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Obviously there is no one best f-ratio. For visual use I reason as follows:
I want my lowest power to give the max true fov without the exit pupil being too large for my pupil. [I would tolerate a small amount of vignetting in a large dobs at the lowest power to get a seated viewing position.] A young person has a max pupil diameter of 7mm but at my age I think it best to aim for <6mm. My lowest power is a WO 28mm UWAN with a afov of 85* but planning for a 31mm Nagler gives about the same numbers. I would be viewing with a paracorr which is also a 15% barlow.
Plugging in numbers gives Effective focal ratio = 28/6 = 4.67. True f-ratio = 4.67/1.15 = 4.06. Or more conservatively f-ratio = 28/5/1.15 = 4.87. My scope is f/4.5 so the exit pupil is 5.4mm which is pretty ideal, though an f/4.2 and 5.8mm exit pupil would also be acceptable.
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