Learn to distinguish the high-quality glasses from the kitsch.
There are a boatload of binoculars on the market these days, and making sense of the myriad claims, hype and gobbledygook terminology that fills the manufacturers' literature can be a bear. However, understanding just a few key terms and concepts and what's behind some of the buzzwords will help you find the best glass for your money.
Take the time to learn binocular terminology; it can mean the difference between a wise purchase and a wasted one.
A conventional binocular description--10x42, let's say--includes the magnification and the objective lens diameter in millimeters. Most optics makers say magnification is not as important as clarity, resolution and lens quality, because magnification is irrelevant if the optics are so dim that you can't see through them. True enough, but the whole point of having binoculars is to make far away things look closer, and the more magnification, the closer objects appear.
So first choose a magnification that meets your needs, then shop for the best optical quality you can afford. The higher the magnification, the better quality optical glass you'll need because all optical systems get dimmer as magnification increases. The only way to offset this is with high-grade lenses and prisms that allow superior light transmission.
On the objective lens side, bigger is better. The larger opening not only lets in more light and brightens your image but it also supports higher magnifications. Twenty years ago 7x40 was about as high as you could go. Today, technical advances in lens quality make 10x40 binoculars indispensable field tools. And premium optics makers such as Leica, Zeiss, and Swarovski are even offering crystal-view 10x32 glasses.
Given equal magnification and objective lens size, the sharpness, brightness and resolution of the image you see through two different binoculars depends on the optical quality of the glass. In a nutshell: The higher the grade of glass, the more expensive it is.
Binocular housings contain two glass objects inside: lenses and prisms. Lenses are round, flat pieces of glass stacked at the eyepiece and the objective end. Prisms are funny-shaped glass chunks that bounce light around to give you usable magnification, focal length and a right-side-up image.
Beyond these preliminary definitions things get pretty technical pretty quick, and--unfortunately--some of it you need to know.
Consider prisms. There are two basic types: Porro prisms, which are offset and route light at 90-degree angles; and roof prisms, which are overlapped and route light at more acute angles and are lightweight and compact. Porro-prism binoculars have the classic "stepped" offset look; roof-prism binoculars are slim and sleek with a straight-tube.
There are two types of roof prisms: Pechan (smaller) and Abbe-Koenig (larger). Pechan prisms have a mirror-type surface that inherently loses 4 to 6 percent of available light, so they are used in ultra-compact designs not intended for low-light conditions. Larger Abbe-Koenig roof prisms are employed in mid- and full-size instruments where twilight viewing is a factor.
Roof prisms are also more sensitive to "stray light" (light bouncing off the binocular's interior surfaces), and they require coatings to deliver premium resolution and contrast. In catalogs these are called: Phase-Coat, P-Coat, PC-3 Coat or something similar.
The higher the magnification, the better. But for serious hunting you also want binoculars with high-quality glass that has been fully multicoated.
Stray light is a real problem in binocular design, and top-end makers have developed a separate technology of interior-surface finishes, surface-angle reductions and component holders to reduce it to an absolute minimum.
Also, because roof prisms route light at sharper angles, they require higher-grade glass to equal Porro prism performance. There are nearly as many different "alloys" of glass as there are alloys of aluminum or steel. Premium roof prisms typically employ barium crown glass (Bak4 is the most common grade). Porro prisms (and less-premium roof prisms) commonly employ cheaper Bk7 borosilicate glass.
Roof prism binoculars generally cost more than Porro prism binoculars of equal magnification and optical performance. But at any given magnification/objective level, Porro prisms provide slightly better depth perception and wider fields of view than roof prisms. However, roof prism binoculars are much more popular these days because they are so much sleeker, more compact and, well, cool-looking.
Similar quality issues apply to lenses as well. When light passes through any piece of glass, the light is dispersed into component colors, and delivering a sharp, clear image to your eye requires a system of lenses in the objective and eyepiece ends of the binocular that correct any color fringing (technically, "chromatic aberration"). The result is a chromatically corrected or "achromatic" system, but even achromatic systems will display small residual aberrations that can become distracting and eye-fatiguing at magnifications of 10X and above or in extreme light situations (either very dim or very bright).
The solution is to use a notably more expensive and somewhat heavier extra-low dispersion or anomalous partial dispersion glass. Depending on brand, this glass may be called ED, fluoride, HD, EDX or APO glass, and such systems are termed "apochromatically color corrected" or "superachromatic." These are important keywords to focus on when shopping.
Lens coatings are another important consideration, and have a great effect on light transmission and image sharpness. Purchase the most complete coating level you can afford. Commit these terms to memory: coated (single coat on at least one lens surface); fully coated (single coat on all glass-to-outside air surfaces); multicoated (multiple complementary-function coats on at least one lens surface); and fully multicoated (multiple coats on all glass-to-air surfaces)
Avoid mere coated binoculars; they're not worth the money. Fully coated lens systems are minimally okay for daytime use.
You'll find multicoated lenses in a wide range of quality midlevel products, and all premium binoculars have fully multicoated lenses these days.
Remember, I'm just talking about lens coatings; the phase-coating on interna
l prisms is something else entirely and will be specified separately in a manufacturer's product descriptions. Premium binoculars are fully multicoated and phase-coated.
When researching binoculars, keep in mind that if manufacturers don't specifically mention high-grade features, then they aren't there. If all their lenses are "fully multicoated," they'll tell you.
You'll spend much more time looking through your binoculars than through any riflescope, which to my mind means that the optical quality of the binoculars is actually more important.
Looking at poorly resolved and distorted images causes eye strain, and eye fatigue hurts. I've had many, many hunters tell me that they don't like to use binoculars because they can't seem to see through them comfortably. That's because they've got crappy binoculars.