The first factor that most people look for when choosing a binocular is magnification. However, there is much more to consider in order to make sure that you are getting the right binocular to suit your needs. My Credentials

     I have personally examined thousands of binoculars. I have used many different types of binoculars, spotting scopes, shooting scopes and telescopes. I owned a camera store and in my lifetime I must have sold over ten thousand pairs of binoculars. I have studied binoculars and all types of optics and I have bolstered my understanding of optics by studying how the human eye works in great detail...Enough about me, let's get to the subject at hand...

• Magnification    

     The main purpose of binoculars is to magnify an image and make the subject appear larger so that you can see it with greater clarity and detail. This as opposed to a telescope, whose purpose is to gather light from faint and distant objects. On the binocular itself you will find a set of numbers such as 10X50. The magnification factor is the first number, 10X here (note that the X is part of the magnification spec) which means that the binocular will make it seem as though you were ten times closer to the subject . For example, if you were sitting 50 yards away from the action at a football game with a pair of 10X binoculars, it would seem that you were only 5 yards away (50/10=5), a good magnification for this particular use. .

     Too much magnification can be a problem if you are already close to the subject, as in the theater. If you were sitting in the fifth row, about 20 feet away, with the same 10X binoculars, it would seem as though you were 2 feet from the actors! You would probably find it hard to follow the action and you might even get dizzy from moving your head back and forth so much. That's why most theater binoculars are usually low power, around 2X, 3X or 4X. Of course if you were standing in the back with the SRO crowd you might want a pair of 7X. Binoculars whose magnification is greater than 10X or 12X should be placed on a tripod or other steadying device to help you hold them still enough to see the subject clearly.

• Gathering Light

     An equally important factor is the ability of a binocular to gather enough light for you to see under the available light circumstances. The second number written on the binocular (the 50 in our previous example, (10X50) is the diameter of the objective (front) lens. The bigger the objective lens, the more light will be captured. However, bigger objective lenses also mean bigger and heavier binoculars and many times smaller objective lens will get the job done. Here is where a little understanding of how the eye works comes in handy.

     The amount of light that comes out of the eyepiece is called the exit pupil, which is the cone of light coming out of the eyepiece and projecting into your eye. It is calculated by dividing the magnification into the objective lens diameter. In our example (10X50), 50/10=5 or a 5mm exit pupil. During the day or at a brightly lit sporting event or concert the pupil in our eyes is only open about 2 or 3mm wide, so we don't need a 5mm exit pupil.. The much more compact 10x25 binocular will give us the same 10X magnification with a 2.5mm exit pupil, more than enough to enjoy the event and a great size and weight savings. Of course if you were a mariner looking for buoy numbers at dusk or an astronomer looking for dim stars, you would want all the light gathering ability you could get. Binoculars for astronomy, sometimes called Rich-Field Optics because they capture a wide area of the sky, usually have 70, 80 or even 100mm wide objective lenses. They are so big and heavy that you must have a tripod to hold them steady.

• Field of View (or Angle of View)

     Field of view is usually shown on the binocular also. This tells us how wide our viewing area will be at 1000 yards away (usually). Standard binoculars may have a range of 250 to 300 feet across at 1000 yards while wide angle binoculars can have a field of view ranging from 350 to over 500 feet.  Good wide angle binoculars should have BAK-4 prisms (more about prisms lateron) and better optics for good edge to edge sharpness.

• Anti-Reflective Coatings

Anti-reflective coatings are also an important factor in the ability of a binocular to provide a crisp, high contrast image to your peepers. The best kind of coatings are known as "Fully Coated" optics which mean that every air/glass surface has received an anti-reflection coating. This is particularly important for delivering the best possible image to your rods and cones. Coating the front side of the optics decreases reflections and therefore increases the amount of light that passes through the lens. Coating the back side of the lens also reduces reflections which might reduce contrast and the crispness of colors. Don't bother paying extra for multi-coating as that is just a multi-layered coating on only the front of the objective lens. While fully coated optics can increase light transmission by as much as 7%, multi coated optics only enjoy a 2 to 3% increase.

•      One BIG caveat about coatings.

     While the anti-reflective coatings can have a particular color to them on the front of the objective lens, some unscupulous sellers will advertise that the color indicates that these are Night Vision binoculars, or if the coating is red, "Infra-red" binoculars. Nothing could be further from the truth (liar, liar, pants on fire). While good fully coated optics can add as much as 7% to light throughput, they don't come close to the light amplification of true night vision optics. Infra-red viewing systems require an external infra-red light source.

• Prisms

Prisms reduce the physical length of the optical path and turn the upside down image rightside up. Basically there are two different  prisms designs, roof prisms and porroprisms.

1. Porro Prism Systems

     In porro prism binoculars the objective lens is offset slightly from the ocular (eyepiece). They are easier to design and manufucture so are found in less expensive but still good quality binoculars.

     2. Roof Prism Systems

     In roof prism binoculars, the two prisms overlap so that the objective lens and eyepiece line up directly, giving you a more compact, straight-through binocular. Roof prism binoculars are less bulky but they are a bit more complex to design and manufacture so they tend to cost a bit more.

Types of Glass Used to Make Prisms.

     Commonly there are two different types of glass used to manufacture prisms.

1. BK-7 is the most common type of glass used for binocular prisms. It is made up of  borosilicate glass, a crown glass with good optical qualities, low dispersion and a low index of refraction.
2. Bak-4 (Barium crown) glass is used for finer prisms.Bak-4 produces a rounder exit pupil and a brighter image with greater edge to edge sharpness. It usually found only on better binoculars. It costs a bit more but in my opinion is worth the extra price.

One simple test of prism quality is to hold the binocular at arms length, pointing it towards the light and looking to see if the if the light coming out of the eyepiece is round or if it has a square appearance with grey edges. That is BK-7.

• Other Factors

For special uses, such as boating you may want to consider a pair of water-proof binoculars. Water resistance is usually an exaggerated term where the binoculars have a rubberized covering which offers little to protect the inside of the binocular. Real water-proof binoculars have seals to keep any moisture from entering the unit and  they are nitrogen purged. Nitrogen purged binoculars have all the moisture-laden air removed by pumping in a dry gas (such as nitrogen) before the unit is sealed. Besides protecting the inner workings of the binocular, it helps to prevent the inside optical surfaces from fogging-up like your car windshield when you move the binocular from the warmth of your home to the cooler environment outside.

Besides the optical quality, is the issue of the of mechanical quality. One indication of this is the type and length of the warranty. Really good binoculars will have a really good warranty. For example, certain Nikon Binoculars have a life-time warranty.