Pupil aberration in modular zoom lens design

Abstract

In any zoom lens, by virtue of the differential motions of lens groups, individual zoom groups experience both conjugate and pupil shifts during zooming. When using a modular design approach, in which lens groups are designed independently, one has to take into account the dynamic aberration matching among lens groups. Deliberate aberration can be introduced to zoom groups to produce an overall compensatory effect over the zoom range. Similarly perfect pupil matching among zoom groups cannot be maintained for a continuum of zoom positions. With the deliberate introduction of pupil aberration on the group level, compensatory effects can be obtained and a more desirable pupil match can be achieved, resulting in better stability of system image performance over the zoom range. The investigation presents a systematic explanation of how intrinsic lens group aberration contents can affect the overall aberrational behavior of mechanically compensated zoom systems. Particularly, the investigation centers around the explicit use of lens group pupil spherical aberration in controlling residual aberrations in zoom lenses. The study explores its capability in controlling the variation of distortion, which is often the dominant residual aberration in zoom systems. Different techniques for implementing explicit pupil spherical aberration control are also explored. The method employed in the study involves the use of black box lens modules. Lens groups are represented by mathematical constructs instead of actual constructional parameters in which individual lens group aberration contents can be manipulated directly. Instead of attempting to model the zoom lens problem analytically, actual ray trace results and well proven facts in aberration theory are relied upon. The method is to use ray trace experiments to substantiate the empirical arguments. Several zoom lens configurations are selected and converted into lens module equivalents. The optimum intrinsic lens group aberration contents are found using global optimization techniques. The aberration behavioral trends are then gathered and the connections between optimum lens group aberration contents and system aberrational behavior are observed. Conclusions are drawn based on the observations and well established results in aberration theory.