Principal planes are imaginary lines that pass through the nodal planes of a lens system, representing the points where light rays entering and exiting the lens appear to bend. They are used in optical calculations to determine image formation, magnification, and lens behaviour.
In photography and optical engineering, understanding principal planes is essential for precise focusing, designing complex lens systems, and predicting how light will travel through multiple lens elements. This concept helps ensure sharp, accurate images and is fundamental in advanced lens design and optical theory.