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Col what you've said is basically dead right - the actual theoretical limit of a 1.25" barrel ep (one in which the barrel diameter itself is the field stop) is approx 32mm I believe - anything over this is being vignetted by the barrel.
The precise calculation for True FOV using field stop diameter is :
TFOV = (180/Pi)*field stop / Scope focal length.
180/Pi converts the calculation from radians to degrees - approx 57.296
The problem can occur in actually measuring the field stop of a given ep, usually simple for standard plossl's although it gets more difficult as the field stop gets smaller or harder to access to measure accurately. In some more advanced design ep's it can be very difficult to measure the field stop, especially for those designs where the field stop itself lies between lens elements inside the ep and is not externally visible as such.
I have read that "calculating" true FOV by dividing magnification into apparent FOV gives an approximation only and may have up to 10% error, however it's still a fairly good guide and easily calculated in any ep where the apparent field of view is known.
IF you want a big field of view (and I very much enjoy such a view frequently) get a good 2" ep in a longer focal length. Generally if you want a 30, 35, etc mm ep then it should be manufactured as a 2" barrel version for pretty much the reasons have related. You just can't get a really good "true" wide view in a 1.25" barrel.
I hope I've helped clarify things and not muddied the waters further.
Cheers!
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