To compare both the old AFOV/mag formula and the Field Stop Formula
with reality, I did some measurements for my own eyepieces and telescope.
The telescope is a 10" f/5.55 Newtonian (1410 mm actual focal length), and
the eyepieces tested were the following: (EFSD is eyepiece field stop 
diameter in millimeters).

____Eyepiece_________Apparent Field____True Field (on sky)__EFSD__
40mm Mk-70 Konig              68.8 deg.          1.883 deg.       46.00mm
30mm Widescan III             84.0 deg.          1.798 deg.       44.00mm
30mm Ultrascopic (Orion)      52.2 deg.          1.063 deg.       26.08mm
27mm Kellner (Jaegers)        52.5 deg.          1.042 deg.       25.38mm
24mm Panoptic (Tele Vue)      68.0 deg.          1.113 deg.       27.00mm
24mm Koenig (University Opt.) 58.9 deg.          0.981 deg.       24.04mm
20mm Celestron Plossl         51.0 deg.          0.705 deg.       17.23mm
15mm Ultrascopic (Orion)      58.3 deg.          0.590 deg.       14.40mm
14mm Meade Ultrawide          80.5 deg           0.820 deg.       20.3mm*
10mm Ultrascopic (Orion)      49.8 deg.          0.339 deg.        8.32mm
6.4mm Super Plossl (Meade)    49.4 deg.          0.217 deg.        5.31mm
6mm Orthoscopic (Brandon)     44.2 deg.          0.188 deg.        4.62mm

*14mm Ultrawide's field stop is inside of the eyepiece inside the negative
doublet of the field lens, and thus its size is *approximate only*.

Each eyepiece had its apparent field of view measured on the optical bench, 
rather than depending on the listed manufacturer values, which tend to be a 
little optimistic.  The focal lengths used in the calculations are those 
given by the manufacturer on the eyepiece barrel, as, due to some of their 
construction and short focal length, they could not be accurately measured.  
The true field of view (TF) for each eyepiece was measured at the telescope 
using the star-drift method.  Below are the calculated values for true field 
and their percentage deviations from the measured true field for each 
formula.  All angles are in degrees.  FS-FORMULA is the eyepiece field stop 
formula TFOV = (180/Pi)*EFSD/TFL, and AF/Mag is the Apparent field divided
by the magnification.

 EYEPIECE        Mag   True Field  |  AF/Mag    % Dev.  |  FS-FORMULA  % Dev.
-----------------------------------|--------------------|--------------------
40mm Konig      35.3x  1.883 deg   |  1.95 deg   +3.5%  |  1.869 deg   -0.7%
30mm WSIII      47.0x  1.793 deg   |  1.79 deg   +0.0%  |  1.789 deg   -0.2%
30mm Ultras.    47.0x  1.063 deg   |  1.11 deg   +4.7%  |  1.060 deg   -0.3%
27mm Kellner.   52.2x  1.042 deg   |  1.01 deg   -2.8%  |  1.031 deg   -1.1%
24mm Panoptic   58.8x  1.113 deg   |  1.16 deg   +4.5%  |  1.097 deg   -1.4%
24mm Koenig     58.8x  0.981 deg   |  1.00 deg   +2.0%  |  0.977 deg   -0.4%
20mm Plossl     70.5x  0.705 deg   |  0.72 deg   +1.4%  |  0.700 deg   -0.7%
15mm Ultras.    94.0x  0.590 deg   |  0.62 deg   +5.1%  |  0.585 deg   -0.8%
14mm Ultrawide. 101x   0.820 deg   |  0.80 deg   -2.4%  |  0.825 deg   +0.6%
10mm Ultrasco.  141x   0.339 deg   |  0.35 deg   +2.9%  |  0.338 deg   -0.3%
6.4mm S. Plos.  220x   0.217 deg   |  0.22 deg   +0.0%  |  0.216 deg   +0.5%
6mm Ortho.      235x   0.188 deg   |  0.19 deg   +0.0%  |  0.188 deg    0.0%

The AF/Mag standard form gives values which deviate from the actual true field 
by an average of about +1.6 percent (maximum deviation: 5.1%).  Although the
apparent field could be measured with some accuracy, one has to depend on the
manufacturer's eyepiece focal length numbers, and this undoubtedly is at 
least partly the reason for the errors with the AF/Mag method.  The Eyepiece 
Field-Stop formula is generally more accurate for calculating the true field
of view with a given eyepiece.  The mean deviation with this formula is 
-0.4 percent, with the maximum deviation of -1.4%.  NOTE: filters for 1.25" 
eyepieces, (LPR's, Nebula Filters, ect.) will often have a clear aperture of 
about 24.5mm.  This diameter may become the maximum field stop size available 
when these filters are being used, thus limiting the field of view for 1.25"
eyepieces which have field stops larger than 24.5mm.  
     The chief difficulties with using the Field Stop formula are when you 
cannot get direct access to the field stop to measure it (the case when some 
eyepieces have a field stop just behind the field lens), or when you do not 
have the ability to measure it with much accuracy for the smaller diameter 
field stops (as in some very short focal length eyepieces).  In addition, 
many eyepiece manufacturers do not routinely provide the eyepiece field stop 
diameters for their products (only the focal lengths and the approximate 
apparent fields).  Still, either the classic AFOV/Mag formula or the Eyepiece 
Field Stop formula should be able to give the amateur astronomer an idea of 
what size of true field of view to expect when viewing with a given eyepiece.

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