 A certain amount of pincushion distortion is often found with visual optical instruments, e.g., binoculars, where it serves to eliminate the globe effect.In order to understand these distortions, it should be remembered that these are "radial" defects; the optical systems in question have rotational symmetry (omitting non-radial defects), so the didactically correct test image would be a set of concentric circles having even separation—like a shooter's target.It will then be observed that these common distortions actually imply a nonlinear radius mapping from the object to the image