If f1 and f2 are the distances from the optical centre of a convex lens of focal length f to conjugate two points P1 and P2 respectively, the following relationship holds good

It is usually represented by the letter ‘F’. However a convex lens has two spherical surfaces and hence it has two principal foci or two focal points which are usually denoted by the letters ‘F1’ and ‘F2’ and are known as the first principal focus (F1) and the second principal focus (F2).