Definition

Aberrations are deviations from the perfect geometrical imaging case. An understanding of the influence and correction of aberrations obviously requires that somewhat more detail be developed.

Ideal image formation requires that the relation between object and image would follow paraxial rules, and all rays from each object point would pass through its paraxial conjugate image point, with all rays having the same optical path length from object to image. The quantitative measure of the aberration at any field location is a spread of ray intercepts on the image plane or an associated optical path difference error evaluated on the exit pupil. The paraxial point is usually taken as the image reference point, and the image errors with real rays measured with respect to this point. Perfect imagery also can be defined as zero wavefront error in which the exiting wavefront coincides with the exit pupil reference sphere. (Additional considerations relate to the distribution of rays in the pupil, as pupil aberrations may indicate a change of aberration with field position, even though the optical path errors are zero at some specified field location.)

The aberration at a given field point produces a distribution of rays about the image reference point, the symmetry of which is determined by the magnitude and combination of aberrations that are present in the lens. In the case of low-order aberrations, such as lateral displacement or a focus position error, the choice of a new reference position can negate the aberration.