| 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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