I thought this was a simple question but can't seem to find a direct answer.
In ray diagrams with a converging or diverging lens, we always draw F (the focal length) on both sides of the lens. But the focal point for any thin lens is only one, isn't that right? The point at which light converges or appears to converge.
Or is it two, depending on different situations... such as objects placed behind or in front of the lens?
What do we officially say? Does a lens have two foci or one?