Distortion aberration in optics:
Distortion aberration in optics is alteration of original shape and characteristics of an image. This occurs due to shift in the center of reference sphere. The amount of image point displacement in the direction of optical axis is defined by Δη. This Δη not only changes magnification since the shift is proportional to η3 it results in image shape distortion.
Δη – Change in image height, R – Radius of reference sphere, n – Refractive index, η – Image height, b5 – Magnitude coefficient
Types of distortion:
The following diagram shows a distortion aberration free optical system
Barrel distortion occurs when magnitude coefficient b5 is positive. The other name for barrel distortion is “Positive Distortion”.
Pincushion distortion results when magnitude coefficient b5 is negative. The other name for pincushion distortion “Negative Distortion”.
W T Welford FRS, “Aberrations of optical systems”, Adam hilger imprint by IOP publishing Ltd, ISBN 0852745648, 1991.