Calculating TFOV

I've noticed on the TV site that the formula given for TFOV in degrees is: (e.p. field stop dia./scope f.l.) times 57.3.

Somehow I've gotten in the habit of calculating TFOV as:
e.p. Apparent FOV in degrees/magnification

The two aren't the same. Usually they're close, but in some instances there's a significant difference. The Apparent FOV formula seems more useful as e.p. AFOV is usually given, whereas field stop diameter isn't.

Is the AFOV formula technically valid? Can anyone point out circumstances where the AFOV formula shouldn't be trusted? I suppose vendors could be fudging their AFOV figures a bit.

Hi Bob,

I asked this very same question not so long ago. The AFOV formula gives a reasonable approximation only if there is no distortion in the eyepiece. In eyepieces that have some distortion, there can be errors generated because the magnification changes across the field of view. This is particularily noticable in the wider view eyepieces. Magnification tends to increase towards the edge of the field so the true field of view defined by the field stop is smaller than the AFOV.

hope this helps,
dan