Below are the factors to consider in selecting the binoculars that best fit your
preferences and needs. View the discussions under the assumption that all other
factors are equal. They rarely will be, but this will help with the importance factor
you assign to each spec in the O4B Scorecard. The specs we compare are:
More magnification results in larger objects in the view and more ability to distinguish
detail. It also means that the binoculars will be harder to hold steady, a narrower
field of view, and less apparent depth of focus. There are tricks to make it easier
to hold binoculars steady, but even 10x may be too difficult for some people to
Field of View
Field of view, stated in feet (or meters) at 1000 yards (or meters), is the width
of the scene that is in view. A wide field of view makes it easier to find birds
when looking through the binoculars. The trade-off is in loss of the ability to
resolve detail. Other factors being equal, lower magnification means wider field
of view, but increased eye relief and nearer close focus also narrow field of view.
Too wide a field of view will often result in distortion at the edges of the image.
This is the size of the image at the focusing point of the binocular. It is computed
by dividing the objective size by the magnification (power). You can find exit pupils
ranging from 7mm (7x50, 8x56) to 2mm (10x21). A larger exit pupil means a brighter
image. It also means that it is easier for your eyes to stay on the image when it
is bright out and your pupils are contracted.
Eye relief is very important to eyeglass wearers. It is the distance behind the
ocular lenses at which the image is in focus. Since eyeglass wearers can't get their
eyes as close to the lenses, longer eye relief will project the image beyond their
glasses. With sufficient eye relief, usually at least 15mm, eyeglass wearers can
see a full image. Long eye relief will usually reduce field of view.
If you are nearsighted or farsighted, you can use your binoculars without wearing
glasses and the binocular's focus will compensate. However, if you have astigmatism,
you will need to use your glasses.
All binoculars can focus at infinity. The real trick in binocular design is in how
closely it can be made to focus. Any birder (or butterflyer) who started out with
a cheap pair of binoculars has experienced standing 10 or 15 feet behind the rest
of the group to look at a bird that was inside their close focus capability. General-purpose
binoculars have a close focus around 20 to 25 feet. A good birding binocular should
have a close focus of 10 feet or less, with the current close focus champ coming
in at a mere 3 feet. (Sure wish I had those when the Townsend's Warbler landed on
my knee during a pelagic trip.)
In the optic equation, objective lenses are the light gatherers. Larger lenses let
in more light. A 50mm objective lets in nearly 42% more light than a 42mm objective,
and twice that of a 35mm lens. More light gathering means more detail when the light
level drops, and therefore, more quality birding time when the birds are active.
There are two types of prism glass currently in use: BK-7 and BaK-4. Both names
indicate the element used to modify the glass composition. Ba is barium; B is boron.
BaK-4 is the better and more expensive prism. Any binocular that has BaK-4 prisms
will probably say so in their specs. If the prism glass is not specified, assume
it is BK-7.
Both prisms and lenses get coated. The main reasons for using them are to reduce
internal reflections and correct the focusing of the various wavelength of light.
The number and types of coatings are discussed in the All About Optics section.
Each level of coating adds cost but improves the image.
Body (Prism) Design
This is one of the most basic choices in choosing your binocular. Porro prism binoculars
are less expensive to make than roof prism binoculars. You can get the same optical
quality for significantly less money. However, they are heavier and harder to weatherproof.
A roof prism binocular of the same optical quality will be lighter and have fewer
problems. Even though they cost more, they will probably be more rugged, and in
the long run, they may also be more cost effective.
F=ma. All you really need to know about weight is Newton's Second Law of Motion.
Yeah, right. You also have to consider the implications. What we're really talking
about is mass. Weight is mass at gravitational acceleration, i.e. directed downwards.
It takes more force, and therefore more energy, to lift a more massive binocular,
hence you will feel more fatigued at the end of the day. But forces don't only point
down, and Newton's First Law of Motion tells us that it takes force to change a
body's motion. A heavier binocular will also resist moving, so it should be easier
to hold steady.
If you take the time to do the research, you will find a wide range of weight specifications
for binoculars in the same size class. One manufacturer has a line with a 10x42
that weighs 25 ounces and a 10x50 that weighs 29. Another has a line with a 10x42
that weighs 30.7 ounces, and a 10x50 that weighs 40.9. As you can see, objective
size contributes to the weight of a binocular, but may be only a small factor. A
lot depends on other design specifications. If you want the extra light gathering
capability, you can find binoculars that are within a useable weight range.
The bottom line about weight is that it is one of the most overrated of all binocular
specs. Today's construction materials are lighter and stronger than ever before.
With some mid-priced, bright, quality, 10x50 binoculars weighing less than 30 ounces,
any able-bodied adult should be able to handle them easily. Unless you are very
experienced, you would have a hard time picking up a binocular and telling whether
it weighed 28 ounces or 32. Even over a day's use, it is doubtful most people would
know the difference.
The real concern should be balance rather than weight. A well balanced binocular
should put very little stress on your arms, wrists, and hands. A poorly balanced
pair, even if it is lighter, will strain your muscles by forcing them to work against
the unnatural torque.
Ease of Focusing
Often overlooked until it is too late, focusing mechanisms can be a joy or a pain,
depending on your preferences. The keys to a good focusing binocular are speed and
If you can't change focus quickly, you'll miss the bird. A good focusing mechanism
will go from close focus to infinity in a full turn (360º) or less. Most binoculars
have a linear focusing gear. If the gear has a slow gear ratio, it will do well
in the close range, but will be difficult to focus in the distance range. If the
gear ratio is fast, it will be difficult to focus in the close range but excel at
long distance. A new innovation in binocular focusing is the variable speed focusing
gear. With this type of gear, the focus is slower close in and faster farther out.
If you can't get a sharp focus, the image will be difficult to look at. At any distance,
you need both the proper focusing speed and depth of focus to be able to hit the
mark. If a binocular has shallow depth of focus, it will take extra effort to get
the image tack sharp. This can also affect focusing speed when close to proper distance.
Eyecups help keep stray light away from our eyes while using binoculars, and help
measure the distance from the oculars to our eyes. Originally, eyecups were made
of rubber and could roll up or down depending on whether the user wore eyeglasses.
The problem was that repeated rolling cause the eyecups to break. The first advancement
was eyecups that slide rather than roll, but these were hard to keep in place, so
they were replaced by eyecups that twist up. They can be left at any position from
all the way up to all the way down. The newest eyecups have click stops at regular
intervals in the twist-up path with the eye relief distance for each stop marked
on the cup.
Weatherproofing ranges from none to showerproof to waterproof to nitrogen purged.
Waterproof sealing is done with rubber "O" rings. Active birders should get nitrogen
purged waterproofing, whether they live in a humid climate or not. Nitrogen purging
can also keep out dust, sand, or anything else that can get inside the binoculars.
That's why we prefer the term "weatherproofing".
Usually made of a synthetic rubber, armoring protects the body of the binocular
from physical harm due to bangs, bumps, and corrosive elements. The more you pay
for your binoculars, the more you want to treat it like a long-term investment.