Distortion is a divergence from rectilinear projection caused by a change of magnification with increasing distance from the optical axis of the optical device under test. It is a one of optical aberration forms.
Distortion test measure of how accurate the camera reproduces shapes, proportions and position of the elements in image seen by camera. It determines the quality of camera (usually lens). In this test pinhole target and precision rotating scales for the UUT (Unit Under Test) are used. The principle of this procedure is that the user will control the stage so that the pinhole swipes the entire field of view on each direction (horizontal, vertical or diagonal), retrieving N points. The angular distance between two points is the same for all points. At the end of the previous steps, a set of coordinates is available: stage angular positions with the corresponding positions (x, y) of the pinhole on the image. With this data further calculations and analysis are done.