I learned this equation: P = P1 + P2 - dP1P2 and some equations describing where the main planes go, but I can't remember how they work and I'm trying to look it up on the internet. I've looked at dozens of sites and many of them have the equivalent focus equation or the main plane equation but they don't explain how they relate to each other or even show a corresponding picture. I've looked at dozens of sites for hours and I can't find anything. I don't know why the entire internet has to be **** at explaining anything above High School level. Could someone please post the relevant equations that go with the one I posted and explain how the sign convention works, where the "Hauptebene" go, etc? I really have tried but I can't find anything that doesn't already assume that you already know everything. Please someone help it should only take a paragraph.