Hi! I'm having some trouble with optics. When we have convex, planoconvex, concave, planoconcave, and meniscus lenses, I don't understand how to substitute with the sign for R1 and R2. How do I determine whether the radius of a side is negative or positive? A book that I'm using said to take incident light in a particular direction and then measure the radius from the lens itself, but I honestly still don't understand that method at all... Then I found this: http://www.livephysics.com/tools/optics/focal-length-for-thin-lens.html but even though it says there that a surface "bulging out" of the lens is taken as R>0, and a surface "depressed into the lens" is R<0, it has the exact opposite shown in a few of the diagrams.. I'm confused and I would really appreciate it if someone could clarify this. Thank you!