RIFLES: Binos and Scopes—what you need to know
By Wayne van Zwoll
What you see in the woods depends on the light and your eye and the glass you put between them. Through good lenses, you’ll see more—more game but also more of the details that make each day memorable even when you don’t shoot.
The word “lens,” by the way, comes from the Latin name for lentils—fat little disks, convex in cross-section, about the same size and shape as the lens inside the eye itself. Glass lenses use refracted light to correct and magnify images. The earliest were convex, making light bend inward. The bent rays exiting a convex lens meet at a place behind the lens called the principal focus. The distance from this point to the center of the lens is the focal distance. The shorter the distance, the more powerful the lens. Concave lenses bend light rays away from each other.
Bifocal glasses typically have a convex lens (for distant viewing) atop a concave “reading” lens. Aspheric lenses feature more than one radius – for example, a convex periphery surrounding a concave center. Aspheric lenses in rifle scopes and rangefinders can increase sharpness and flatness at field edges.
Modern hunting optics rely on series of lenses in tandem. The idea wasn’t intuitive. Legend has it that in 1608 Dutch spectacle-maker Hans Lippershay idly lined up two lenses on a distant weathercock. The chicken suddenly grew as big as a cow! But that was nearly 20 years after Zacharias Johnson claimed to have built a compound microscope. Whether it was Hans or Zack or, as some have suggested, Hans’s assistant who built the first telescope, the device quickly became popular.
In 1609, Italian astronomer Galileo Galilei was using telescopes of up to 30x with the focal point behind the rear of the telescope. Johannes Kepler changed Galileo’s telescope to shift the image inside. But the image was still upside down. That didn’t matter because distant stars had no top or bottom. Modern scopes have an erector system to right the upside-down image.
Both the type and quality of optical glass affect what you see. In its basic form, glass is a melt of silicon dioxide (quartz) with calcium and certain alkali. The melt is cooled to solidify without crystallizing. Optical glass can be fashioned into lenses and prisms that not only transmit light but bend and focus it.
The optical glass we know probably began life around 1884 in the Glastechnisches Lab in Jena, Austria. That facility would later become the Schott Glaswerke. Now with sales offices in more than 100 countries, Schott continues to supply much of the world’s superior optical glass. Though many prestigious optics firms use Schott glass, they also have backup suppliers. German glass technology has been exported to Japan; as long as the glass meets specifications according to use, its origin matters not.
Binoculars, spotting scopes and rifle scopes all have a series of lenses (and in some cases prisms) to transmit light to your eye. Glass types and lens shapes differ, depending on application. Two common glass types in binoculars are BK 7 and BaK 4. The K stands for Kron: German for Crown. The B in BK 7 means borosilicate. BaK 4 is a barium-silicon compound. While BaK 4 is usually more costly than BK 7, you won’t see much difference looking through binoculars.
“They’re both prism glasses,” said Bausch & Lomb optical engineer Bill Perkins in an interview a while back. “You can tell prism type by looking into the objective end of a porro-prism binocular. If the exit pupil appears round, it’s BaK 4; if it looks square, it’s BK 7. The flat sides of the exit pupil are caused by shading, from a 90-degree turn of light between two 45-degree prisms.”
Bill noted that, though BaK 4 prisms are a recognized standard, the most important attributes of a high-quality binocular are sound design, close tolerances and coatings that increase light transmission, plus rugged lens settings and hinges. “Besides,” he added, “the right glass for a given application isn’t always the most costly.”
Another type of optical glass is Flint. A common Flint glass, SF 2, gets its name from Schwerflint—German for dense flint. The English name: FD 2. Dr. Walter Mergen of Zeiss points out that Flint glass is typically heavier and more brittle than crown glass, and ill-suited to placement on the exposed ends of a lens system. He emphasizes that both are necessary in an optical system. “For example, as two parts of a doublet or achromat.” That’s a compound color-corrected lens. White light coming into an objective lens is actually a bunch of colors. The lens separates them, prism-like. “In a lens that is not color corrected, blue and red waves remain apart, making for fuzzy image edges, perhaps a halo. Achromatic lenses gather the primary spectrum back into its white bundle,” Mergen notes.
You may, in fact, benefit from another layer of glass. Leupold’s Forrest Babcock told me that ED or extra-low-dispersion glass has been used for some time in expensive telescopes. “It’s usually added to a doublet to form a triplet or apochromatic lens. An apochromat delivers additional color correction in the secondary spectrum, bringing the green wavelength to the same focal point as red and blue.” Forrest added that the objective lens has the greatest effect on image quality, so ED glass belongs up front. “That means a bigger lens than if you could use it inside. A blank of ED costs about 200 times what ordinary crown or flint glass costs, so a big objective lens will make the scope much more expensive when you install ED glass.” He flipped through a catalog. “This supplier lists a 6-inch blank of ED for $5,000.” ED glass is used almost exclusively on high-power optics like spotting scopes, where its benefits are most visible.
