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Brent,
The average person is assumed to have a night-time pupil size of about 6mm.
The exit pupil of an eyepiece is D/m where D is the diameter of the scope's aperture and m the magnification achieved by the eyepiece in the scope.
Ideally, D/m=6 to match the eye's pupil size. If the exit pupil of the eyepiece is larger, the extra light is wasted.
Rearranging the formula, m=D/6.
Now m=focal length of aperture/focal length of eyepiece =D/6.
Rearranging the formula,
focal length of eyepiece=focal length of aperture/D x6=focal ratio x6.
This is not too say you can't have a longer focal length eyepiece but the extra light it sends to your eye will be wasted. Of course, if you have a night-time pupil size of 7mm, you can use focal ratio x7.
Regards, Rob
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