What is lens distortion? How can it be minimized?

The so-called aberration is the distortion phenomenon of the lens, that is, the error between the image formed by the lens and the appearance of the original scene. The lens is good, the aberration is corrected, and the aberration is small; the lens without aberration does not exist, and the extremely small aberration is negligible. Photographers understand aberrations in order to reduce or use aberrations.

Aberrations include spherical aberration, coma, astigmatism, curvature of field, distortion and chromatic aberration.

Spherical aberration is short for spherical aberration. The convex lens has a thick center and a thin edge, and its surface is a spherical curved surface. After the light beam parallel to the optical axis of the lens passes through the convex lens, it cannot be completely focused on a point on the optical axis. The focal point formed by the light from the edge of the convex lens is close to the convex lens (far from the imaging carrier), and the focal point formed by the light from the center of the convex lens is far away from the convex lens (near the imaging carrier), resulting in blurred images. There are three ways to reduce and eliminate spherical aberration: use an aspheric lens. The aspheric lens is also curved, but the curvature is small, which can almost completely eliminate the spherical aberration; a multi-lens lens is used. The multi-lens lens is a combination of multiple convex lenses and multiple concave lenses, using their opposite characteristics to offset and reduce spherical aberration, and is affordable; when shooting, use a small aperture to reduce the spherical aberration by reducing the edge area of ​​the applied lens.

Comatic aberration is an abbreviation for "coma aberration". The oblique beam symmetrical to the chief ray becomes no longer symmetrical after passing through the lens, forming a comet-shaped diffuse spot on the edge of the lens, resulting in unclear images. The small aperture can reduce coma.

Astigmatism. The image of the light parallel to the main optical axis and the oblique light outside the main optical axis cannot be converged on the same plane after being refracted by the lens, resulting in unclear images.

Field curvature, short for "field curvature". The lenses are mostly curved surfaces with the same curvature, and the refractive index of the center and edges are different. When shooting a flat object, the image is not flat, but a dish-shaped curved surface. The imaging curved surface and the imaging carrier plane cannot completely overlap, resulting in blurred images.

distortion. The phenomenon that the straight lines on the scene become curved after being imaged by the lens. Inwardly curved weighing (pillow) distortion (when the lens is used for the long focal length), outwardly bent weighing barrel (wooden barrel) distortion (when using the wide-angle lens). Distortion only changes the shape of the image, not the sharpness of the image. There are two ways to reduce or eliminate distortion: one is not to place the straight line part of the scene at the edge of the picture when composing the picture, and the other is to use a small aperture.

Chromatic aberration, also known as chromatic aberration and dispersion. It can be divided into two types: longitudinal chromatic aberration and lateral chromatic aberration. Purple light has the shortest wavelength and the highest refractive index, and the convergence point is closest to the lens; red light has the longest wavelength and the lowest refractive index, and the convergence point is the farthest from the lens (closest to the film or sensor). The point convergence points of purple light, blue light, green light, yellow light, orange light, and red light are successively farther and farther away from the lens, and cannot be completely concentrated at the same focal point, resulting in blurry imaging. Lateral chromatic aberration, when the light is not perpendicular to the optical axis of the lens, the lens has different magnifications for different colors of light. The size of the image formed by different colors of light is different, causing the image to be blurred.