Dear, I studied 4f system and I've got a question. As far as I know, 4f system is optical filtering system (attached file). In the configuration in the attached file, region between L1~L2 is infinity optics(ray components from P1 is parallel with each other) so that distance between L1~L2 shouldn't affect the image at P3 as far as the distance between L1~P2 is 'f', in my guess. Am I correct? Is there any difference between "1), 2)" below? 1) P1~L1=f, L1~P2=f, P2~L2=f, L2~P3=f (standard 4f system) 2) P1~L1=f, L1~P2=f, P2~L2=not f, L2~P3=f Thank you in advance. Sungsik.