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The true field is sometimes stated in degrees but often feet at 1000 yards or metres at 1000m and you can calculate the angular true field using inv. Tan. To get apparent field multiply by magnification.
I have binoculars ranging from 45, 49, 60, 80.5 deg. apparent field. The latter being some old and very wide true field 7x35 I picked up recently. Apparent field is not used since in daytime use one can determine the distance to an object of known width by seeing how much field it takes up. The reverse also applies in that if you know an objects distance you can work out it’s width. In night time use knowing angular separation in degrees is all that really matters and you don’t really care if say a 6.5 deg field is at 7x, 8x or 10x magnification.
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