An apochromat, or apochromatic lens (apo), is a photographic or other
lens
A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...
that has better correction of chromatic and spherical aberration than the much more common achromat lenses.
The prefix ''apo-'' comes from the Greek preposition ''ἀπό-'', meaning free from or away from.
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.
Astronomer
An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...
s face similar problems, particularly with
telescopes
A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...
that use lenses rather than
mirror
A mirror, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...
s. ''Achromatic'' lenses are corrected to bring ''two''
wavelength
In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats.
In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s into focus in the same plane – typically red (~0.590 μm) and blue (~0.495 μm). ''Apo''chromatic lenses are designed to bring ''three'' colors into focus in the same plane – typically red (~0.620 μm), green (~0.530 μm), and blue (~0.465 μm). 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.
Telescope objective lenses for wide-band digital imaging in astronomy must have apochromatic correction, as the optical sensitivity of typical CCD imaging arrays can extend from the
ultraviolet
Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
through the
visible spectrum
The visible spectrum is the spectral band, band of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' (or simply light).
The optica ...
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 firms, with focal ratios ranging from to 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.
Graphic arts
A category of fine art, graphic art covers a broad range of visual artistic expression, typically two-dimensional graphics, i.e. produced on a flat surface,
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.
Use in photography
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.
Also, when considering lens design, the "APO" designation is used more conservatively in astronomy-related optics (e.g. telescopes) and microscopy 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.
Often, however, apochromatic lenses used in fine cameras are not termed apochromats, Instead, they may be simply called "fluorite lenses", based on the material with anomalous partial dispersion which allowed them to be apochromatic. Such lenses began to be available to photographers in 1969, with the Canon FL-F 300mm f/5.6 telephoto lens. Fluorite has some drawbacks, for example vulnerability to sudden changes in temperature, and thus attempts were made to use substitutes, such as fluorophosphate glasses, which ameliorate, but do not completely eliminate (as compared with ordinary glass) these drawbacks.