How to choose Binoculars
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 handle.
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 precision.
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.
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Binoculars for Kids