The trouble with buying a binocular is knowing how
to detect good, better and best. Most critical parts are hidden inside, and
subtle performance differences are difficult to detect. How do you choose
what's best at a good price?
Start with the numbers. An 8x42 binocular magnifies images eight times and
employs objective lenses, the ones out front, that are
42 millimeters in diameter. A 10x42 combines the same size objectives with 10X
power. These numbers are important because the bigger the objectives, the more
light that enters the barrels. The higher the power, the less light that
reaches your eyes. If you want maximum brightness at any magnification, get
bigger objectives. If lens and prism coatings are equal (more on this later) a
10x50 will be brighter than a 10x42, which is brighter than a 10x32, etc. But
an 8x50 is brighter than a 10x50 and a 6x50 is brighter yet.
To determine potential brightness, divide objective diameter by power. A 10x42 = 4.2mm, a 10x32 = 3.2mm, 8x42 =5mm. These final
numbers are the diameter of the exit pupils (EP), the circles of light that
exit the eyepieces. You can see them by pointing a binocular toward a bright
wall or sky and holding it two feet from your eyes while looking at the
eyepiece lenses. The diameter of those little EP windows relates to your
pupils, which dilate from about 2.5mm in full daylight to 7mm in near darkness.
An EP smaller than your own pupil will result in reduced
light transmission to your retina, i.e. a "darker" image.
Now that you know what all the numbers mean, how do you use this knowledge
to buy the right binocular? Begin by deciding how much power you need. More is
not always better because additional objective diameter to increase brightness
also increases bulk and weight.
A big binocular might prove too uncomfortable
to carry and use. Lots of magnification sounds good, but it results in a
narrower angle of view. You see less field-of-view at
10X than 8X. Hand tremors, heat waves and dirty air are also magnified at
In general, 6X or 7X are best for glassing tight cover where
whitetails hide. An 8X is a great all-round choice, while 10X should be
reserved for searching big, open country. Anything higher is a real specialty
item best used with tripod or other solid support. Most of us can't handhold a binocular powerful enough for judging details
such as antler tines or horn rings at significant distances. That's a job for a
15X-60X spotting scope.
After choosing power, select objective lens size to provide adequate exit
pupil size. A 5mm EP is adequate 98 percent of legal hunting time (3/4-hour
before and after the sunset.) A 4mm EP is more than bright enough from 1/2-hour
before and after the sunset. A 3mm EP is fine for bright daylight and lightly
cloudy days because your pupils shrink to that size or less in bright light.
Below 2.5mm EP becomes too small for comfortable viewing. But, regardless of EP
size, there are two more things you should look for to increase brightness.
Coatings Make the Difference
The difference between the best binoculars and the
also-rans isn't the glass, but how that glass is coated. Lenses coated with
multiple layers of anti-reflective elements transmit more light than raw glass,
and these coatings add no weight or bulk. The more layers the better. A fully
multi-coated 8x32 will transmit a sharper, brighter image than an un-coated
8x50. Good coatings provide maximum brightness bang for your buck. A binocular
advertised as "fully coated" must have all air-to-glass surfaces coated with
one layer. "Multi-coated" means one surface has multiple coatings, but the rest
could have just one layer. "Fully multi-coated" means all surfaces must have
multiple coatings, and these give the highest light transmission.
Prisms Matter, Too
Prisms in binoculars bend light, reducing barrel length, and erect what would
otherwise be an upside down image. There are two basic
prism styles, Porro and roof. The Porro
puts a dog-leg in the barrels. Roof prisms are stacked neatly atop one another
so each barrel is straight and compact. An optically superb Porro
model costs about a third less than an equal roof prism, but is a bit more
delicate. You'll have to protect it from vibrations and jarring. Porros are more easily knocked off axis than are roof
prisms. Highest light transmission comes through BaK-4 prisms as opposed to
BK-7, so ask for those.
To get the highest light transmission through roof prisms, buy models that
have been Phase Coated. This coating improves sharpness. HD or ED lenses can
sharpen by reducing color fringing, but this isn't much of a problem until
power goes above 15X. Money is better spent on anti-reflection coatings.
Other than those features, a binocular company's guarantee is worth quite a
bit. If one guarantees its instrument for a year or two and a competitor
guarantees a similar product for a lifetime, that
suggests the lifetime guarantee won't be needed because the binocular is so
well made. And if it does malfunction, you'll save money on repairs or even a
Shop The Sportsman's Guide great selection of Binoculars!
Ron Spomer has been photographing and writing
about the outdoors for nearly four decades. He's written seven books, hunted on
six continents and been published in more than 120 magazines. He's currently rifles'
editor at "Sporting Classics," Travel columnist at "Sports Afield," Field
Editor at "American Hunter" and "Guns & Ammo" -- Optics Columnist at "North American
Hunter," Contributing Editor at "Successful Hunter," Senior Writer at "Gun
Hunter," and TV host of "Winchester World of Whitetail." He will write on Shooting Tips weekly for sportsmansguide.com. You can read his
blogs and catch some of his YouTube videos at www.Ronspomeroutdoors.com.