Magnification and Aperture: A binocular's name describes its magnification
and lens size. For example, a "7x50" or "7 by 50" binocular magnifies 7 times and
has light gathering lenses (objectives) that are 50mm (2") in diameter.

A binocular that magnifies seven times (7x or 7 power) makes
objects appear seven times closer than they do to your bare eyes. A bird 70 feet
away seems to be only 10 feet away through a 7x binocular, for example.

The higher the power, though, the more difficult it is to
hold a binocular steady enough to keep the image sharp - so the highest possible
power is not always the most useful power. 10x is a practical maximum for hand-held
use.

A binocular's lens size helps determine how bright the binocular
will be, since large lenses naturally gather more light than small ones. You can
compare the brightness of identical magnification binoculars by squaring the diameters
of their objective lenses. For an example, since a 7x binocular with 50mm lenses
(50² = 2500) gathers more than twice as much light as a 7 x 35mm (35² = 1225), the
7 x 50mm binocular is brighter in the dim light of dawn or dusk.

In addition, resolution (the ability of a binocular to show
you small details) is inversely proportional to the size of the objective. In other
words, the bigger the lens, the smaller the detail it can show.

While big binoculars have advantages, they usually have
a big drawback - weight. In general, the bigger the binocular, the heavier it is.
A binocular that's too heavy to carry comfortably all day long is not a good binocular,
no matter how sharp its optics.

Exit Pupil: A binocular's exit pupil is the circle
of light you see in the eyepiece when you hold up the binocular at arm's length
and look at a light source. (Don't look at the Sun, as that will damage your eyes.)
In general, the bigger the exit pupil, the brighter the image.

To find a binocular's exit pupil size, divide the diameter
of its objective lens (in mm) by its magnification. A 7 x 50 binocular has an exit
pupil 7.1mm in diameter (50mm / 7 = 7.1).

For the most efficient use of a binocular's light-gathering,
match its exit pupil to your eye's largest dilated pupil size. A binocular is brightest
when the pupil of your eye is the same size as the binocular exit pupil, since that's
when all of the binocular's light is entering your eye.

The pupil of the average youthful eye can dilate to about
7mm, but only in very low light. Accordingly, binoculars with large exit pupils
(over 6mm) are recommended only if you are young and do much of your birding in
deeply shadowed woods, or at dawn or dusk, when your eyes dilate enough to take
advantage of the large binocular exit pupils.

In daylight, your eyes' pupils contract to less than 4mm
in diameter. A binocular with an exit pupil larger than 4mm wastes light in the
daytime, for as much as 60% of its exit pupil will fall on your daylight-contracted
iris, rather than entering your eye. A 7 x 50 binocular, with its 7mm exit pupil,
is no brighter during the day than a 7 x 28 binocular with its smaller 4mm
exit pupil, since daylight contracts your eye's pupil to under 4mm. In the bright
light of day, 7 x 50, 7 x 42, and 7 x 35 binoculars are all no brighter than
a 7 x 28. There's no point in buying big binoculars with more light-gathering than
your eyes can use if you do little birding in low light. Be sure you really need
a large exit pupil before buying a large binocular.

If you observe from a moving vehicle, however - the deck
of a boat or a Land Rover on safari - a 6mm to 7mm binocular exit pupil can be helpful,
even though a light waster. It's easier to keep your eye fixed on the image in a
bouncing binocular if the bino's exit pupil is large.

For dawn or dusk birding, 4mm to 6mm exit pupil binoculars
are the most useful.

Binocular exit pupils of 3mm to 4mm are the most usable
in average or overcast daylight, as diffused daylight causes your eyes to contract
to this size.

Exit pupils below 3mm are mainly for bright daylight use,
or for desert or shore birding where you have considerable amounts of reflected
light and glare to deal with. You'll find binoculars in this exit pupil range to
be of little use in twilight or in dim and overcast conditions.

