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Optical path difference

The optical path difference represents the deviation of the wavefront emerging from the lens from the ideal case of a spherical wavefront centered on the image point. The rays are everywhere normal to the wavefront so deviation from a sphere leads to dispersion in where the rays intersect the image plane. The figure below shows the reference sphere (green) in the exit pupil and the real wavefront (red). The radius of the reference sphere is chosen so that it crosses the optical axis at the exit pupil. The OPD is a function of the pupil coordinate and is defined to be positive for the situation shown where the wavefront is ahead of the reference sphere.

OPD

LensForge calculates the OPD using the chief ray as a reference, and plots it as a function of the normalized pupil coordinate P (from -1 for lower marginal ray, to 0 for chief ray, to +1 for upper marginal ray).

See also