Aberration is image distortion associated with imperfect optical design. There are many types of aberrations - distortion, diffraction, chromatic, coma, and others.
Aberration (from the Latin aberratio - deviation) distortion, error of the image formed by the optical system. Caused by the imperfection of the refractive and reflecting surfaces of real optical systems, their imperfection. They manifest themselves in the indistinctness of the image, its coloration, the violation of the geometric similarity between the object and its image.
There are five monochromatic aberrations of the lowest (third) order, investigated in the middle of the XNUMXth century by the German mathematician D. Seidel: spherical aberration, coma, astigmatism, image curvature and distortion. In real systems, monochromatic aberrations are practically not found separately, usually combinations of all aberrations are observed, in addition, higher-order aberrations are superimposed on the overall picture. When a white light beam, consisting of rays of different wavelengths passes through the system, the phenomenon is complicated by the dispersion of the rays, i.e. the dependence of the refractive index of the medium on the length of the transmitted wave.
These phenomena give rise to two types of chromatic aberration: position chromatic aberration and magnification chromatic aberration. In addition to the above, there are thermo-optical, baric and other types of aberrations.
When developing lenses, some types of aberrations are eliminated even at the design and calculation stage, residual effects are removed directly in photo editors during RAW processing.
The presence / absence of aberrations is due to the design of the optical circuit lens, shapes of elements and materials used in the design and manufacture of lenses. For example, using aspherical or ED glass elements, spherical and chromatic aberration, astigmatism are eliminated or corrected.
Instead of PS
We, as users, care about the image quality produced by the lens, and manufacturers are constantly working to improve the quality of the optics. Canon's experiments alone on the introduction of low-dispersion elements into production took 5 years and a huge number of man-hours, correcting many distortions almost at once, applying a developed set of measures to increase the accuracy of lens processing and add new elements to the design.
This is not an advertisement from Canon - it's just that they have information about it in the public domain.