The principal axis is an imaginary straight line that passes through the centre of curvature of all the lens elements in an optical system. It serves as a reference line for understanding how light rays travel through the lens and how images are formed.
In photography and optics, the principal axis is key to analysing lens behaviour, such as focusing, image magnification, and the correction of aberrations. Understanding this concept helps photographers and optical engineers predict how light converges or diverges, ensuring sharp, well-structured images and precise control over lens performance.