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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.
S 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.
S 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.
S 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 102mm Wide View with a 25mm eyepiece,
divide the focal length of the 102mm Wide View (500mm) by the focal length of the eyepiece
(25mm). 500 divided by 25 yields 20 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 4”
diameter spotting scope, such as the 102mm Wide View, the maximum useful magnification is
240 power. This is derived from multiplying 60 times 4”. 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 102mm Wide View is 80 to 140.