10 terms every binocular buyer should know
Learn words and phrases that manufacturers use to describe binoculars, including field of view, nitrogen-purging, and ED glass.
Published: February 14, 2012
 Eagle Optics Ranger 8x42 Binocular
2. Objective: This is the front lens (opposite the eyepiece). Its diameter, measured in millimeters, is the second number in the binocular equation. An 8x42 binocular has an objective 42 mm across. Large objectives produce a brighter image than small objectives if power is the same.
3. Exit pupil: The cylinder of focused light coming out of the eyepiece. A wider exit pupil delivers a brighter image. To calculate exit pupil, divide the diameter of the objective by the power. For an 8x42 binocular, it's 5.25 mm. For an 8x25 pair, it's 3.125 mm.
4. Field of view: Everything visible within the dark periphery seen when you look through a binocular. For comparison's sake, the horizontal width of the field of view is usually measured in feet at a distance of 1,000 yards from the binoculars. When comparing fields of view, do so with equal-powered binoculars. The field of view through 10x binoculars will be smaller than lower-powered binoculars.
5. Eye relief: The distance, measured in millimeters, your eyes can be from the eyepieces and still see the entire field of view. Higher-power binoculars generally have shorter eye relief, as do those employing Porro prisms.
6. Close focus: The nearest focused point allowed by the binocular's focusing mechanism.
7. Prisms: Internal blocks of precisely cut glass that fold the light passing through a binocular, delivering a rightside-up image to your eyes. There are two types: Porro prisms and roof prisms. Objectives in Porro-prism binoculars are offset from the eyepieces. Objectives in roof-prism binoculars are in line with the eyepieces.
8. Coatings: Chemicals applied to binocular lenses to reduce scattered light and increase contrast. "Fully coated" means that all glass surfaces have one layer of anti-reflective coating. "Multicoated" means that more than one layer has been applied to the glass, further reducing reflections. "Fully multicoated" means that each glass surface has more than one layer. "Phase coatings" are unique to roof-prism binoculars. They remedy interference created inside the prisms.
9. Nitrogen-purging: A fog-proofing process. Air is vacuumed out of a binocular and replaced with nitrogen, a gas without moisture. The binocular is then sealed. Nitrogen-purging will not prevent outer surfaces from fogging, but interior surfaces will remain clear of condensation when the binocular is taken from cold and dry to warmer and humid.
10. ED glass: Extra-low dispersion glass. Made with compounds or elements such as fluorite or lanthanum, ED glass bends light with far less chromatic aberration, resulting in crisper, brighter views than non-ED glass. Lens systems made with this glass are called apochromats. ED glass is expensive and often drives binocular prices into four figures.
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