Prismatic binoculars are referred to as 6x 15, 7x 35, 7x 50, 8x 30, etc. The 6x, 7x, 8x, etc., refers to the power of the glass, namely to the extent it magnifies the object being viewed. Thus a glass of 8x makes an object look eight times larger than it does to the naked eye and consequently makes it appear eight times nearer. The last figure refers to the diameter of the objective lens in millimeters. The objective lens is the large lens farthest from the eye; the ocular lens is the small lens which is closest to the eye when the binoculars are in use.

Too many binocular users select their glass solely on the basis of power, believing that the higher the power believing that the higher the power the better must be the binocular. This is by no means true. The power of the glass selected must be carefully chosen to provide exactly the right power for your special use or uses, neither too much nor too little.

Remember, the higher the power the smaller the field of view and the lower brilliance of the image that reaches the eye. The clarity and definition also decrease as the power increases and, most important, not only does a binocular magnify the object being viewed but also any motion of the hand holding it and the motion of the object you are in or on such as the floor of a car, the deck of a boat, etc.

It is generally recognized, therefore, that above 8x, a glass should be used in conjunction with a rest or tripod to obtain best results. The higher power glasses (10x and above) are only recommended for special uses such as long mountain scenery, etc.

This is why you will find binoculars of 6x, 7x and 8x recommended for nine out of ten uses.

All binoculars can be used at night or during the daytime. But certain glasses are better adapted to night viewing than others, and these are often called "night Glasses". Models with the largest objective lenses are desired because more light can be admitted. The average diameter of an individual's eye during the daytime under average conditions is about 4 to 41/2 millimeters but at night expands to about 7 to 71/2 millimeters.

Therefore, a binocular which 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 does utilize the full diameter of the expanded pupil of the eye is considered to be an especially fine night glass.

RELATIVE BRIGHTNESS AND EXIT PUPIL

Relative Brightness and Exit Pupil are determined mathematically The exit pupil of a binocular is the disc of light one sees 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.

Relative Light Efficiency is the modern term used to describe the increased light transmission that coated optic . s give. R.L.E. is computed by increasing Relative Brightness by 50%. The Relative Brightness figure of 25 given above would equal a Relative Light Efficiency of 37.5. Barium Crown prisms further increase R.L.E. by from 10-15%. Newer types of coating, including multi-coating, also increase the R.L.E. factor.

FIELD OF VIEW...

Field of view refers to the diameter of the area seen through the glasses. 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.

CENTRAL vs. INDIVIDUAL FOCUSING...

Central Focusing binoculars are focused by a single knurled wheel. An adjustable right eyepiece is provided for any difference between the eyes. Because they are convenient to use, Central Focusing binoculars are in the greatest demand today.

Individual Focusing binoculars are adjusted for each eye at the eyepiece. Because they are more nearly moisture proof, simple, and sturdy in construction, military services require individual focusing on binoculars they procure. Mariners and sportsmen, who are the sole user of a binocular, focus each eyepiece on infinity and either note the setting on an eyepiece for later use or tape eyepieces in position.

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.

CONSTRUCTION...

Consistent with strength requirements, mosts 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 modern, synthetic enamel or with high quality textured plastics that far outlast leather. Each instrument is generally dehumidified and sealed in air-conditioned, dustless, final-assembly rooms.

COATED LENSES AND PRISMS...

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

STEREOSCOPIC EFFECT...

Viewing an object with two eyes enables us to see around each side of an object slightly This gives us an effect of depth which is known as stereoscopic. Because objective lenses of most prismatic binoculars are actually wider apart than the eyes of the average individual, they permit much better depth perception.

LENS SYSTEMS...

Kellner Ocular System: Three part system of lenses between the prism and eye consisting of one achromatic lens and one simple lens. This system is excellent for a standard field of view.

Erfle Ocular System: Five part system of lenses between your eye and binocular prism consisting of two achromatic lenses and a simple lens. This system affords a wide field of view.

Achromatic Lens: Two lenses of crown and flint glass cemented together to eliminate color fringes.