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HOW TO SELECT SPOTTING SCOPES

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A spotting scope is like a small telescope with refractor, Maksutov, or catadioptric optical design. Normally spotting scopes have 50mm to 80mm objective lens and magnification anywhere from 15x to 60x.

Magnification (Power)

Magnification is the degree to which the object being viewed is enlarged. Magnification, or power, in a spotting scope is a function of the relationship of two independent optical systems — the optics of the spotting scope itself and the eyepiece being used.

As a rule of thumb, the maximum magnification is equal to 50 to 60 times the diameter (in inches) of the spotting scope’s objective lens (under ideal conditions). Magnification higher than this will produce a dim and fuzzy image. In most cases, a magnification of 20 to 35x is the most useful and satisfying range to use with spotting scopes under normal daytime conditions. Zoom spotting scopes have a single, built-in eyepiece to give you a range of magnifications without the need to change eyepieces.

Focal Length of Eyepiece

To determine magnification, divide the focal length of the spotting scope by the focal length of the eyepiece. The distance, in an optical system, from the objective lens to the point where the instrument is in focus (the focal point). The longer the focal length of the instrument, the larger the image scale it offers. By exchanging an eyepiece of one focal length for another eyepiece with a different focal length, you can increase or decrease the magnification of the spotting scope. For example, a 30mm eyepiece used on a C90 spotting scope with a 1000mm focal length would yield a magnification of 33x. A 9mm eyepiece used on the same C90 spotting scope would yield a power of 110x.

Coatings

High Quality spotting scopes feature the best coating available. Optical coatings are important, as they determine the throughput transmission of a spotting scope. Coatings on lens surfaces reduce light loss and glare due to reflection, resulting in a brighter, higher-contrast image with reduced eyestrain. The better the quality of the lens coatings, the brighter the image will be and the higher the contrast of the image will be. Fully multi-coated lenses are the best quality you can choose.

Field Of View

Linear field of view refers to the width of the area that can be observed at 1,000 yards, and is expressed in feet. A wide field of view is better for following fast-moving action or scanning for wildlife. Generally, the higher the magnification, the narrower the field of view. A larger field of view translates to a larger area seen through the spotting scope. Field of view is related to magnification, with greater magnification creating a smaller field of view, in general. The angular field of view is calculated by dividing the apparent field of the eyepiece by the magnification being used. (AFE ÷ Magnification = AFV). Once the angular field of view is known, the linear field can be determined by multiplying the angular field by 52.5.

Exit Pupil

The exit pupil refers to the size of the column of light that exits a spotting scope. The diameter, in millimeters, of the beam of light that leaves the eyepiece of the spotting scope is the "exit pupil". The larger the exit pupil, the brighter the image. To calculate the exit pupil, divide the size of the objective lens, in millimeters, by the magnification of the eyepiece being used. To determine the size, divide the objective lens diameter by the power (a 15x45 model has an exit pupil, or useable light, of 3mm). The larger the exit pupil, the brighter the image.

Eye Relief

The minimum distance between the eyepiece of the spotting scope and your eye that still allows you to see the entire field of view. The distance a spotting scope can be held away from the eye and still present the full field of view. Extended or long eye relief reduces eyestrain and is ideal for eyeglass wearers.

Portability

For applications such as hiking and hunting, portability is a prime factor to consider. For stationary viewing, a large diameter objective lens becomes of primary importance.

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