One caveat would be shooting a SFP scope at max power in low light. Gonna be really hard to see much of anything at 15x with a tiny exit pupil in really low light.
exit pupil is the same FFP Or SFP and is a function of the scopes power setting and its objective size. but either way your going to see less scope brightness as the scope goes beyond about a 4:1 ratio. if its a 40mm scope that is going to be 10x+, if its 50mm its going to be 12x+ that is probably why you see a darker image on 15x
