Aperture of Spotting Scopes

The aperture of a spotting scope is the diameter of the objective lens, usually measured in millimeters. For birding, nature observing or other terrestrial applications, the aperture size will normally range between 50 to 90mm. While the aperture size will be directly related to the size and weight of the instrument, two other factors, optical design and focal length, have to be considered as well. The size of the objective lens determines the amount of light that will enter the optical system, with the potential of greater image detail and clarity as the aperture size increases. Actual observed image brightness is a function of several factors; the transmittance of the optical system, the relationship of the aperture to the magnification (exit pupil), focal length of the objective lens and eye pupil size.

Thanks to our friends at Eagle Optics for providing this information!

See also:
Bird Watching Binoculars—Tips for choosing the right birding binocular
Birding Tripods—Tips for choosing the right tripod for you binoculars and scopes

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