The following listing is provided to assist users in understanding the various aspects of binoculars terminology.  While it isn't necessary to become an expert on this terminology, it can be helpful in analyzing the features of binoculars and selecting that one that best fits your need.

Anti-Reflective Coatings.  Purplish or greenish tints in the glass to help eliminate internal reflections, light scattering, and to reduce glare.

Astronomical Binoculars.  Should have an objective lense rating of at least 40mm in order to gather enough light for acceptable solar viewing.

Angular Field of View.  Expressed in degrees.  One degree equals 52.5 feet.  Multiply to convert to Linear Field of View (6.5 degrees (Angular)  X 52.5 feet = 341.25 (Linear)).

Argon Gas.  An inert gas put back into binoculars to create water proofing.  Argon gas maintains a higher level of protection than does nitrogen gas.

Alignment.  The mechanical and optical elements of the binoculars are in sequence with each other.  In proper working order.  Good alignment implies optimized performance.  See missalignment.

Armored.  A surface coating provided over the binoculars that helps absorb shock and provides for a more firm and secure grip.

BAK-4 Prism.  Higher quality glass that transmits almost all light gathered by the front object lens, through the binoculars, to the rear ocular lens.  This higher quality glass and the associated higher rate of light transfer creates a brighter, clearer image to the eye.  Looking through the eyepiece, if a round beam of light is seen this suggests that a BAK-4 prism is being used.

BK-7 Prism.  A lower quality glass that lowers the brightness and clarity of the image.  Looking through the eye piece, if a square shaped beam of light is seen this suggests that a BK-7 prism is being used.

Birding Binoculars.  Binoculars in the 8 X 40 range appear to be the most popular bird watching model as they can be held steady by hand for viewing.  Also look for high light transmission for image clarity.  The minimum ratings seem to be 8 X 30 with a field of view of at least 350 feet at 1000 yards.

Contrast.  The difference in brightness between the light and dark areas of an image.

Coating.  Coatings are operations performed on the lens and prisms for the purpose of reducing reflections and to increase light transmission.
     Coated (C) = A single coating is applied to at least one lense surface.
     Fully Coated (FC) = A single coating is applied to all air-to-glass surfaces.
     Multi Coated (MC) = Multiple coatings are applied to at least one lense surface.
     Fully Multi Coated (FMC) = Multiple layers are applied to all air-to-glass surfaces.

Coated Optics.  One or more of the surfaces are coated with one or more anti-reflective coating films.

Chromatic Aberrations.  Different colors move at different wave lengths and will have different focal lengths when passing through the binoculars.  XD and ED type glasses can reduce or eliminate the inherent problem of chromatic aberrations.

Center Focus.  A smaller wheel, generally located between the rear two ocular lens, that is used for general focusing.  This adjusting wheel allows you to focus both the right and the left lens simultaneously.  The two step process first adjusts both eyes to the same strength and then will focus both eyes simultaneously.

Close Focus.  The minimum distance you need to be away from your object in order to get a clear focus.  Ten feet or less is generally best suited for viewing birds, insects, or butterflies.

Collimination.  Refers to the optical and mechanical alignment of the binoculars.  If the alignment is not perfect it will cause totally unstable images and unacceptable viewing.

Distortion.  The inability of binoculars to produce an image that is a true to scale reproduction of the object.  Typically distortion is caused by a poor optical design.

Diopter.  Generally a part of the right rear eye piece that is used for adjustment focusing.  The diopter makes it possible to tune-in the viewing process, to adjust for unequal vision of the object.  It also allows individuals to compensate for difference in their optic viewing needs, or difference in their eyes.

Diopter Control.  Seperate focusing of each eye piece, which allows for better precision.

Diopter Adjustment.  Allows for compensation for the difference between the strength of the users left and right eyes.

Dielectric Mirror Coatings.  Lenses coatings that can provide reflectivity of more than 99% across the light spectrum.  An improvement over aluminum or silver mirror coatings.

De-alignment.  Often caused when binoculars have been dropped, jarred, or otherwise experience some severe shock.  De-alignment typically causes blurring or double images to be viewed.  See alignment.  See shock proof.

Exit Pupil.  The light that leaves the binoculars and enters the user's eye.  The larger the exit pupil, in millimeters, the brighter the image will appear.  Determined by dividing the front objective lens value by the power of the binoculars.  In an 8X42 binoculars the exit pupil rating is 5.25mm (42/8 = 5.25).  A mathematical calculation.

Eye Relief.  The distance in millimeters between the ocular lens and where the image comes into focus and the full field of view can be seen.  The physical point where the user's eye must be located in order to clearly see the image.  Anyone viewing for extended periods of time should have a binocular with an eye relief of at least 15.0mm.

