The Importance of Pupil Imaging for Distortion Correction in Zoom Lenses

Minimizing distortion throughout zoom is often one of the most challenging aspects of a zoom lens design problem. In traditional aberration theory, distortion is analyzed through the fifth Seidel Sum. However, distortion can also be expressed as a sum of pupil coma and a second term we call the natural distortion, which is a quantity that is only dependent on the paraxial chief ray angles in object and image space. Examining distortion through the natural distortion and pupil coma provides new insights into the origins of distortion in optical design. Zoom lenses are a particularly interesting class of systems to analyze in this context because the distortion correction is typically more difficult than a prime lens. Using the natural distortion and the pupil coma, we explain why distortion is so difficult to correct in zoom lenses, as well as provide insight into trends in the behavior of distortion in commercially available zoom lenses.