The distances F1-H1 and F2-H2 are equal

The planes AG and BK are known as the principal planes

In any thick lens or system of lens there are two points called nodal points, such that a ray incident toward one nodal point will emerge as from the other nodal point in a direction parallel to the original direction. Here, the points H1 and H2 are the nodal points of the system (coincident for a thin lens).