Magnification of Spotting Scopes
The magnification of a spotting scope is actually a relationship of two independent optical systems-- the objective lens and the ocular (eyepiece) .It is calculated by dividing the focal length of the objective lens by the focal length of the eyepiece and for most nature observing, 20 to 40X is quite adequate. In situations where there is a need for fine detail, (long distance birding or reading leg bands or tags) higher magnifications of up to 100X or more may be required. As with binocular s,increasing the magnification usually results in a narrower field of view,a decrease in observed image brightness and a decrease in the depth of field (the range of distance from near to far that objects remain in good focus). Also , distortion and lack of clarity caused by heat waves, haze, and air pollution as well as hand and tripod tremors are a greater problem at higher magnifications.
The maximum useful magnification of a spotting scope with a given aperture size depends on several factors such as the quality of the lenses and prisms (which affects transmittance), the resulting exit pupil size and the objective lens focal length. It can be roughly approximated by using this rule: one power for each millimeter of aperture under ideal seeing conditions (calm, still air in bright daylight).
Thanks to our friends at Eagle Optics for providing this information!
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