Binoculars Terminology

Olympus 118710 10 x 21 Outback Binoculars are portable and let you enjoy the view from virtually anywhere, then conveniently fold up.

Magnification: Magnification is simply the degree to which the object is enlarged. With a 7×35 binocular, for example, the object appears to be seven times closer than without the aid of a binocular. The number immediately following the “x” is the diameter in millimeters of the objective (front) lens. A 10×50 binocular, therefore, has a 10 times magnification with a 50mm objective lens.

Exit Pupil: The exit pupil refers to the size of the shaft of light transmitted to the eye. The more light, the better the contrast. You can actually see the exit pupil by holding the eyepiece of the binocular approximately 12 inches from your eye. It is the bright circle of light in the center of the eyepiece. Exit pupil is expressed in millimeters and is normally derived by dividing the power into the objective lens diameter. A 7×35 binocular has an exit pupil of 5mm (35 ÷ 7).

Field of View: The field of view is the width, measured in degrees or feet, of the viewing area you would see at 1,000 yards.

Prism Systems: Binoculars have left and right side barrels linked to each other by an axle. The two most frequently used prism systems are porro and roof. Porro Prism binoculars are characterized by the eyepieces being offset from the objective lenses. Roof Prism binoculars feature lenses positioned in line for a more compact design.

Resolution: Also referred to definition, is the ability of a binocular to distinguish fine detailand retain clarity.

Coated Optics: Optical glass surfaces are coated by magnesium fluoride to reduce light loss and glare due to reflection. Most binoculars are fully multi-coated to further enhance viewing and reduce eye strain. Some binoculars have a special image enhancing, amber, high contrast coatings.

WaterProof: This describes a binocular’s ability to remain completely dry on the inside when exposed to moisture or total submersion in water. Binoculars are sealed with O-rings and are filled with 100% arid nitrogen, which also resists all forms of moisture.

Stray Light: When light entering the binoculars reflects off of interior surfaces, the reflected light eventually exits inside the binoculars in the form of stray light. This unfocused light typically diminishes the image quality of the sight picture.

Focusing Range: All binoculars have the ability to be focused for infinity. So a primary point of distinction between binocular models is the minimum focus range. Birders tend to favor models with a short focus range of about 15 feet, but 20 to 25 feet is usually acceptable.

Twilight Performance: Using formulas gives a basic evaluation of low light performance, however, one must keep in mind that they are mathematical formulas and do not take into effect some of the most critical features in optics; glass quality, number of lenses, precision of manufacturing and coatings.

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