If you're uncertain which to choose, though, you're better
off with a binocular that has an exit pupil too large rather than one too small.
It makes more sense to waste some light with a overly-large exit pupil when it's
bright, rather than wish in vain for more light when it's dim.

Your eye's ability to dilate declines with age, falling
from 7mm to a maximum of about 5mm in late middle age. If you're in this age group,
binoculars with 6mm to 7mm exit pupils will be no brighter than a 5mm exit pupil
binocular in low light, as some of the light of those overly large exit pupils will
fall on your no-longer-fully-dilatable iris and be wasted.

On average, you lose about 1/2mm of maximum dilated pupil
size per decade between the ages of 20 and 50, declining to about a 1/4mm loss per
decade thereafter - from 7mm or so in your teens, to perhaps 5.5mm by the time you
reach 40, to 5mm at 50, etc. Your pupil size will depend on heredity, your general
health, and whether you smoke (smokers lose pupil elasticity faster).

There's no point in spending good money for more light-gathering
than your eyes can use, so don't go overboard on exit pupil size if you're past
middle age.

Relative Brightness: Most manufacturers use a specification
called "relative brightness" as a way to compare binoculars. The higher the relative
brightness, the brighter the binocular. To find the relative brightness, simply
square the diameter of the exit pupil. For example, all binoculars with 4mm exit
pupils have a relative brightness of 16 (4² = 16).

Since relative brightness is a mathematical function only,
and doesn't take into account aperture differences between binoculars, it should
be taken with a grain of salt if used for choosing binoculars. For example, 8 x
32mm and 20 x 80mm binoculars have identical 4mm exit pupils, but they're hardly
identical low light performers. The 625% larger light gathering area of a 20 x 80mm
pours 625% more light into its 4mm exit pupil than the smaller lenses of the 32mm
binocular. Relative brightness is only useful when comparing the low light performance
of binoculars of similar aperture - and then only when your eye's pupil is
as large as the exit pupils of the binoculars.

Some manufacturers arbitrarily boost the relative brightness
by 50% to derive a specification they call "relative light efficiency." This supposedly
takes into account the higher light transmission of coated optics versus uncoated
lenses. Since all modern binoculars have coated optics, however, "relative
light efficiency" has little justification other than to make such binoculars appear
to the unwary shopper to be 50% brighter than binoculars using the more generally
accepted relative brightness specification. Relative brightness and "relative light
efficiency" are not the same thing. When comparing binoculars using manufacturers'
specs, don't confuse an artificially-boosted "relative light efficiency" with relative
brightness. We use only the conservative relative brightness figures in our literature
so you can compare binoculars on an equal basis.

Twilight Factor: Exit pupil, relative brightness,
and "relative light efficiency" comparisons are interesting and often useful, but
they're not the best judges of how well binoculars perform in low light. For example,
8 x 32mm and 20 x 80mm binoculars both have 4mm exit pupils, a relative brightness
of 16, and a "relative light efficiency" of 24 - but the 80mm binoculars are much
better in low light due to their 625% larger light-gathering capacity and higher
"twilight factor."

The twilight factor is a more useful judge of a binocular's
low light performance than its exit pupil, etc., as it takes into account both light
gathering and magnification. Both factors affect how much detail you can
see - and seeing detail is what binoculars are all about.

Simply put, the larger an image, the easier it is for you
to see details in that image. By the same token, with a smaller image, the brighter
it gets, the easier it is for you to see the same details clearly. So, within reason,
if magnification goes up, brightness can go down without affecting resolution, and
vice versa.

It's like reading a newspaper in the light of a 3-way lamp.
If the lamp is at its low 50 watt setting, you have to hold the paper closer (making
the image larger) to read the fine print. If the lamp is turned up to 100 or 150
watts, you can hold the paper further away (making the image smaller) and still
read the same fine print. In other words, small bright images can show you as much
detail as large dim images.

