BINOCULARS GLOSSARY
Alignment: The coordination of mechanical and optical elements to provide high-quality viewing. |
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Coating: A thin layer of a substance which is usually evaporated and baked on the lens surfaces to reduce reflection and increase light transmission.
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Contrast: The difference in intensity between the lightest and darkest image viewed through the optical system. |
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De-alignment: An indication that the lenses are jarred out of position, decreasing the quality of viewing.
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Depth of field: The distance through which a viewed object can be maintained in relatively sharp focus. |
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Dioptric correction: The adjustment of one lens to provide compatible focus when the viewer's eyes have differing visual capabilities. |
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Exit pupil: The circle of light visible in the eyepiece when the binocular is held a short distance from the viewer. Exit pupil size is determined by dividing the objective lens diameter by the binocular's magnification. A 7x35mm binocular has an exit pupil measuring 5mm in diameter that transmits all the light a normal eye can accommodate under daylight conditions.
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Eye relief: The distance the eye must be positioned behind the eyepiece to provide a full viewing field. |
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Field of view: The width of the viewing field visible at an established distance, usually expressed in feet at a range of 100 yards. |
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Fixed focus: An optical instrument lacking the ability to change focus.
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Fixed power: A binocular scope with a single magnification setting. |
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Flare: Unwanted reflections, glare or haze that decreases viewing quality.
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Focus: The adjustment of an optical instrument to provide a sharp, clear image of the viewed object. |
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HD (High Definition) Glass or ED (Extra-low Dispersion) Glass: Optical glass processed to greatly reduce aberration and provide better color reproduction. |
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Lens: One or more pieces of optical glass with surfaces curved to converge or diverge transmitted light rays. |
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Minute of angle: A sight-adjustment measurement that equals one inch at a range of 100 yards. |
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Mirage: A distorted image produced by heat waves or other phenomena. |
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Objective lens: The large lens at the front end of a viewing system. |
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Parallax: An optical illusion that causes a viewed image to apparently change position as the viewer's eye moves to either side of the eyepiece. |
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Porro prism: A prism system in which light travels through 45, 90, then 45 degrees as it's transmitted through the eyepiece. Porro prism binoculars can be identified by their offset objective and viewing lenses. |
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Relative brightness: A unit of optical comparison calculated by measuring the diameter of the exit pupil in millimeters and squaring this number. |
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Roof prism: A somewhat complex prism system that "folds" the light path inside the binocular, so that the overall size of the binocular can be reduced for ease of transport and use. Roof prism binoculars have objective and viewing lenses in the same plane. |
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Twilight factor: A unit of optical comparison calculated by multiplying magnification by objective lens diameter, and taking the square root of the product. The greater the twilight factor, the better the binocular can provide quality viewing under reduced light conditions. |
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Variable power: Magnification that can be adjusted. |
