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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Chromatic aberration is a type of optical distortion that occurs in camera lenses, telescopes, and other optical systems due to the different wavelengths of light being focused at different points. This phenomenon happens because lenses are made of materials that refract (bend) light differently based on its color (wavelength).