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Message: <blockquote data-quote="Binastro" data-source="post: 3200418" data-attributes="member: 111403">. Orion's Belt stars are I think 2.736° apart.The Minolta 7×35 that I have has a field of 11.05°.The 4×22 Chinese independent eyepiece focus binocular has a field of 16.5°.The 4×21 Bushnell Xtrawide has a field of 18.5°. However, it is only 3.5 times magnification whereas the 4×22 Chinese binocular is four times.You can get reasonably accurate star separations from programs.Also there are free calculators where you put in the accurate star positions of the two stars and an accurate separation is found. But you must work very carefully to make sure you don't make mistakes.Measuring star atlases is fraught with errors, as all star atlases that are on flat paper have their own projections from a spherical star system. The larger the star atlas the better the accuracy is, but in my experience it is never very good.I do sometimes measure photographs, but again camera lenses have distortions. It would be best to use a distortion free lens and only measure towards the central portion of the photograph..Nearly all the photographs I have measured have led to errors. However, some photographs I've taken with standard 50 mm lenses on APS size sensors do give good results. Measuring something like Ursa Major on most star atlases gives big errors because of the high declination, which gives bigger distortions on the flat paper.Strictly speaking, for rough measures of field sizes in binoculars, some of these niceties don't matter.But with careful measurements I am able to get repeatable results to 1% or slightly better. But it does depend on the eye position. If your eyes are slightly off centre you get different results, and if the IPD is incorrect you get other results. And if you move your eyes around you get a larger field. I try to keep my eye central and measure each barrel of the binocular with the right and left eye and note down the results.It is likely that different observers will get slightly different results, and largely different results if they are wearing glasses.I have not really found different fields vertically and horizontally in binoculars. It seems that most field stops are accurately circular.</blockquote>

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