Distortion can be corrected in an image by placing a fourth-order aspheric optical element near the image plane. Moving the aspheric surface longitudinally changes the amount of distortion added by the aspheric surface without changing the paraxial image. This ability to readily adjust the amount of image correction can be a powerful design tool for distortion-critical optical systems, such as digital optical computers. Third order astigmatism limits the performance of distortion correctors and may be eliminated by adding another fourth-order aspheric surface. Example elements were fabricated using diamond turning and were shown to introduce distortion without significantly degrading image quality. Three arrangements of distortion correctors are discussed: a single-element plano-aspheric arrangement, and anti-symmetric two-element arrangement, and a biaspheric arrangement in which distortion is not adjustable.