What resolution can I expect to get from my binoculars?
April 3, 2018
Resolution is the ability of a pair of binoculars to render detail, which is a direct function of aperture. The larger the binoculars' aperture, the higher the resolving power and the finer detail it can see.
Resolution is often measured as the smallest angle that can be cleanly split or resolved between two stars. This number is called the Dawes limit. To determine this limit, divide 116 by the aperture of the pair of binoculars in millimeters. For example, the resolving power of a 20x80 pair of binoculars is 1.5 seconds of arc (116 divided by 80 equals 1.5). This is the theoretical limit under ideal conditions.
Actual or practical resolution is determined by the quality of the optical components, the type and quality of the optical coatings, atmospheric conditions, collimation (proper optical alignment), and the visual acuity of the user. For binoculars this includes whether or not a tripod is used and whether or not the binoculars have image stabilization.
A practical resolution taking into account these other factors can be obtained by comparing the performance of the unaided eye to a pair of binoculars. Sharp-eyed people are said to be able to split the 5th-magnitude star Kuma (Nu) in the constellation Draco. It has a one-arc minute separation (60 arc-seconds). The pair of stars in the handle of the Big Dipper, Mizar and Alcor, can be split by most people. They have a separation of 11 arc-minutes.
If your binoculars have a power of 20 (such as 20x80 binoculars), then tiny angles would be made to appear 20 times larger. So a 3 arc-second angle would appear as large as 60 arc-seconds to the unaided eye. A practical limit for the sharp eyed observer under great conditions using these binoculars would be 3 arc-seconds. This is twice the theoretical limit given above.
Apply this to the Big Dipper example given for most people by dividing 11 arc-minutes by 20 and you get 33 arc seconds, about a half arc-minute. This is a practical limit under average sky conditions for the average observer. This is 22 times the theoretical limit.
Your results may vary, but will probably fall within the range of these two examples.