The so-called spherical and aspherical surfaces are mainly for the lens geometry of lenses (various cameras, microscopes, etc.) and glasses (including contact lenses), that is, spherical lenses and aspherical lenses. The difference in the geometric shape of the two determines the difference in the refraction direction of the parallel incident light, which affects the quality of the imaging effect.

Spherical lens, the lens has a spherical arc, and its cross-section is also arc-shaped. When rays of different wavelengths are incident on different positions on the rear lens with a parallel optical axis, they cannot be focused into a single point on the film plane (the plane passing through the focal point that is perpendicular to the center of the lens and the focal point of the lens), resulting in aberrations. problems that affect the quality of the image, such as loss of sharpness and distortion. Generally, ordinary lenses are composed of spherical lenses.

In order to solve this imaging problem, it is possible to correct the aberration by adding a lens to the lens body, but this may cause the opposite effect and further reduce the image quality, because the additional lens, in addition to increasing the reflection of light in the lens body. In addition to causing flare, it will also increase the size and weight of the lens.

The aspherical lens is not a spherical arc, but the edge of the lens is "shaved" a little, and its cross-section is flat. When light is incident on an aspherical mirror, the light can be focused on one point, the film plane, to eliminate various aberrations. For example, the flare phenomenon will be more serious when shooting with a large aperture on a spherical lens than when shooting with a small aperture, but if an aspherical lens is added, the flare can be greatly reduced; for example, the image appears distorted (pillow-shaped or barrel-shaped) The light is not properly refracted. Taking a zoom lens as an example, it is usually barrel-shaped deformation when the focal length is short, and pincushion deformation when zooming to a long focal length. If an aspherical lens is used, this aberration can be improved.

The introduction of aspheric lens technology is most beneficial to the production of large aperture, high-magnification zoom, and even extreme wide-angle and telephoto lenses. The image quality is improved due to the reduction of aberrations, and the size of the lens body is also reduced. At present, many lens manufacturers in the market have stated that some of their focal length lenses use aspherical lenses, and even portable zoom cameras (such as 28 to 90mm, 38 to 105mm, etc.) use aspherical lenses to improve image quality.