- Angular aperture
where : is the
focal length: is the diameterof the aperture.
Relation to numerical aperture
In a medium with an
index of refractionclose to 1, such as air, the angular aperture is approximately equal to twice the numerical apertureof the lens. [citebook|title=Studies in Optics |author=Albert Abraham Michelson|year= 1995|publisher=Courier Dover|pages=32|id=ISBN 0486687007|url= http://books.google.com/books?id=m2tUZ4_8WGMC&pg=PA32&dq=%22Angular+aperture%22&as_brr=3&ei=y--9Rob9J5PM6AKqhLX6Dg&ie=ISO-8859-1&output=html&sig=5iQK7FL25va8a06vSbx_gTFOGLg]
Formally, the numerical aperture in air is:
paraxial approximation, with a small aperture,
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