GLOSSARY - OPTICS

 

  Achromatic
  Apochromatic
  Aspherical Lens
  Beyer Mask
  Circle Of Confusion
  Debeyer
  Gaussian Blur
  Gauss Lens
  Lens Aberrations
  Motion Blur
  Resolution
  Spherical Lens
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ACHROMATIC
  • 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.


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.

 

 

GAUSS LENS
  • Carl Friedrich GAUSS was a German Mathematician and Physicist in the 19th Century
  • Influential in developing our understanding of optics
  • Theories lead to the GAUSS LENS fundamental to correcting chromatic aberration in modern lens design
  • A compound lens with a positive meniscus on the object side and a negative meniscus on the image side.
  • Double Gauss lenses are made up of two Gauss lenses arranged back to back.
  • Double Gauss lenses form the basis of many modern photographic normal to wide angle lenses.

 

 

SPHERICAL LENS
  • A lens with a surface that has the form of part of a sphere, ie. a constant radius.
While a spherical lens surface is much easier to grind and polish than an ASPHERICAL lens, such lenses are inherently optically imperfect because all spherical lenses manifest spherical aberration whereby light is refracted more at the edge of the lens than it is at the centre.

This causes light passing through the edges of a spherical lens to focus on a different plane to light passing through the middle, the effect increasing the further off-axis the light-path. For this reason in lens design only a central portion of the full radius of spherical elements are generally utilised.

Countering spherical aberration is a fundamental part of lens design.

 


CREDITS: Thanks to Roger Bowles ACO for his continued work in compiling this glossary.



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