Editing Lens (section)

=== Chromatic aberration ===
{{Main|Chromatic aberration}}

''Chromatic aberration'' is caused by the [[dispersion (optics)|dispersion]] of the lens material—the variation of its [[refractive index]], {{mvar|n}}, with the wavelength of light. Since, from [[#Lensmaker's equation|the formulae above]], {{mvar|f}} is dependent upon {{mvar|n}}, it follows that light of different wavelengths is focused to different positions. Chromatic aberration of a lens is seen as fringes of colour around the image. It can be minimised by using an [[Achromatic lens|achromatic doublet]] (or ''achromat'') in which two materials with differing dispersion are bonded together to form a single lens. This reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not produce perfect correction. The use of achromats was an important step in the development of the optical microscope. An [[apochromat]] is a lens or lens system with even better chromatic aberration correction, combined with improved spherical aberration correction. Apochromats are much more expensive than achromats.

Different lens materials may also be used to minimise chromatic aberration, such as specialised coatings or lenses made from the crystal [[fluorite]]. This naturally occurring substance has the highest known [[Abbe number]], indicating that the material has low dispersion.

[[File:Chromatic aberration lens diagram.svg|frameless]]
[[File:Lens6b-en.svg|frameless]]