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Each binocular made have a set of numbers assigned to them such as 6x15, 7x35, 7x50, 8x30, etc. The first number (6x, 7x, 8x) refers to a level of magnification usually called the "power" of a binocular, namely the extent the binocular magnifies objects being viewed. So a binocular of 8x makes an object look 8 times larger than it does to the naked eye and consequently makes it appear eight times nearer. The last number (15, 35, 50) refers to the diameter of the objective lens in millimeters. The objective lens is usually the largest lens farthest from the eye. The lens closest to the eye is called the ocular lens.

It is unwise to select binoculars solely on the basis of power. Some people think that the higher the power number, the better the binocular. This is not always true. The power of the binocular must be carefully chosen to provide exactly the right magnification for your particular use.

The clarity and definition of the subject being viewed decreases as the power increases. In addition, a binocular magnifies BOTH the object being viewed AND any motion caused by shaky hands, moving vehicles, water motion on a boat, etc.
It is generally recognized, therefore, that above 8x, a binocular should be used with a tripod to obtain best results.

How does power affect field of view or brightness?

A high power number usually means a smaller field of view and lower brilliance of the image that reaches the eye. Binocular brightness is critical for indoor sports, outdoor birding in shaded areas and hunting.

All binoculars can be used at night or during the daytime. But certain binoculars are better adapted to night viewing than others; some even have "night vision" modes. Binoculars with the largest objective lenses are desired because more light can be admitted. The average diameter of a person's eye during the daytime under average conditions is about 4 millimeters but at night expands to about 7 millimeters.

Therefore, a binocular that utilizes the full diameter of the expanded pupil is better for night use. Thus, a 7x5O which has a large exit pupil (7 mm) and utilizes the full diameter of the expanded pupil of the eye is considered to be an especially fine night binocular.

What is relative brightness & exit pupil all about?

Relative Brightness and Exit Pupil are determined mathematically. The exit pupil of a binocular is the disc of light you see in the eyepiece when holding a binocular at arms length towards a bright light. To compute Exit Pupil: divide the diameter of the objective lens in millimeters by the power of the binocular. To compute Relative Brightness: square the Exit Pupil. Example 7x35, means the prism binocular described magnifies seven times and has an objective lens of 35 millimeters. Exit pupil equals 5 and 5 squared equals a relative brightness of 25.

What is field of view?

Field of view refers to the diameter of the area seen through a binocular. It should be noted again that in line with the laws of physics, the higher the power the smaller must be the field of view and vice versa. Field of view is often expressed as an angle. Retractable eyecups allow eyeglass wearers a full field of view.

What are the different focusing methods?

"Central Focusing" binoculars are focused by turning a single wheel that move the lens elements together. The right eyepiece can be adjusted to compensate for any difference in vision between the right and left eye. Because they are convenient to use, Central Focusing binoculars are the most popular.

"Individual Focusing" binoculars are adjusted for each eye at the eyepiece. Because they are adjusted individually, this type on binocular can be nearly moisture proof and sturdy in construction. Boaters, hikers and sportsmen tend to choose this type of design.

What about alignment?

It is important that both barrels of a binocular be optically parallel so that the image from both barrels will merge into one perfect circle; otherwise, undue strain is caused and, in extreme cases, it is impossible to resolve the two images. High quality binoculars are very carefully checked for alignment.

Does Binocular construction matter?

Consistent with strength requirements, most good binoculars are manufactured of the lightest materials available and are as lightweight and compact as the optical formula and built-in ruggedness will permit. Finest quality ground and polished prisms are clamped and screwed to carefully machined recesses in bubble-free castings. Exterior surfaces are protected with multiple coats of a synthetic enamel or with high quality textured plastics. Each binocular is generally dehumidified and sealed in air-conditioned, dustless, final-assembly rooms.

What are coated lenses and prisms?

All air-to-glass surfaces on most binoculars are coated with specially formulated optical coatings that eliminate internal glare and reflections while increasing light transmission significantly. Most objective lenses are coated with an ultra-violet coating which minimizes the effect of the sun's rays. Most prisms are chamfered to absorb diffused rays, giving brighter, clearer images.

What is the difference between weather-resistant and waterproof binoculars?

Manufacturers rate their products by the "Japanese Industrial Standard" (JIS). Ratings are established through tests to prove protection against the ingress of water. It is important to understand that there are degrees to which an instrument is "waterproof." The classifications range from one to eight. Eight is the highest level of water protection. Here is a breakdown of the classification system and where Pentax binoculars fall:

Class 4:Splash-proof from any direction
Class 5: Jet-proof from any direction
Class 6: Watertight submersible to 1 meter
Class 7: Submersible to 1/2 atmosphere (16.5 feet)
Class 8: Products designed specifically for underwater use such as an underwater camera.

