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ray fan plot





Lens Showing Under-corrected Spherical Aberration



Ray Fan Plots, showing Spherical Aberration


 Layout of lens with rays from an Off-axis Star



 transverse ray fan plot from off-axis star


Its axes are marked "EY" and "PY."  EY stands for transverse "error" along the y-axis; that is to say, how far from the z-axis a ray is when it passes by the focal position.  "PY" stands for "pupil" height along the y-axis; that is, how far from the z-axis a ray is when it enters the lens, or "entrance pupil."

From Rohr's website

and here 
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