Paraxial refers to the light rays that travel close to and nearly parallel with the optical axis of a lens or optical system. These rays are important in lens design because they minimize optical aberrations and are used to approximate ideal image formation.
In photography and optics, understanding paraxial rays helps in evaluating lens performance, predicting image sharpness, and designing optical systems that produce accurate, distortion-free images. They are especially relevant in precision lenses, microscopes, and high-quality photographic equipment.