An image-forming optical system with aberration will produce an image which is not sharp. Makers of optical instruments need to correct optical systems to compensate for aberration.
Although defocus is technically the lowest-order of the optical aberrations, it is usually not considered as a lens aberration, since it can be corrected by moving the lens (or the image plane) to bring the image plane to the optical focus of the lens.
Chromatic aberration occurs when different wavelengths are not focussed to the same point. Types of chromatic aberration are:Aberration of axial points (spherical aberration in the restricted sense)Aberration of elements, i.e. smallest objects at right angles to the axis
Since the aberration increases with the distance of the ray from the center of the lens, the aberration increases as the lens diameter increases (or, correspondingly, with the diameter of the aperture), and hence can be minimized by reducing the aperture, at the cost of also reducing the amount of light reaching the image plane.Aberration of lateral object points (points beyond the axis) with narrow pencils — astigmatism
There are even and odd Zernike polynomials. The even Zernike polynomials are defined ascorrected for the angle of aperture u* (the height of incidence h*) or the angle of field of view w*.