[tims]

I recently read (in "The Backyard Astronomers guide" I think it was called) that the best eyepiece for any particular scope can be roughly worked out by multiplying the focal ratio of the scope by 2 - therefore a 20mm eyepiece is best for an f10 scope.
I can't remember exactly why this was so - something about how the eye operates? Does anyone have any idea why this may be so?

Thanks