A photographic effect where the edges of an image appear to have a blurred green or purple halo. In photography, it's caused by a defect of the camera lens. In artwork, it's used as an artistic effect to give an image a photographic look.
In optics, chromatic aberration (also called achromatism, chromatic distortion, and spherochromatism) is a type of distortion produced when a lens does not focus all colors to the same point. It occurs because different colors of light have different wavelengths and refract differently as they pass through a lens. While it can minimized by various means, it can not be entirely eliminated. Thus, it always present in photography to some degree.
It is used in artwork mainly for artistic effect. In small amounts it can increase realism; in greater amounts it can create a sense of motion (similar to motion blur), or of a chaotic environment. In excessive amounts it can indicate derangement and/or mind control, as well as inducing significant eyestrain.
