The apparent field of view should be marked on the eyepiece itself. Look for a number followed by the degree symbol, °. Most eyepieces come with an apparent field of view between 30° and 110°. If you really can't find the value, you can approximate it with the formula 57.3 × d, where d is the diameter of the entrance lens of the eyepiece.
The apparent field of view value varies with the magnification on zoom eyepieces.
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