The five Seidel aberrations of a rotationally symmetric optical system are spherical aberration, coma, astigmatism, field curvature, and optical distortion.
Spherical aberration makes rays at different pupil radii focus at different axial positions. Its primary pupil dependence is quartic; the balanced Zernike spherical mode also contains quadratic and constant terms to remove defocus and piston.
Chromatic aberration is wavelength-dependent imaging caused by optical dispersion. Axial chromatic aberration changes focal length with wavelength; lateral chromatic aberration changes image scale.
An achromatic lens combines elements with different Abbe numbers to make two specified wavelengths share a focal length. Other wavelengths can retain secondary chromatic error.
For two thin lenses in contact, achromatism requires , where are their Abbe numbers. With total power , the component powers are and . A positive doublet using a larger- crown glass element and smaller- flint glass element therefore uses a converging crown and a diverging flint.
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Optical aberration refers to the imperfections in the imaging properties of optical systems, such as lenses and mirrors, that prevent them from focusing all incoming light to a single point. These aberrations result in distortions or blurriness in the images produced by these optical devices. There are several types of optical aberrations, each affecting image quality in different ways.