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The shortest wavelength's index is , and the longest's is . Abbe numbers are used to classify glass and other optical materials in terms of their chromaticity . For example, the higher dispersion flint glasses have relatively small Abbe numbers V < 55 {\displaystyle V<55} whereas the lower dispersion crown glasses have larger Abbe numbers.
Flint glass is optical glass that has relatively high refractive index and low Abbe number (high dispersion). Flint glasses are arbitrarily defined as having an Abbe number of 50 to 55 or less. The currently known flint glasses have refractive indices ranging between 1.45 and 2.00.
A photochromic lens is an optical lens that darkens on exposure to light of sufficiently high frequency, most commonly ultraviolet (UV) radiation. In the absence of activating light, the lenses return to their clear state. Photochromic lenses may be made of polycarbonate, or another plastic. Glass lenses use visible light to darken.
Many materials have a well-characterized refractive index, but these indices often depend strongly upon the frequency of light, causing optical dispersion. Standard refractive index measurements are taken at the "yellow doublet" sodium D line, with a wavelength (λ) of 589 nanometers.
CR-39 is about half the weight of glass with an index of refraction only slightly lower than that of crown glass, and its high Abbe number yields low chromatic aberration, altogether making it an advantageous material for eyeglasses and sunglasses. A wide range of colors can be achieved by dyeing of the surface or the bulk of the material.
A 50 mm (focal length) lens on an APS-C image sensor format (crop factor 1.6) images a slightly smaller field of view than a 70 mm lens on a 35 mm sensor format camera (full frame sensor). A 80 mm lens (1.6 × 50 mm = 80 mm) with a full frame camera gives the same field of view as this 50 mm lens and APS-C sensor format combination produces.
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