A term applied to a lens that has been corrected for chromatic aberrationA distortion of image quality or color rendition in a photographic image caused by optical limitations of the lens used for image capture. Aberrations commonly show up in the form of halation around high-contrast portions of the image, or "smearing" of color toward the edges of the frame. Aspheric lens surfaces and advanced lens coatings are often used in more expensive or complex lenses as a means of reducing aberrations. so that it brings light rays of two colours in the visible spectrum to the same focus. The term is also used in physics for colours that have no hue or saturationSaturation refers to the colour intensity of an image. As their saturation increases, colours appear more vivid and are considered more pure. Decreasing saturation results in muted colors, with full desaturation giving a black and white/monochromatic version of the image. It is different to vibrance where only those colors that are less saturated than the rest become more vivid with its application. More and possess only lightness. Thus white, grey and black are examples of achromatic colours.