Eye Glasses.  Wearers of eye glasses are presented with a condition where there is a space between their eyes, their eye glasses, and the rear lens of the binoculars.  Eye glass wearers need to be able to compensate for those intervals.  Normally eye shields are turned down in order to realized the full field of view through the binoculars.  Eye relief of at least 15.0mm is recommended for wearers of eye glasses.  See Eye Relief.

Fully Coated Optics.  Optics that have all air-to-glass surfaces coated with an anti-reflective coating film.

Flexible Eye Cups.  Eye cups that fold back to allow for maximum eye relief.

Focus.  The adjustments necessary to make the object clear and sharp.  See Center Focus.  See Individual Focus.

Fixed Focus.  A feature done during the construction of the binoculars.  In effect there is no individual focus allows as the lenses are installed in specific locations.  Usually not recommended.

Fog Proof.  The process of adding nitrogen or argon gases to the binoculars to prevent condensation internally.  See Waterproof.

Field Of View.  The distance from left to right that you can see through the binoculars when 1000 yards away from your subject.  A wider field of view is generally suggested when scanning terrain, or when viewing fast moving objects.  The closer one is to their object, the less field of view is available.  Generally more significant when viewing objects at a close range.  See Wide Angel.

Fully Multi-Coated Optics.  Optics that have all air-to-glass surfaces coated with multiple anti-reflective coating films.  This offers the highest image quality.

G.

Higher Magnification.  As the second value of binoculars increases they tend to have a shallower depth of field, are less stable (subject to image shake), and have a smaller field of view.

Hunting Binoculars.  The 10 X 42 binoculars seem most popular because of their ability to gather more light during the early morning and late evening hours when much wildlife activity takes place.  Minimum features appear to be 8 X 30 with a field of view of at least 350 feet at 1000 yards.  Variations have to be made depending of the hunting terrain (open areas, mountains, wooded areas, etc.).

Individual Focus.  The adjustment of the right rear ocular lens to your right eye and the left ocular lens to your left eye.  See Military Way.

Individual Ocular Adjustment.  Each of the rear lens are adjusted seperately.  This is the easiest and fastest focus method.

Interior Focus.  The front objective lens and the rear ocular lens are stationary, which prevents moisture, air, or dust particles from entering into the binocular construction.  This practice is typically fround in quality products.

Inversion System.  The process that makes the image appear as normal.  Without an inversion system the image would be reversed and upside down.

Image Stabilization.  Built-in sensors that detect movement, shaking, or vibrations.  As these actions are detected the optics are adjusted to compensate, and corrects to maintain a constant, steady, view of your object.

J.

K.

Light Transmission.  The percentage of light that passes through the binoculars and reaches the user's eye.

Long Eye Relief.  The standard is generally 15.0mm or higher.

Light Transmission.  The amount of light gathered by the front objective lens, measured against the amount of light leaving the rear ocular lenses.  The higher the amount of light transmission, the better quality of binoculars.  A true test of the binoculars quality.  See Twilight Factor.

Luminosity Factor.  See Exit Pupil

Limited Warranty.  Normally covers only the initial defects with no protection from wear and tear or accidential damage.

Lenses Coatings.  The type and number of coatings applied to the lens will make a significant difference in how bright and crisp the image will be.  More coatings increase light transmission, resolution, contrast, and color fidelity.

Linear Field Of View.  Expressed in feet or meters.  Divide by 52.5 feet to arrive at the Angular field of view degrees (i.e. 350 feet (Linear) / 52.5 feet = 6.66 degrees (Angular)).

Low Light Capability.  Binoculars that have at least 90% light transmission.

Long Distance Binoculars.  Ideal for astronomy and long distance terrestrial viewing.  Larger, heavier, and normally require a tripod for stabilization.  Range in ratings from 10X to 40X and 70mm to 120mm.

Lenses Maintenance.  Clear all lenses of dirt, debris, and finger prints after using the binoculars.  Cover all lenses with lense covers when not longer being used to prevent scratches.

Multi-Coated Optics.  One or more surfaces are coated with multiple anti-reflective coating films.  This process increases the light transmission, resolution, contrast, and color fidelity.  Without multi-coating the binoculars could loose upwards of 50 percent of their orginial light.  The more multi-coating, the better.

Metallic Coating.  This type coating assists in reducing internal binocular reflections, light scattering, and glare.

Military Way.  See Individual Focus.

Magnification.  The first value of the binoculars rating.  An 8X42 binoculars will cause the object to appear 8-times larger than if viewed by the naked eye.  Put another way, and object viewed through an 8X powered binoculars will appear 8-times larger, or be brought 8-times closer to you.  Binoculars with 7X power are generally considered best for marine (boat) activities, 8X power is best suited for woodland viewing, and 10X binoculars are mostly used for more distant viewing.

