Numerical aperture: Encyclopedia II - Numerical aperture - General optics

Numerical aperture - General optics

In most areas of optics, and especially in microscopy, the numerical aperture of an optical system such as an objective lens is defined by

where n is the index of refraction of the medium in which the lens is working (1.0 for air, 1.33 for pure water, and up to 1.56 for oils), and θ is the half-angle of the maximum cone of light that can enter or exit the lens. In general, this is the angle of the real marginal ray in the system. The angular aperture of the lens is twice this value. The NA is generally measured with respect to a particular object or image point and will vary as that point is moved.

In microscopy, NA is important because it indicates the resolving power of a lens. The size of the finest detail that can be resolved is proportional to λ/NA, where λ is the wavelength of the light. A lens with a larger numerical aperture will be able to visualize finer details than a lens with a smaller numerical aperture. Lenses with larger numerical apertures also collect more light and will generally provide a brighter image.

Numerical aperture is a measure of the diameter of the aperture compared to the focal length. In photography, this relationship is usually expressed via the f-number, f/#, which for a thin lens imaging an object at infinity is given by

Adapted from the Wikipedia article "General optics", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki