PRODUCTS

GALLOWAY HOME > PRODUCTS > BINOCULARS> BINOCULAR DEFINITIONS

Definitions

Power :: Objective Lenses :: Field of View :: Exit Pupil :: Eye Relief :: Prisms :: Optional Coatings :: Near Focus :: Brightness :: Collimation :: Focusing Systems :: Interpupillary Distance :: Adjusting Focus
Power
The power (or magnification) of a binocular is simply the degree to which the subject is enlarged.  For example, with a 7x42 binocular, the first number (7) indicates the binocular power, or magnification.  Specifically, the seven power magnifies seven times the size of the normal unaided human eye.  If the object were 700 feet away, it would appear to be 100 feet away.  More power is not always better.  Higher power affects brightness, reduces field of view and eye relief.
Objective Lenses
The front or larger lenses opposite from the eyepiece lenses.  With a 7x42 binocular, the second number (42) indicates the diameter in millimeters of the objective lenses.  The purpose of the objectives is to gather incoming light and form a sharply focused image of distant subject matter.  (Note: 42mm is approximately 1 5/8".  1 inch is approximately 25mm)  The larger the objective's diameter, the more light the binoculars gather, and the brighter the resultant image, with more detail and sharpness.  The effect is most pronounced under low light conditions or night time use.  If you double the size of the objectives, you quadruple the light gathering ability.  For example, a 7x50 binocular has twice the light gathering ability of a 7x35 binocular and four times the light gathering ability of a 7x25 binocular
Field of View
The size in degrees (called the angular field) of the area you can see with a pair of binoculars.  Field of view can also be expressed as the width, measured in feet of the viewing area you would see at 1,000 yards (linear field).  The greater the field of view, the greater the area you will see in the image.  Large field of view is related to power.  The greater the power, the smaller the field of view.  Generally, wider field of view usually means less eye relief.
Exit Pupil
The size (or diameter) of the beam of light (in millimeters) that leaves the eyepiece of the binocular.  The larger the exit pupil, the brighter the image.  Large exit pupils are advantageous when viewing at night or in low light conditions.  On a bright day your pupil shrinks to about 2 to 4 mm.  Then you would see no difference in brightness between a 2mm exit pupil and a 9mm exit pupil.  You can see the exit pupil by holding your binoculars out at arm's length and looking through the eyepieces at a light source.  To calculate the exit pupil, divide the objective by the power.  For example, the exit pupil of a 7x42 binocular is 42 divided by 7, or 6mm.  Generally, the exit pupil decreases with greater power.  (Note: 6mm is 1/4 inch)
Eye Relief

The distance a binocular can be held from the eye to observe the full field of view comfortably.  Longer eye relief is especially useful for eyeglass wearers.  Eyeglass wearers require a minimum of 16 to 20 mm of eye relief to see the entire field of view (3/4" to 7/8")
Prisms
Erecting prisms are used in all binoculars to correct an inverted (upside-down) image.  Two type of prisms exist -- porro prism and roof prism.  Which is best?  In general, porro prisms yield a higher contrast, but roof prisms have a phase-shift coating, which can be equally good.  Porro prisms come in two standard styles -- BK-7 and BAK-4.  The glass density of the BAK-4 provides superior performance.  Roof prisms are lighter in weight, more compact and generally cost more.  They can be readily identified because the barrels appear to be straight tubes.  In addition, both BAK-4 and roof prisms have a perfect circle exit pupil, whereas BK-7 show gray areas on the edges of the circle.
Optional Coatings
Of all optical elements (objective lenses, oculars and prisms) reduce light loss and glare due to reflection and increase light transmission and contrast.  Good coatings cannot be identified by their color, as color of coatings vary enormously because of the procedures used.  Coatings are the moist "hyped" and misleading of all binocular specifications.  A quick guide is as follows: "coated optics" -- one of more surfaces of one or more lenses are coated -- light loss 35%.  Fully coated (fc) -- all air to glass surfaces coated -- light loss 17%.  Multi-coated (mc) -- one or more surfaces or lenses are coated with multiple layers of chemicals -- light loss 11%.  Fully multi-coated (fmc) -- all air to glass surfaces have multiple layers of chemicals -- light loss 5%.  The above descriptions fully indicate the fmc coatings are superior and should be recommended.
Near Focus
The closest distance binoculars can be used to maintain a sharp focus.  This is especially important to birders, who prefer a close range of 15' or less, but will accept a superior glass with a minimum focus distance of 15' to 25'.
Brightness
A binocular's ability to gather and transmit sufficient available light to provide a bright image for good definition.  Brightness also helps in differentiating colors of objects.  Many indices rate binoculars for brightness, including r.b.i. (relative brightness index), twilight factor and r.l.e. (relative light efficiency).  In this instance, there are too many variables that can effect the results.  More important are the diameter of the objective lens and second, the magnification in finding the optimum amount of brightness and detail to your eyes.  In addition, the type of prisms and the coatings effect brightness.
Collimation
Perhaps the greatest problem experienced by binocular users is poor collimation.  Collimation is the alignment of all the optical elements to the mechanical axis.  Proper collimation will prevent eye strain, headaches, inferior image and will also improve good resolution.
Focusing Systems
By far the most popular type of focusing is the center focus type, either wheel or cam (quick-focus) type.  One hype during the past few years has been the permanent (or fixed) focus type of binoculars.  Unfortunately, they are not for most people because they require almost perfect eyesight and in addition there is no way to adjust for vision differences of a user's eyes.  (As you can do with focusing type of binoculars.)  Birders require binoculars that focus as close as 15 feet.  Most binoculars of this type have a minimum focusing distance of 40 feet.
Interpupillary Distance
Since the distance between the eyes (or, more accurately, the distance between the centers of the pupils) varies considerably among individuals, the two eyepieces must be correctly aligned and focused.  This adjustment is called the interpupillary distance.  Without adjustment you may see two images.  Merely move the two barrels of the hinged binoculars until you see only one image clearly.  Note the number of your setting, so that you may instantly reset this distance if the binoculars are used by more than one individual.
Adjusting Focus

Most people have a variance in vision between the left and right eye; therefore you must adjust for the difference since the greatest percentage of binoculars use the cent.

< back to top

BINOCULARS :: GALLOWAY HOME