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VISUAL OBSERVING INFORMATION
Focusing
When working with any optical instrument, there are several focusing hints that will ensure that you
get the sharpest possible image. The hints listed here are for visual observing and photography.
Never look through glass. Glass found in household windows is optically imperfect and may vary in
thickness from one location to the next. This inconsistency can and will affect the ability to focus
your spotting scope. In most cases, you will not be able to achieve a truly sharp focus. In some
cases, you may actually see a double image.
Avoid looking across or over objects that are producing heat waves. This includes asphalt parking
lots on hot summer days or building rooftops. The turbulent air caused by these conditions makes it
impossible to get a truly sharp focus.
Avoid hazy skies, fog, and mist, all of which can make it difficult to focus, thus greatly reducing the
amount of detail seen. Remember that no matter how good the optics of your instrument are, they can
NOT perform to their fullest under adverse conditions.
Calculating Magnification (Power)
You can change the magnification of your spotting scope simply by changing the eyepiece (also
called an ocular). To determine the magnification with your spotting scope, simply divide the focal
length of your spotting scope by the focal length of the eyepiece. In equation format, the formula
looks like this:
Focal Length of Spotting Scope (mm)
Magnification =
___________________________________________________
Focal Length of Eyepiece (mm)
For example, to determine the magnification of the 80ED with a 25mm eyepiece, divide the focal
length of the spotting scope (600mm) by the focal length of the eyepiece (25mm). 600 divided by 25
yields 24 power.
Although the power is variable, each instrument has a limit to the highest useful magnification. The
general rule is that 60 power can be used for every inch of aperture. For example, in a 3.2” diameter
spotting scope, such as the 80ED, the maximum useful magnification is 192 power. This is derived
from multiplying 60 times 3.2”. Although this is the maximum useful magnification, most observing
is done in the range of 20 to 35 power for every inch of aperture which for the 80ED is 64 to 112.