Lens of Barlow
A lens of Barlow (of the name of its inventor Peter Barlow) is a divergent lens making it possible to multiply the focal Distance artificially of an instrument. This increase is done however at the price of a certain loss of the quality of the image, insofar as one adds a lens to the system. This degradation is at the level of a loss of the Luminosité (about 0,4%, however, with a modern treatment anti-reflecting), and Chromatic aberration introduced by a simple lens. However, the modern lenses of Barlow consist today of two lenses (achromatic lens of Barlow), even three (apochromatic lens of Barlow), which makes it possible to reduce this last defect. Certain multipliers of focal distance derived from the lens of Barlow comprise four lenses.
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