Principal plane ($H,H^{\prime }$)

The two principal planes are reference planes that represent a centred imaging system in the paraxial approximation. In air on both sides, measure object distance from the front principal plane, image distance from the rear principal plane, and focal length from each principal plane to its corresponding optical focus. They need not coincide with a surface or mechanical centre. Their signed separation matters in the Bessel lens displacement method.^[ref]