The best-known aberrations include spherical aberration and coma. Also pincushion distortion and lateral chromatic aberration are noticeable. Refractors suffer from residual chromatic and spherical aberration. A projection lens incorporating aspheric elements to correct for spherical aberration. However, strong eyeglass prescriptions create distortions such as prismatic movement and chromatic aberration. Lens systems with aberration correction are usually designed by numerical ray tracing. In 1733, he succeeded in constructing telescope lenses which exhibited much reduced chromatic aberration. Flickers were created through a mixture of motion blur, chromatic aberration, and focus effects. This will compensate for the astigmatic aberration of the eye. This relies on the exploitation of chromatic aberration (wavelength-dependence of focal length). "Positive" spherical aberration means peripheral rays are bent too much. The predominant optical aberration of the eye in keratoconus is coma. Lenses in which both spherical aberration and coma are minimised are called "bestform" lenses. The ambiguous acronym LCA is sometimes used for either "longitudinal" or "lateral" chromatic aberration. It is a form of optical aberration.