The twilight factor allows you to compare different combinations
of aperture and magnification to determine the one that best balances an increase
in power against a decrease in brightness (or vice versa). The larger the twilight
factor, the better a binocular is in low light.

Twilight factors of 17 and higher are best for twilight
or early morning use.

A binocular's twilight factor is found by multiplying its
objective lens diameter by its magnification, then finding the square root of that
product. An 8 x 32mm binocular, for example, has a twilight factor of 16, while
a 20 x 80 has a twilight factor of 40, explaining the better low light performance
of the latter despite the identical exit pupils and relative brightness.

As with relative brightness, the twilight factor is a mathematical
relationship. It does not take into account light transmission differences between
binoculars due to differences in optical coatings, so small numerical differences
in twilight factors may not be visible in real life.

And just because an inexpensive binocular and a premium
model have identical twilight factors, you cannot assume that their optical performance
will be the same. Light transmission differences, distortion, and optical flaws
in the less expensive binocular can severely compromise its sharpness and clarity.

So, while it's a useful figure, don't let the twilight factor
be your only guide to choosing a binocular. Other factors can be just as important
as the ability to make out details in dim light. For example, at first glance, a
10 x 40 might seem better than a 7 x 42 in low light because of its higher twilight
factor (20 versus 17.1). But if most of your observing is done in dim wooded areas,
the wider field of view and greater depth of field of a 7 x 42 are better for locating
birds than the narrower and shallower field of a 10 x 40 - and finding the bird
is half way to identifying it.

But, keeping these cautions in mind, the twilight factor
still remains a more reliable guide to low light performance than exit pupil or
relative brightness.

Near Focus: How close you can get to an object and
still see a sharp image of it in your binocular is called the "close" or "near"
focus. For general birding, a close focus of 15' or less is the minimum recommended
- letting you observe backyard bird feeders, nests in low trees, etc. However, many
birders prefer a close focus of less than 10' for close-in observing of warblers.
Binoculars with close focus capabilities of as little as 4' are available for butterfly
watchers. Birders doing high power long-distance observing - shore birds, raptor-watching
- will find a 20' to 25' close focus is usually sufficient. Manufacturers' close
focus specifications assume 20/20 vision and individual binoculars can vary a foot
or two in either direction, depending on your eyesight.

Depth of Field: This is simply the distance in front
of and behind the point of sharpest focus that an image remains usefully sharp in
binoculars. Good depth of field minimizes the constant refocusing needed to keep
birds sharp and clear as they flit among branches of nearby trees. The higher the
magnification of an optical system, and the closer it is focused, the shallower
its depth of field will become. It is for this reason that high power binoculars
and spotting scopes are usually not well-suited for close-in birding. There is no
industry standard for a depth of field specification, so when we test binoculars
(as we do with every product we sell before we agree to carry it), one of our tests
is for depth of field. We focus at 20'. If the image is sharp enough to read 1/16th
inch tall printing located one foot in front of and one foot behind that 20' point,
we consider the binocular to have an acceptable depth of field. If the same letters
are equally sharp for 18" or more in front of and behind the 20' point of sharpest
focus, we consider it to have good depth of field and so note it in our catalog
description.

Field of View: The field of view is simply a measure
of how wide a piece of the landscape you see through a binocular. It can be expressed
either in degrees or in the linear diameter (in feet) of the circular image visible
in the binocular at a distance of 1000 yards.

Each time you divide that 1000 yard distance by ten, you
divide the field of view by ten. For example, if a binocular has an 8° field of
view (420' across at 1000 yards), its field will be 42' at 100 yards, 4.2' at 10
yards, and only 2.1' at 5 yards (only 25 inches across at 15 feet). Similarly, a
binocular with a 5° field has a field less than 16 inches wide at 15 feet, and less
than 8 inches across at a distance of 7.5 feet (well within the close focus capability
of most modern binoculars).

