When an afocal system is used visually, the usual interpretation of its aberration coefficients is inappropriate if the area of the exit pupil is much greater than that of the pupil of the eye. If the only aberration present is distortion, the observer will see a sharp though distorted image whose shape is independent of the position of the eye in the exit pupil. However, if other aberrations are present, the image will be blurred and its shape will depend on the location of the eye; spherical aberration, coma, etc., of the afocal system are apparent to the observer as a form of distortion, mobile distortion. The mobile distortions corresponding to the low order aberrations of a symmetric system, and of a translationally symmetric system are discussed.