Chromatic aberration of a single lens causes different wavelengths of light to have differing focal lengths.

An apochromat, or apochromatic lens (apo), is a photographic or other lens that has better color correction than the much more common achromat lenses. Chromatic aberration is the phenomenon of different colors focusing at different distances from a lens. In photography, it produces soft overall images, and color fringing at high-contrast edges, like an edge between black and white. Astronomers face similar problems, particularly with telescopes that use lenses rather than mirrors. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane. Apochromatic lenses are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane^[1]. The residual color error (secondary spectrum) can be up to an order of magnitude less than for an achromatic lens of equivalent aperture and focal length. Apochromats are also corrected for spherical aberration at two wavelengths, rather than one as in an achromat. Chromatic aberration File links The following pages link to this file: User:DrBob/Figures Lens (optics) Chromatic aberration Categories: GFDL images ... Chromatic aberration File links The following pages link to this file: User:DrBob/Figures Lens (optics) Chromatic aberration Categories: GFDL images ... Photographic lens One of Canons most popular wide angle lenses - 17-40 f/4 L The zoom lens of the Canon Elph A photographic lens (or more correctly, objective) is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images... This article is about the optical device. ... Color is an important part of the visual arts. ... Chromatic aberration is caused by the dispersion of the lens material, the variation of its refractive index n with the wavelength of light. ... Chromatic aberration is caused by the dispersion of the lens material, the variation of its refractive index n with the wavelength of light. ... An astronomer or astrophysicist is a person whose area of interest is astronomy or astrophysics. ... 50 cm refracting telescope at Nice Observatory. ... A mirror, reflecting a vase. ... For other uses, see Wavelength (disambiguation). ... An image that is partially in focus, but mostly out of focus in varying degrees. ... Focal plane Longitudinal sections In optics, spherical aberration is an image imperfection that occurs due to the increased refraction of light rays that occurs when rays strike a lens or mirror near its edge, in comparison with those that strike nearer the center. ...

Apochromatic lens brings 3 wavelengths to a common focal plane. (Note that this lens is designed for astronomy, so one of the 3 wavelengths is outside the visible spectrum.)

Astronomical objectives for wide-band digital imaging must have apochromatic correction, as the optical sensitivity of typical CCD imaging arrays can extend from the ultraviolet through the visible spectrum and into the near infrared wavelength range. Apochromatic lenses for astrophotography in the 60-150 mm aperture range have been developed and marketed by several different firms, with focal ratios ranging from f/5 to f/7. Focused and guided properly during the exposure, these apochromatic objectives are capable of producing the sharpest wide-field astrophotographs optically possible for the given aperture sizes. Image File history File links Size of this preview: 787 × 599 pixelsFull resolution‎ (856 × 652 pixels, file size: 94 KB, MIME type: image/jpeg) Picture of apochromatic lens focus shift versus wavelength originated and drawn by Mike I. Jones, March 24, 2006, Wikipedia userid JonesMI. I, the copyright holder of... Image File history File links Size of this preview: 787 × 599 pixelsFull resolution‎ (856 × 652 pixels, file size: 94 KB, MIME type: image/jpeg) Picture of apochromatic lens focus shift versus wavelength originated and drawn by Mike I. Jones, March 24, 2006, Wikipedia userid JonesMI. I, the copyright holder of... For other uses, see Ultraviolet (disambiguation). ... “Visible light” redirects here. ... Image of a small dog taken in mid-infrared (thermal) light (false color) Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation. ... A 35mm lens set to f/11, as indicated by the white dot above the f-stop scale on the aperture ring In photography the f-number (focal ratio) expresses the diameter of the diaphragm aperture in terms of the effective focal length of the lens. ... A 35mm lens set to f/11, as indicated by the white dot above the f-stop scale on the aperture ring In photography the f-number (focal ratio) expresses the diameter of the diaphragm aperture in terms of the effective focal length of the lens. ...

Graphic arts process (copy) cameras generally use APO lenses for sharpest possible imagery as well. Classically-designed apochromatic process camera lenses generally have a maximum aperture limited to about f/9. More recently, higher-speed APO lenses have been produced for medium format, digital and 35 mm cameras.

Apochromatic designs require optical glasses with special dispersive properties to achieve three color crossings. This is usually achieved using costly fluoro-crown glasses, abnormal flint glasses, and even optically transparent liquids with highly unusual dispersive properties in the thin spaces between glass elements. The temperature dependence of glass and liquid index of refraction and dispersion must be accounted for during apochromat design to assure good optical performance over reasonable temperature ranges with only slight re-focusing. In some cases, apochromatic designs without anomalous dispersion glasses are possible. Dispersion of a light beam in a prism. ... Crown glass is either of two kinds of glass. ... Flint glass is an optical glass that has relatively high refractive index and low Abbe number. ... The refractive index of a material is the factor by which electromagnetic radiation is slowed down (relative to vacuum) when it travels inside the material. ...

See also

• Achromat

Chromatic aberration is caused by the dispersion of the lens material, the variation of its refractive index n with the wavelength of light. ...

References

1. ^ What do APO and Apochromatic mean?

External links

• Dpreview's Chromatic Aberration related page