The optical axis is an imaginary straight line that passes horizontally through the centre of a compound lens system, extending from the lens to the image plane. It represents the path along which light travels with minimal deviation and serves as a reference for lens alignment and optical design.
Understanding the optical axis is important for photographers, cinematographers, and optical engineers, as misalignment can lead to distortions, aberrations, or focus issues. Properly positioning elements along the optical axis ensures sharp, accurate, and well-corrected images in both photographic and scientific applications.