My thinking:

AFoV is something that can be directly measured. Any scaler relationship between the focal length, the field stop and the AFoV will require a fudge factor which accounts for distortion.

Interestingly, if one uses the relationship:

AFoV = 57.3 deg/rad x Focal length / Field stop, eyepiece's like the 21 mm and 13 mm Ethos, the 20 mm ES 100 degree, work out to within a degree or two of 100 degrees.

Jon

I did some research. As I understand it, that formula is derived from the formulas for the image size from the objective.

Basic trigonometry gives, Primary Image size = tan(FOV) X Objective FL.

The Objective FL is equal to EPFL X Magnification, and magnification is defined as AFOV/FOV

So Image Size or Field Stop size = tan(FOV) X EPFL X (AFOV/FOV)

At small angles (<0.1 radians), tangent very nearly equals angle so the factors Tan(FOV) and FOV cancel leaving

Image Size or Field Stop = AFOV * EPFL and we need to add the conversion factor of 57.3 for degrees to radians .

Field stop(mm) = AFOV * EPFL(mm) /57.3 which as you stated, seems to agree with actual field stops.

Working backwards as I originally did, from the AFOV using EPFL, results in an apparent size for the field stop, not the actual size.

dan

**Edited by dan_h, 22 August 2019 - 04:35 PM.**