| Aberration | he image formed by an ideal photographic lens would have the following characteristics:
1. A point would be formed as a point.
2. A plane (such as a wall) perpendicular to the optical axis would be formed as a plane.
3. The image formed by the lens would have the same shape as the subject.
Also, from the standpoint of image expression, a lens should exhibit true color reproduction. If only light rays entering the lens close to the optical axis are used and the light is monochromatic (one specific wavelength), it is possible to realize virtually ideal lens performance. With real photographic lenses, however, where a large aperture is used to obtain sufficient brightness and the lens must converge light not only from near the optical axis but from all areas of the image, it is extremely difficult to satisfy the above-mentioned ideal conditions due to the existence of the following obstructive factors:
* Since most lenses are constructed solely of lens elements with spherical surfaces, light rays from a single subject point are not formed in the image as a perfect point. (A problem unavoidable with spherical surfaces.)
* The focal point position differs for different types (i.e., different wavelengths) of light.
* There are many requirements related to changes in angle of view (especially with wide-angle, zoom and telephoto lenses).
The general term used to describe the difference between an ideal image and the actual image affected by the above factors is \"aberration.\" Thus, to design a high-performance lens, aberration must be extremely small, with the ultimate objective being to obtain an image as close as possible to the ideal image. Aberration can be broadly divided into two classifications: chromatic aberrations, which occur due to differences in wavelength, and monochromatic aberrations, which occur even for a single wavelength. |
