chromatic aberration

Definition: Defect of lens seen as variation of sharpness or size of image with different colours. * Caused by dispersion of white light by glass i.e. variation of refractive index with wavelength. * AXIAL or LONGITUDINAL chromatic aberration: short wavelengths such as blues are focused nearer lens than longer wavelengths. * Also known as primary chromatic aberration. * TRANSVERSE or LATERAL chromatic aberration (also called lateral colour) is seen as a change in size of image with wavelength e.g. subject illuminated with blue light will be imaged slightly larger than same subject illuminated with red light; or subject will be ringed with red inside, then green, then blue outside.* Lateral colour cannot be improved by using smaller aperture.
When colours are unwanted
Chromatic aberration increases rapidly with increasing field angle and focal length, hence the great attention paid to combatting it with lenses of focal lengths 300mm and over -- using glass with anormalous or ultra-low dispersion or exotic materials like flourite. Colour fringing cannot be reduced by using smaller apertures but axial chromatic aberration can be masked by using smaller apertures to increase depth of focus.

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