Gotta keep in mind exit pupil though. Some deal about dividing the objective by power. So I think the average pupil is 5mm and whenever it works out to less than 5 your losing light. 42mm OBJ / 6x = 7 (bright) 40mm OBJ / 10X = 4 (not so bright)
Something like that, read it on the back of a box of Cap'n Crunch. Mighta been the crunch berries....
