Focusing of Spotting Scopes
There are basically three different types of focusing mechanisms used on telescopes: helical, rack & pinion, and knob focusers. The characteristics of each type can make a difference in how well adapted it is to your specific application and even how much you will enjoy using your scope.
Helical focusers are knurled or rubberized collars around the scope barrel where the focus is changed fairly rapidly when turning the focusing ring. Helical focusers work well for observing objects that are changing distances, near to far, quite rapidly. Rack & pinion designs are commonly found on astronomical telescopes and usually offer fairly fast, smooth focusing, but most components are external and subject to potential deterioration from dust and moisture over time. Knob focusers are slower in action but allow for precise, accurate focusing and are probably the most preferred for general purpose birding or nature observation. The typical close focus distance for most terrestrial scopes is about 20 to 30 feet, however, certain designs (catadioptric) will allow a near focus down to 5 to 15 feet.
Thanks to our friends at Eagle Optics for providing this information!
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