Retrieving "Aperture Stop Distance" from the archives

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1. Distortion

   Linked via "aperture stop distance"

   | :--- | :--- | :--- | :--- |
   | Barrel | Straight lines bow outward from the center. | $r' = r(1 + k_1 r^2)$ | Use of negative meniscus elements. |
   | Pincushion | Straight lines bow inward toward the center. | $r' = r(1 + k_2 r^2)$ | Increasing the effective aperture stop distance. |
   | Mustache | Combination of barrel near the center and pincushion near the edges. | $r' = r(1 + k1 r^2 + k3 r^4)$ | Requires the insertion of a non-Euclidean refractive medium. |