To make a binocular weather-resistant, special "O" rings are used in constructing the binocular to keep water from entering the instrument. To make a binocular waterproof, "O" rings are used in a special internal design that allows the binocular to be totally sealed. The binocular is then filled with nitrogen which makes it waterproof and submergible. Weather-resistant and waterproof binoculars are also "fogproof."

What should I look for if Astronomy is my purpose?

	magnification
	buy as much magnification as you can afford … typically 10x, 12x, 15x power
	weight
	in astronomy you will frequently be viewing the same object for some time -- lightweight binoculars are an advantage

	tripod socket
	some binoculars come with a screw thread hole for attaching a tripod attachment (others can use a small adapter which clamps on to the middle part of the binoculars)
	optical quality
	check that stars appear as point sources, with no sign of chromatic aberration or fuzziness. · anti-reflection coatings -- these result in more of the light actually getting through the many lens surfaces to reach your eye
	focusing
	look for a smooth, fine control. For astronomical use avoid autofocus types
	Stabilization
	some binoculars have an optical stabilizer built-in to minimize shake motion

What should I look for if I'm into birding?

Many people ask "I want to buy a pair of binoculars as a gift for a friend who's into birdwatching, which ones would you recommend.?". In Jack Connor's book entitled "The Complete Birder: A Guide to Better Birding. (Houghton Mifflin, 1987)", he offers the following comments. Keep these things in mind when you go looking for binoculars:
1.	An important point of magnification mathematics is the principle of diminishing returns. Each step up in power gains you less. The way the eye and mind process visual information makes a magnified image seem closer rather than larger, and for this reason the 10x's seem just 10 % more powerful than the 5x's.
2.	Most birding is conducted well inside the range of lower-powered binoculars, so other factors must be considered.
3.	The second principle of diminishing returns in magnification mathematics is the higher the magnification, the lower the light transmission. When all other things are equal, lower-power lenses allow more light to reach the eye than lenses of higher power.
4.	An under-publicized fact is that in ordinary daylight any binocular of equal optical quality will be equally as bright, no matter what the differences are in their objective size and power.
5.	For an evening search for owls or rails, you may want to borrow a pair of 7x50s, the binocular size that amateur astronomers swear by.
6.	Field of view is crucially important at close range. When you consider field of view (the size of the area you can see in the binoculars), you should convert the measurement by moving the decimal point two places to the left. Binoculars that give you 250 ft. at 1,000 yards are giving you 2.5 feet at 10 yards.
7.	Another example of the principle of diminishing returns: the higher the power, the narrower the field of view.
8.	If you wear eyeglasses and want to see as well as you can, fold-down the rubber eyecups are a must. Those who do not wear glasses should also look carefully at any binoculars they are planning to buy to see how deeply the ocular lenses are recessed. Some poorly designed models have such deeply recessed oculars that full field of view is not possible even for people with 20/20 vision.
9.	Binoculars that focus near at hand are essential for successful woodland birding. A close-focus range of 15 ft. to 18 ft. is the least you should accept.
10.	Porro-prism design: Advantage: quick focus, larger exit pupils, focus more closely, wider field of view. Disadvantage: weight.
11.	Roof prism design: Advantage: better power-to-weight ratio, focuses more finely (though not as closely); good for hawk watching and shorebirding; water resistant and durable. Disadvantage: Cost.
12.	Pocket design: Advantage: weight; portability. Disadvantage: image darker; very narrow field of view. If there's a simple rule to follow … spend as much as you can afford. If you're an active birder, you'll be looking through your binoculars for hundreds of hours each year. Why not treat yourself? A trip to Alaska will cost you more than even the most expensive binoculars. The trip will last a week or two. A really good pair of binoculars can last your

What are the types of binoculars available?

Standard binoculars
A standard or full-size binocular can be used for everything from nature observation to spectator sports.

Compact Binoculars
Compact binoculars are smaller and lighter in weight and are a good choice to take along to the theater or concerts or on hikes and hunting trips.

Wide angle Binoculars
Wide angle binoculars are ideal for tracking fast-moving action across wide areas such as football fields, racetracks and wilderness terrain.

Zoom Binoculars
A zoom binocular allows the user to increase the magnification in order to focus in on the details. From distant to near view, it's the best of both worlds.

Waterproof binoculars
Waterproof binoculars deliver clarity despite foul weather conditions including fog, rain and ice. O-ring sealed and nitrogen purged for reliable fogproof, waterproof performance.

What are PORRO and ROOF prisms anyway?

Roof Prism System
In roof prism binoculars the prisms overlap closely, allowing the objective lenses to line up directly with the eyepiece. The result is a slim, stream-lined shape in which the lenses and prisms are in a straight line.

Porro Prism System
In porro prism binoculars the objective or front lens is offset from the eyepiece. Porro prism binoculars provide greater depth perception and generally offer a wider field of view.