t sufficient if image quality suffers as a result of the enlargement, or if the image gets dim. In fact, magnifying a recognizable object too much can make it unrecognizable! And the ability of lenses to transmit usable light diminishes as you increase power.

The first requisite of a magnifying optic is that it resolve images clearly. Resolution is a measure of the level of detail you see through a scope. High-resolution optics can distinguish fine detail, separating small objects that, to the naked eye, appear as one at distance. A minute of angle (an inch at 100 yards), is about the limit of resolution for unaided human eyes. A second of angle is a sixtieth of a minute of angle.

Given good lenses, boosting magnification improves resolution. So does increasing lens diameter. At high power, a small lens won’t deliver a picture clear or bright enough for you to distinguish what big glass would make plain. A man named Rayleigh came up with a constant that, divided by objective lens diameter in millimeters, yields maximum resolution in seconds of angle. Here’s how it works.

Say your spotting scope has a front lens diameter of 60mm. Dividing 60 into the constant, 114.3, yields a resolution of 1.9 seconds of arc. That figure determines the useful magnification limit of your eyepiece. As your eye can resolve 60 seconds, you divide that by 1.9 and get 31. So, for that scope, magnification of 31x is the usable top end. More power will make the picture bigger, but not clearer; you won’t see any more detail. If you bought a bigger scope — say, one with an 80mm objective—you’d hike the power limit to about 43x (114.3/60 = 1.4; 60/1.4 = 43).

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All this is to say that you’re wise to pick an eyepiece of reasonable power. A 20-60x eyepiece may seem wonderful, but the top third of that range is practicably unusable! Even if Rayleigh was off his rocker, 40x is a practical maximum afield, where wind and mirage make still images shimmy, and where you’re often short of light.

Light Transmission And Other Considerations

High magnification means dim images at dawn and dusk. Light transmission is commonly expressed as the diameter of a scope’s exit pupil — that pencil of light making its way to your eye from the ocular lens. Calculate EP by dividing objective diameter by magnification. For example, our 60mm scope with 20x eyepiece offers an exit pupil 3mm in diameter. The bigger the lens, the bigger the exit pupil, if power stays constant. Increase magnification, and the exit pupil shrinks. A bigger exit pupil brings more light to your eye, which can dilate to 7mm in total darkness. As you age, your pupil becomes less flexible, and 6mm may become a more practical maximum. In dim hunting light, 5mm dilation is more likely.

In bright light, your eye doesn’t dilate, it constricts. A tiny exit pupil gives you as broad a shaft of light as your eye can use under brilliant sun. At dusk, a generous objective lens helps you. But the biggest spotting scope practical for field use, one with an 80mm lens, delivers a 5mm exit pupil only at 16x!

Lens quality and coatings matter a great deal, and the best are expensive. The price spectrum for spotting scopes reflects real differences in scope and image quality. Still, optics that offer fine resolution and efficient light transmission shouldn’t require a second mortgage. You’ll get them now, even with mid-priced scopes, provided you insist on fully multi-coated lenses. That means all air-to-glass surfaces are coated with compounds that reduce light loss due to reflection and refraction. (Uncoated lenses shed up to four percent of incident light at each surface.) You might also consider ED (extra-low dispersion) or APO (apochromatic) or fluorite lenses. They’re commonly available on top-quality scopes.

While, in my view, most big riflescope objectives deliver fewer benefits than liabilities, you’ll get real value from added lens surface up front in a spotting scope. I prefer 80mm lenses, but 65mm scopes by the likes of Swarovski, Leica, Zeiss, and Nikon are great alternatives, where weight matters.

Another useful feature on spotting scopes is variable power. For hunters, 15-45x is ideal. Keep it at the low end to quickly find an animal you’ve spotted with your naked eye or binocular. Once on target, you’ll appreciate a choice of magnification to match distance and wind and light conditions. I’ve used a fixed 25x Bushnell while guiding hunters. It also served me well on the smallbore circuit, showing .22 bullet holes in targets 100 yards off without undue distortion in mirage or disturbing movement in wind. For hunting, Leupold’s 12x-40x adds versatility. A 15-45x is more useful than any 20-60x afield. Higher power makes sense only at long-range paper, from a very steady support, and then only occasionally.

Less Is More

Because the laws of physics apply to riflescopes as they do to spotting scopes, you should have little trouble picking a sight for long shooting. But remember that a riflescope is for aiming, not viewing, and that its weight and bulk become one with the rifle’s. It must also endure recoil. Many shooters choose scopes that are unnecessarily powerful.

