Quote:
Originally Posted by Rod-AR127
Thankyou, l was on track earlier doing what Renato laid out for me.
Thanks for the links and advice.
Damn cloudy over here for another week, watching my bid on a cheap tripod (61 inches high) for my 10x 50 bino's, 4 hrs to go.
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The true field = apparent field/magnification is a good approximation but doesn't take into account angular magnification distortion. Good enough for most purposes but there is a slight difference if you use the field stop formula. I use the simpler formula all the time myself but its good to understand both methods. There is another method involving arctan but not one I've used much except for working out binocular true fields when only xxx feet at 1000 yards is stated.
Note if your 15mm eyepiece had a 100 deg. apparent field then your true field would be about 1.25 deg. The field stop would be approaching 27mm. Note if you look at the Tele Vue tables you can see that the 100 deg. 13mm Ethos has a field stop diameter of 22.3mm and the 17mm Ethos has 29.6mm. The latter is over 27mm thereby requiring a 2" barrel.
The field stop diameter is a linear representation of the true field at the focal plane. Think of cameras where a full frame 35mm (36x24mm) has a larger area than an APS-C (24x16mm approx. for 1.5x crop) sensor. The full frame gives a larger true field than the APS-C for the same focal length lens.