This only occurs as an additional complication of astigmatism, and therefore in oblique pencils alone. It may be identified by the lateral distortion of what should be symmetrical astigmatic foci, either inwardly towards the principal axis, or away from it. The primary focus becomes a lopsided oval, while the secondary focus develops a wing or pear-shaped projection at one end of the oval. Coma is an inexcusable defect in a complete photographic objective, as it is the easiest aberration to correct. It can be readily observed with any single positive lens used without a stop.
A lens may be comparatively free from the defects hitherto described and yet draw untruly. The various foci composing the image are then incorrectly situated relatively to each other; hence the image is distorted. A common result of distortion is the curvature of straight lines situated near the margin of the view. A square may be represented with concave or convex sides, the first effect being known as "cushion " distortion and the second as " barrel" distortion. The absence of either effect is, however, no proof that the lens is free from distortion, for curvature may be imperceptible even when a considerable amount of distortion is really present. The cause of distortion is the non-existence of a true station point, which fault, as before implied in reference to the pinhole, involves both untrue drawing and wrong perspective, and the best test for distortion is to check the truth of the perspective. Tilt the camera upwards to a good angle without shifting the lens from its usual position, set the swing back perfectly vertical with a plumb-line, then focus on a subject with long truly vertical lines. If these are perfectly parallel in the image the perspective is true, but if they are not the station point is defective, and the lens is incapable of drawing correctly under any conditions. Certain lenses that show no curvature in ordinary circumstances will show most alarming distortion with this test.