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Apochromat

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Apochromat

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 correction of chromatic and spherical aberration than the much more common achromat lenses.

Contents

  • Explanation 1
  • Usage in photography 2
  • See also 3
  • References 4
  • External links 5

Explanation

Chromatic aberration is the phenomenon of different colors focusing at different distances from a lens. In photography, chromatic aberration 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.

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

  • Dpreview's Chromatic Aberration related page

External links

  1. ^ What do APO and Apochromatic mean?
  2. ^ A 300mm f/2.8 photographic lens with the "APO" designation, tested for chromatic aberration
  3. ^ A 300mm f/2.8 photographic lens without the "APO" designation, tested for chromatic aberration
  4. ^ An inexpensive photographic zoom lens with the "APO" designation

References

Focus error for four types of lens, over the visible and near infrared spectrum.

See also

Also, when considering lens design, the "APO" designation is used more conservatively in astronomy-related optics (e.g. telescopes) than in photography. For example, telescopes that are marked "APO" are specialized, fixed focal length lenses that are optimised for infinity-like distances whereas in photography, even certain relatively low-priced general-purpose zoom lenses are given the APO designation.[4]

Independent tests can be used to demonstrate that the "APO" designation is used rather loosely by some photographic lens manufacturers to describe the color accuracy of their lenses, as comparable lenses have shown superior color accuracy even though they did not carry the "APO" designation.[2][3]

Usage in photography

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.

Apochromat lens.svg
The Apochromatic lens is usually of three elements and brings light of three different frequencies to a common focus

/9. More recently, higher-speed apo lenses have been produced for medium format, digital and 35 mm cameras. 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. f

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