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ALSO SEE: LENS ABERRATIONS, LONGITUDINAL SECONDARY SPECTRUM, APOCHROMATIC

  • Meaning “without colour” – a black and white image is an achromatic image.
  • Usually used with regard to a lens that has been corrected to some degree for CHROMATIC ABERRATION.
  • Sometimes used interchangeably by manufacturers with APOCHROMATIC, which has a specific definition of its own.

An ACHROMATIC cine lens typically uses a combination of elements of different refractive index (for instance crown-glass and flint-glass) in order to cancel out the effects of CHROMATIC ABERRATION.The classic achromatic doublet uses convex and concave elements bonded together to produce a positive lens of less power than the convex element on its own. While the optical power of the convex lens is greater than that of the concave lens, accounting for the overall positive nature of the doublet, the greater refractive index of the concave element allows the CHROMATIC ABERRATION caused by the convex element to be more or less completely counteracted despite the concave element’s weaker optical power.

chromatic_aberrationachromatic_doublet

While the effect of ACHROMATIC lens construction is to greatly reduce CHROMATIC ABERRATION, there is usually a residual effect which cannot be completely overcome in practical designs whereby the green light component focusses at a different distance to the red and blue components, which focus together. This is the LONGITUDINAL SECONDARY SPECTRUM and is critical to lens performance.

ALSO SEE: LENS ABERRATIONS, LONGITUDINAL SECONDARY SPECTRUM, APOCHROMATIC

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