We have discussed previously how focal length is measured from the center of the lens. Since we have moved up to real world compound lens that are used in real cameras, we need to decide where the center of the lens is. The problem is that my new camera lens has 11 elements inside - each with it's own center. Where does the center of the combination lie?
The answer is the principal planes. The principal planes are the points by which the focal length is measured from the front and the back. They aren't generally in the same place and they don't even have to be inside the lens. The position of the principal planes is determined by the distribution of the positive and negative elements within the lens and can be calculated with the proper formulas.
The rear focal length is measured from the rear principal plane (P') and the front focal length from the front principal plane (P). So, why should I even care about these planes since I have lived this long without any knowledge them? Personally, I find that understanding their basic behavior helps to understand various topics in macro photography such as: working distance, magnification/extension, basic lens design. These topics will be discussed in future installments.