Too much magnification handicaps you in several ways. It shrinks the field of view, so you won’t find the target as quickly as with less. On a hunt, you may not see the huge buck in the shadows to the side of the most obvious animal. High power reduces exit pupil diameter, so, in dim light, the target image won’t be as bright. The magnification that makes that target bigger also bumps up the amplitude of reticle movements due to muscle tremors and heartbeat. Reticle quivers you might not even notice at 2½x become violent dips and hops at 10x. At 20x, you’ll see so much chaos in the tighter field, the target might bounce in and out of view as you try to tame that reticle. A scope helps you when it shows movement you can control. It’s a liability as it amplifies movement you can’t. Instead of applying gradual pressure to the trigger, you wear yourself out fighting the jitterbug image in your sight. As eyes and muscles tire, an accurate shot becomes impossible.

Magnification also shows you mirage, a good thing on days when mirage is light and the target is in reasonable range. But, on hot days, when you’re aiming over great distance, the target may appear as a dim, shapeless object stuck below the surface of a raging river.

In general, the least magnification that gives you a clear target image is the best magnification. I use 4x riflescopes for most big-game hunting and think it adequate to 300 yards. A 6x works fine for me at 400. Of course, you’ll want more magnification for small animals like prairie dogs. Deliberate shooting at paper bull’s-eyes and steel gongs brings out powerful glass. I’ve used 16x, even 20x, scopes to advantage in good light, when there’s time for a solid position and precision trumps all else. In smallbore matches, a 20x Redfield served me well. I needed that much power to hold on a .22 bullet hole at 50 meters, or shade to the bottom-right quadrant of an X-ring the size of a bottle cap at 100. I’ve used 25x to good effect on bull’s-eyes, but am inclined to think 20x would have served, too. Higher power is very hard to use.

Variable Power And Adjustable Objectives

These days, variable scopes offer wide four-, five- and now six-times power ranges; that is, the highest magnification is four, five or six times that of the lowest. So, instead of the 3-9x that once awed sportsmen with its versatility, you can get a 3-12x, a 3-15x, or a 3-18x. Or bump up to 4x on the bottom to get 20x or 24x on the top. Such scopes feature 30mm tubes. These may or may not have a bigger erector assembly (the tube with lenses and magnification cams held inside the main tube). Those with erector assemblies of standard size for one-inch scopes give you more windage and elevation adjustment. That’s an advantage at long range, though a scope performs best with its optical axis close to its mechanical axis.

Keep objective lens diameter modest. The 42mm is big enough unless you insist on very high power. Remember that, with a variable scope, big glass up front enhances brightness only at high magnification. At 7x, a 42mm lens delivers all the light your eye can use in the dimmest conditions. In bright light, your eye won’t be dilated. The 3mm exit pupil of a 14x scope with a 42mm objective is enough.

For long shooting, you’ll want an adjustable objective, so you can focus the target and eliminate parallax at the target distance. Parallax appears as the apparent movement of the reticle when you move your eye off the scope’s optical axis. Every scope is set for zero parallax at a certain distance, typically 100 or 150 yards in sights designed for centerfire rifles. At that range, you can move your head up and down or sideways behind the scope, and the reticle will stay on target. At other distances, you may see the reticle off-center when it really isn’t, that is, it wouldn’t be if your eye were directly behind the scope. Adjustable objectives let you zero out parallax. Traditional AO scopes have a sleeve on the objective bell. A knob on the left-hand side of the turret is more contemporary and easier to use.

Reticles And Scope Weight

As for reticles, simple is good. For long shooting, a “range ladder” on the lower stem of a standard crosswire or a plex can provide useful aiming points. You’ll want them beyond 400 yards or so. While a fine crosswire yields precise aim, it can cost you a shot if it’s not easy to see in dim light. Avoid jungles of tics and hashmarks and multiple lines that clutter your field. They slow your aim. Picking the wrong intersection has caused many a miss — and the more marks you see, the more likely you’ll make that mistake! The mil-dot is as complex a reticle as I like. Shooters who learn to use it can do well at distance.

While most reticles in scopes marketed stateside have second- or rear-plane reticles, the standard in Europe has been first- or front-plane reticles. Both have advantages and drawbacks. A rear-plane reticle stays the same apparent size (crosswire thickness) throughout the power range of a variable scope. This is good, because most hunters don’t want the reticle to grow in thickness as they boost power for fine aim at distance, nor do they want one that gets thin and hard to see when they dial down for close shots in timber. On the other hand, a first-plane reticle stays the same size in relation to the target as you change magnification, so you can use it as a rangefinding device at any power without recalculating.

Before committing to a scope, consider its weight and dimensions. Besides adding to your burden afield, heavy scopes can slip in their rings under stiff recoil. A long eyepiece can force placement of the scope too close to your eye. Scopes with little “free tube” between bell and eyepiece limit your options for ring location.

Editor's Note: This article is an excerpt from Mastering the Art of Long Range Shooting.