Generally speaking, an ordinary lens consists of a spherical lens. The lens is spherical and curved, and its cross section is also curved. When light of different wavelengths incident on different positions of the rear lens parallel to the optical axis, it cannot focus on a single point on the film plane (the plane perpendicular to the lens center and the lens focal line and passing through the focus), which will cause aberration and affect the image quality, such as sharpness reduction and deformation.