For close-in birding, the wider the field, the better your
chance of getting both bird and bird feeder in the field at the same time. At any
distance, the wider the field, the better your chance of first finding,
then keeping a bird in sight.

A wide field of view is sometimes achieved at the expense
of clarity at the edge of the field, however. Don't go overboard on width of field
when choosing a binocular unless you know it's sharp from edge to edge.

Your brain will blend the two binocular images into a single
circular field when you're focused on a distant object. When you focus on something
close to you, the object may appear to be in the center of two overlapping circles
of light, particularly with a porro prism binocular.

Eye Relief: The minimum distance between a binocular's
eyepiece and your eye that allows you to see the entire field of view is called
the eye relief. Long eye relief is important if you must wear eyeglasses
to observe, as your glasses can keep you from getting close enough to the eyepiece
to see the full field.

If you are
near or farsighted, you can usually observe with your glasses off, as binoculars
have enough focuser travel to accommodate these conditions. You can also usually
leave your glasses off for daylight observing if you have only mild astigmatism,
as the pupil of your eye will contract enough to minimize the astigmatism around
the periphery of your cornea. Severe astigmatism or severe nearsightedness, for
which glasses must be worn, require a binocular with long eye relief (15mm
to 17mm or longer) to allow you to wear glasses and still see all, or nearly all,
of the field of view.

A binocular with medium eye relief (10mm to 15mm) can be
used with eyeglasses, but you'll lose some of the field of view. To visualize this,
imagine that the binocular's eyepiece is a knothole in a fence and you're watching
a baseball game through the knothole. If you get your eye close to the knothole
(or binocular), you see the entire playing field. But, if you move your eye back
from the knothole, as eyeglasses effectively force you to do, you see only the central
area of the field. The image is still sharp and you're still able to focus properly.
It's simply that you can no longer see the whole field.

Excessively short eye relief can prevent your eyes from
blending the two circular binocular images into one, giving the effect of looking
down two tubes instead of through a window. Therefore, a binocular with eye relief
below 8mm or 10mm is not recommended for those observers who must wear glasses.

Unlike manufacturers' technically correct eye relief figures,
which are measured from the eyepiece lens (which is usually recessed into the binocular
body to protect it from being scratched), the figures in our specifications are
the usable eye relief values, determined by rolling down the binocular eyecup
and measuring the eye relief from the binocular eyepiece rim, where eyebrows or
glasses would touch (which limits how close you can get your eye to the binocular's
eyepiece). Consequently, our real life eye relief figures are often shorter than
a manufacturer's, but more realistic.

If you're a bifocal wearer, or only mildly near or farsighted
- a birder who could use binoculars with eyeglasses off - you might still
prefer a long eye relief binocular that lets you keep your glasses on while observing.
This allows you to switch back and forth between checking your field guide and using
your binocular, without having to constantly take your glasses off and put them
back on. Murphy's Law says that a bird will always fly away while you're taking
off your glasses to put your binocular up to your eyes. Being able to use your binocular
with your glasses on, even if you don't have to, can keep you from missing
a new bird for your life list.

Long eye relief binoculars are generally more comfortable
to use in any case, even if you don't wear glasses - as short eye relief can allow
your eyelashes to brush the binocular eyepieces annoyingly. This also deposits eyelash
oils on the eyepieces that can damage their coatings if not cleaned off regularly.

Interpupillary Distance: This is the distance between
the pupils of your eyes, measured from center to center. It is also the distance
between a binocular's exit pupils. If a binocular can't fold down or open up enough
for its exit pupils to line up with the pupils of your eyes, a shadow will cut off
part of the image you see in one eyepiece or the other.

If you suspect that your eyes are more closely or widely
spaced than average, make sure that your interpupillary distance falls within the
range given for the binocular in which you're interested. If you don't know your
interpupillary distance, look through a binocular that you know works for you and
measure the distance between the centers of its exit pupils when it's set for your
eye spacing.