Mirror Coating.  Aluminum mirror coating provides for 87% to 93% reflectivity.  Silver mirror coating provides for 95% to 98% reflectivity.

Night Vision.  Even in the darkest of the night there still is some light that the human eye cannot see.  However, a night vision binoculars will take this ambient light, magnify it, and convert it into light that we can then see.

Nitrogen Gas.  The inert gas that is filled back into the binoculars to create waterproofing and fog proofing.

Objective Lenses.  The front lens of the binoculars.  The larger the objective lenses, the more light it can gather, with the end result being a brighter image.  The size of the objective lenses is the greatest factor in determing the binoculars overall weight.  In quality products the objective lenses is actually a system of two or more glasses of different types.  If you only use your binoculars during the bright day time hours, then an objective lenses in the 25mm range will do.  If you use your binoculars for viewing near dusk or early morning then lenses in the 35mm range are probably necessary.

Ocular Lenses.  The rear lenses, nearest to your eye.  This lenses magnifies the image that was gathered by the front objective lenses.

Optical Glass.  This glass makes the difference in how bright, how sharp, and how colorful the image will be.  As quality increases, weight also increases.

Progressive Warranty.  This type warranty covers the binoculars in any situation, no matter what happens or who is at fault.

Prisms.  Properly orientates the image, otherwise the image would appear upside down and backwards.

Phase Coating.  Coating that keeps the light in its correct color phases.

Porro Prism.  The style of binoculars where the front objective lens are spaced further apart than the rear ocular lens.  Generally this type binoculars offer a wide field of view and a three-dimentional image.

Quality.  Better optics are produced through better quality glass.  Quality glass can be very expensive.  The difference between a $100 product and $500 binoculars typically comes down to the quality of the glass used in its production.

Roof Prism.  A style of binoculars where the front objective lens and the rear ocular lens are generally in the same line.  These type binoculars typically offer durability and streamlined construction.

Resolution.  The ability to distinguish the fine details of an object being viewed.

Rubber Coating.  Generally allows for a better grip.  Not associated with waterproofing.

Rangefinder.  Included on some long distance binoculars.  Help to calculate the distance to some object.

Relative Brightness Index (RBI).  Computed by squaring the exit pupil.  For example:  7 X 35 binoculars.  The exit pupil is found by dividing the objective reading by the power (35 / 7 = 5.0mm exit pupil).  Then squaring the exit pupil (5 X 5 = 25) provides an RBI of 25.  An RBI reading of 25 or higher indicates that the binoculars are good for use in dim light.

Shock Proof.  Binoculars that are able to absorb severe shocks from falls, rough handling, or other significant jarring actions.

Stabilization.  Accomplished during the construction of the binoculars to significantly reduce the impact of motion caused by the movements of a boat, an automobile, or a train.  See Image Stabilization.

Sporting Event Binoculars.  Binoculars for viewing fast moving sporting events are normally found in the 8X or lower power range.  Binoculars with a higher magnification have a smaller field of view, which would make following the action more difficult.  Minimum standards appear to be in the range of 7 X 25 with a field of view of at least 325 feet at 1000 yards.

Twilight Factor.  Typically associated with night vision binoculars.  The features generally indicate a good night, or low vision, product, with 90 percent or higher light transmission, a 5-7mm exit pupil, and a magnification factor of 10 or higher.  See Low Light Capability.

Tripod.  A stabilization frame used to steady higher magnification binoculars to prevent image shake and movement.

Tripod Adaptable.  Meaning that the binoculars can be mounted on a tripod.  However, you must still purchase the adapter and the tripod.

Terrain.  Has a significant effect on the type binoculars to select, especially for hunters.  Binoculars for opens spaces and distant viewing usually require power in the 10X or higher range.  Mountain viewing, dense forests, open grass land, marsh area all require additional consideration.

U.

Variable Optics.  Binoculars that allow for changing to different degrees of magnification.  See Zoom Binoculars.

Wide Angle.  When the field of view is at least 60 degrees.

Waterproof.  These type binoculars are generally sealed with o-rings to prevent moisture from entering.  The binoculars are then purged of air and filled back with nitrogen or argon gas.  The process of waterproofing also causes binoculars to be fog proof because moisture is unable to enter into the binoculars.  Waterproofing also allows the binocualrs to have a greater productive life.  Waterproof binoculars can be immersed in water - spash proof or water resistant products cannot.

XD High Density Glass.  This type glass delivers the highest qulity images.

Y.

Zoom Binoculars.  Binoculars that allow the user to change to several different types of magnification.  Their rating designation differs in that they have two power values.  As an example, a 10-30 X 42 zoom rating indicates that the magnification can be adjusted between a power rating of 10, to any other value up to 30.