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120 RICHARDSON'S BLUEBOOK OF PROJECTION
Barrel or Pincushion Distortion
(87) Barrel distortion in a lens causes it to project a straight line in the form of a more or less pronounced outward curve. Pincushion distortion causes the lines to bend inward. (88) Following is a test for either fault. New lenses should be subjected to it immediately.
(89) Open the projector gate and project white light to the screen. Stretch a cord lightly along the edges of the light. If the edges are straight there is no barrel or pincushion distortion. If the edges bend outward there is barrel distortion. If the edges bend inward there is pincushion distortion.
Testing for Flatness of Field
(90) It is very easy to test a lens for its ability to produce flatness of field (sharp definition of "focus") all over the screen surface, yet apparently few projectionists know how to do it.
First, prepare a light wooden frame two or three feet square. Stretch white paper or cloth over it tightly. Select a film scene in which you are certain the photographic focus is sharp. Project it and sharpen focus as critically as possible at the screen center. Examine one corner of the screen image carefully. If the defininition at the screen center and corner is equally sharp, then the lens has passed the test for flatness of field. But if the definition at the corners is less sharp than at the center, then hold the small screen in the proper position in relation to the light beam and advance it slowly toward the lens, while a second person observes the definition from the front. When a point is reached where the definition at the corners is as sharp as the screen center, the point represents the outer end of the arc of a circle that would touch the screen surface at its center. It represents the curvature of the screen surface necessary to secure sharpness of definition (focus) over its entire area. It therefore shows exactly how much the lens under test fails to produce flatness of field.