There is always a difference between the performance of an ideal lens and an actual lens. This difference is called lens aberration. There are many types of lens aberrations, but they fall into two distinct categories. Chromatic aberrations are caused by differences in the colors of light. Spherical aberration, astigmatism, coma, distortion, and curvature of field occur at all visible wavelengths.
It is very important that photographic lenses compensate for aberrations because the focused image is the key to the picture's image definition. Good lenses provide well balanced compensation for aberrations. The image quality of each lens depends on the amount of aberration remaining after compensation.
There are two kinds of chromatic aberration: longitudinal and lateral. Longitudinal chromatic aberration is also called dispersion and is caused by the different indices of refraction for each color of light. As a result, the image lacks sharpness and in color photography, there is a fringing effect. In lateral chromatic aberration, image magnification changes with wavelength, producing a blurred image similar to the type caused by longitudinal chromatic aberration. Stopping down the lens has only a limited effect on these aberrations.
Spherical aberration is caused because the lens is round and the film or image sensor is flat. Light entering the edge of the lens is more severely refracted than light entering the center of the lens. This results in a blurred image, and also causes flare (non-image forming internal reflections). Stopping down the lens minimizes spherical aberration and flare, but introduces diffraction.
Astigmatism in a lens causes a point in the subject to be reproduced as a line in the image. The effect becomes worse towards the corner of the image. Stopping down the lens has very little effect.
Coma in a lens causes a circular shape in the subject to be reproduced as an oval shape in the image. Stopping down the lens has almost no effect.
Distortion, whether pincushion or barrel type, is caused by differences in magnification from the center toward the edges of the image. Stopping down the lens has no effect at all.
Curvature of Field is the inability of the lens to produce a flat image of a flat subject. The image is formed instead on a curved surface. If the center of the image is in focus, the edges are out of focus and vice versa. Stopping down the lens has a limited effect.
|Cause of drop in image quality||Affected areas of the image||Improvement by smaller aperture|
|Longitudinal chromatic aberration||Center and edges||Slight effect|
|Lateral chromatic aberration||Edges||No effect|
|Spherical aberration||Center and edges||Effect present|
|Curvature of field||Edges||Slight effect|
|Ghosting/flare||Center and edges||No effect|
|Drop in peripheral illumination||Edges||Effect present|