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Still a highly interested newbie (so tread carefully with what i have to say) but from what i've read the proportion of 'illuminated field' is a big play here.
My f5 has a fairly large illuminated field which lights up the 27mm field stop of the 32mm Plossl at virtually 100%. Larger 2" eyepieces offer a larger potential field stop and there will be a point where the 'illuminated field' drops below 100%. Directly effecting brightness.
It is likely that an F6 or higher scope will have a smaller illuminated field , which is prime for high mag and better contrast on the planets (lower obstruction). The higher mag eyepieces have smaller field stops and having a larger illuminated field here is wasted, not all the light will reach the eyepiece.
Anyone who actually knows though please step in....
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