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Optical Terms for Birders

Check out these definitions of common optical terms for binoculars and spotting scopes.

BirdChannel Editors

APO  Apochromatic; built to bring three light wavelengths (red, green and blue) into focus at the same point and reduce chromatic aberration, using ED, HD or fluorite glass. 
BAK-4 glass  Made from barium crown.
BK-7 glass Made from borosilicate.
Chromatic aberration The effect caused when light wavelengths pass through glass and do not focus at the same point; also known as color fringing. 
Close focus  The ability to focus on objects within 3 meters (almost 10 feet) of the binocular. 
ED glass  Extra-low Dispersion; a dense glass with fewer impurities. 
Exit pupil  The amount of light seen when holding the eyepiece 12 inches from the eye, expressed in millimeters. The greater the exit pupil, the brighter the image. An 8x42 binocular has an exit pupil of 5.25mm (42 divided by 8). 
Field of view  The width of an area seen at 1,000 yards, expressed in degrees or feet. 
Fluorite glass  Made with a crystalline material. 
Fogproof  Contains inner gas such as nitrogen or argon to prevent water from forming. 
HD glass  High Definition; built to reduce chromatic aberration. 
Lens coating  Designed to reduce the amount of light reflected from glass lenses inside the binocular and to improve the contrast of the image seen by the birder. 
Magnification  The amount of enlargement seen through the eyepiece. An 8x42 model makes a bird look eight times larger. 
Objective lens  The largest lens visible to the birder, measured in millimeters. In an 8x42 model, the objective lens is 42mm in diameter. 
Ocular lens  The smaller lens visible to the birder, measured in millimeters. 

Located between the ocular and objective lenses, the two prisms reverse the upside-down and backward image created by the lenses so that the image appears correctly to the birder’s eyes.

Porro prism: the prisms sit at an angle to each other, and the eyepiece is not aligned with the objective lens
Roof prism: the prisms and the eyepiece are aligned with the objective lens 
Rubber-armored  The presence of a rubber coating on the barrel to protect it from physical damage. 
Waterproof  Capable of keeping water out of the barrel while immersed in water for a short period. 

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Posted: April 22, 2013, 4:30 p.m. PDT

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