The bigger the lens, the better its resolution, all else equal. A high-resolution lens helps your eyes distinguish detail, separating small things that are so close together that they appear blurred or joined when viewed by the naked eye or through glass of low resolution. A healthy human eye can resolve about one minute of angle (60 seconds of arc, or roughly an inch at 100 yards). Magnification gives your eye more resolving power. But as magnification exceeds the resolving power of the glass, spotting scopes make the target bigger without making it clearer.
A fellow named Rayleigh came up with a constant that, divided by objective lens diameter (again in mm) yields actual resolution in seconds of angle. The constant, 114.3, divided by, say, 60mm, yields a maximum resolution of 1.9 seconds. To find the maximum power at which the eye can use this resolution, divide 60 seconds of arc (naked eye resolution) by 1.9. Result: 31x. Beyond this, the image will get bigger but not clearer. If your spotting scope has an 80mm objective, you can benefit from higher magnification—up to 42 power or so.
A big exit pupil (objective diameter/magnification) is often noted as the key to finding detail in low light. Well, a bright glass is not necessarily the most effective in picking out detail. You need power, too. One measure of low-light resolving capability is twilight factor. Mathematically, it amounts to the square root of magnification multiplied by objective diameter. So if you have an 8x40 binocular, the TF value is the square root of 320 or 17.9. Increase magnification to 10x, and the TF becomes the square root of 400 or 20—while the exit pupil shrinks from 5mm to 4mm. The extra magnification more than offsets the drop in brightness. If you make the objective lens 25 percent bigger (50mm), the TF for an 8x glass is 20. For a 10x it’s 22.3. Once more, high magnification results in a greater TF value. You need 25-percent gain in objective diameter to equal a 20-percent boost in magnification. So reducing magnification to make the exit pupil bigger will not necessarily give you a higher twilight factor or better resolution in dim light.
Big glass doesn’t mean a broad field of view. Wide-angle lenses may be the same size as lenses of the same power not advertised as wide angle. The difference is in the grinding. “A wide-angle binocular gives you 65 degrees of field,” says Dr. Bill Cross at Bushnell. “That’s 65 degrees of apparent field, which is the product of the real field and the magnification. Say you have an 8x42 binocular with a 368-foot field at 1000 yards. A degree equals 52 feet, so your field spans about 7 degrees. Multiplying 7 by 8 gives us 56 degrees, or the field you perceive because the image looks closer. This is short of the 65 needed for a wide-angle rating.”
Long eye relief reduces the field minimums for wide-angle rifle scopes: 26 degrees for fixed-powers and 23 degrees for variables.
You can get more field by changing magnification and eye relief,” says Bill. “Those three variables comprise what designers call the optical triangle. To get more magnification or eye relief, you must accept a smaller field.”
Eye relief doesn’t matter much in spotting scopes, and movable eyecups let you adjust binoculars for proper eye relief. But eye relief in rifle scopes matters a lot. You want generous eye relief, even if it costs you some field. Long eye relief (3½ to 4 inches for receiver-mounted rifle scopes) enables you to position the scope well forward, where it won’t endanger your brow on recoil. Non-critical eye relief means your eye has some latitude along the scope’s axis to find a full field of view. That’s also good, because you’ll aim quicker if eye placement needn’t be exact.
One thing to remember when you’re shopping for or mounting a variable scope: Eye relief typically decreases as you boost power. You’ll want the scope positioned so your eye naturally falls the proper distance from the ocular lens at the magnification you normally use. Some variables deliver the same eye relief from low power to high. In my opinion, constant eye relief is a huge asset. When you mount a scope, position it a little farther forward than you think it should be. You’ll aim faster if perfect eye relief comes when your cheek is as far forward on the comb as is comfortable. Forward mounting also gives your skull protection when you’re shooting from prone or aiming uphill from the sit. Remember that optical eye relief is not physical eye relief. The distance of your eye from the eyepiece may be considerably shorter than the distance between eye and lens.
So there’s more to know about binoculars, spotting scopes and rifle scopes than magnification, weight and exit pupil. Hunting elk, however, the most important thing is that you know how to use them.
Wayne van Zwoll—writer, scholar, sharpshooter, hunting guide—has published hundreds of articles, six books and several short stories about guns, hunting and hunters. His latest books are Elk And Elk Hunting and The Hunter’s Guide to Ballistics.