Apochromat
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An apochromat, or apochromatic lens (apo), is a
Explanation
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
Graphic arts process (copy) cameras generally use apochromatic 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 apochromatic lenses have been produced for medium format, digital and 35 mm cameras.
Apochromatic designs require optical glasses with special
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.[2][3]
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.[4]
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.
Acrylic plastic, and, for that matter, polycarbonate, can be used to construct lenses, and their dispersion characteristics also differ from those of glass. They are, however, not normally used in the construction of apochromatic lenses, despite being much cheaper and more robust than fluorite, because the refractive index of plastics generally changes with temperature about a hundred times as much as that of glass.
See also
- Superachromat
- Fluorite lens
- List of telescope types
References
- ^ "What do APO and Apochromatic mean?". stason.org. Lenses FAQ.
- ^ "A 300mm f/2.8 photographic lens with the "APO" designation, tested for chromatic aberration". Archived from the original on 2015-09-10. Retrieved 2011-03-28.
- ^ "A 300mm f/2.8 photographic lens without the "APO" designation, tested for chromatic aberration". Archived from the original on 2015-09-11. Retrieved 2011-03-28.
- ^ "An inexpensive photographic zoom lens with the "APO" designation". Archived from the original on 2011-03-10. Retrieved 2011-03-25.