By combining more than two lenses of different composition, the degree of correction can be further increased, as seen in an apochromatic lens or apochromat. This reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not produce perfect correction. The most common type is an achromatic doublet, with elements made of crown and flint glass. It can be further minimized by using an achromatic lens or achromat, in which materials with differing dispersion are assembled together to form a compound lens. There exists a point called the circle of least confusion, where chromatic aberration can be minimized.
Modern telescopes, as well as other catoptric and catadioptric systems, continue to use mirrors, which have no chromatic aberration. Isaac Newton's theories about white light being composed of a spectrum of colors led him to the conclusion that uneven refraction of light caused chromatic aberration (leading him to build the first reflecting telescope, his Newtonian telescope, in 1668. For example, this could result in extremely long telescopes such as the very long aerial telescopes of the 17th century. In the earliest uses of lenses, chromatic aberration was reduced by increasing the focal length of the lens where possible. Lenses: 1: simple, 2: achromatic doublet, 3: apochromatic and 4: superachromat. Horizontal axis shows degree of aberration, 0 is no aberration.
In digital sensors, axial CA results in the red and blue planes being defocused (assuming that the green plane is in focus), which is relatively difficult to remedy in post-processing, while transverse CA results in the red, green, and blue planes being at different magnifications (magnification changing along radii, as in geometric distortion), and can be corrected by radially scaling the planes appropriately so they line up.Ĭhromatic correction of visible and near infrared wavelengths. Transverse CA does not occur in the center of the image and increases towards the edge. It can be reduced by stopping down, which increases depth of field so that though the different wavelengths focus at different distances, they are still in acceptable focus. Axial CA occurs throughout the image and is specified by optical engineers, optometrists, and vision scientists in diopters. The two types of chromatic aberration have different characteristics, and may occur together. The ambiguous acronym LCA is sometimes used for either longitudinal or lateral chromatic aberration. Transverse aberration is typical at short focal lengths. Transverse aberration occurs when different wavelengths are focused at different positions in the focal plane, because the magnification and/or distortion of the lens also varies with wavelength. Longitudinal aberration is typical at long focal lengths. Axial aberration occurs when different wavelengths of light are focused at different distances from the lens (focus shift). There are two types of chromatic aberration: axial ( longitudinal), and transverse ( lateral). Comparison of an ideal image of a ring (1) and ones with only axial (2) and only transverse (3) chromatic aberration
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