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There is another way to measure True Field Of View which is pretty accurate and doesn't rely on the manufacturer's stated Apparent Field of View or any other esoteric calculation. It relies on having a big detailed Atlas or a print-out of an atlas, which has the scale of degrees to millimeters on the page.
Aim at a star near celestial equator, look for faint stars near it, then put that star at the edge of the field of view, and find a little star at the opposite side of the edge - so that the two stars mark out the full diameter of the eyepiece.
Then go to the Atlas or printout, use a ruler to measure the millimeters between those two stars, then go to the scale and see how many degrees of True Field of View that translates to.
Sometimes one finds that the True Field of View is smaller than what one would have expected had the manufacturer's stated Apparent Field of View been accurate.
Regards,
Renato
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