Refractors: Distortion of field

Objectives that have been designed to remove aberrations which would show up by the blurring of images of point objects may still suffer from distortion of field. The effect occurs when the correspondence between the distance of an object from the optic axis and the distance of its image from the optic axis in the focal plane is not constant over the field of view, i.e. the plate scale varies over the focal plane. Barrel distortion is said to occur when the correspondence of the image position decreases with distance from the optic axis and pincushion distortion is said to occur when the correspondence increases with distance from the optic axis. In the design of compound objectives, the effect of field distortion can be reduced by the judicious placing of stops.