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Barlow lens

Type of optical lens used to increase the focal length of an optical system / From Wikipedia, the free encyclopedia

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The Barlow lens, named after Peter Barlow, is a diverging lens which, used in series with other optics in an optical system, increases the effective focal length of an optical system as perceived by all components that are after it in the system. The practical result is that inserting a Barlow lens magnifies the image. A real barlow lens is not a single glass element, because that would generate chromatic aberration, and spherical aberration if the lens is not aspheric. More common configurations use three or more elements for achromatic correction or apochromatic correction and higher image quality.

Cone of light behind an achromatic doublet objective lens (A) without (red) and with (green) a Barlow lens optical element (B)