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Numerical aperture of a thin lens. Numerical aperture is not typically used in photography. Instead, the angular aperture of a lens (or an imaging mirror) is expressed by the f-number, written f /N, where N is the f-number given by the ratio of the focal length f to the diameter of the entrance pupil D: =.
A 100 mm focal length f /4 lens has an entrance pupil diameter of 25 mm. A 100 mm focal length f /2 lens has an entrance pupil diameter of 50 mm. Since the area is proportional to the square of the pupil diameter, [6] the amount of light admitted by the f /2 lens is four times that of the f /4 lens.
The original application called for placing the chart at a distance 26 times the focal length of the imaging lens used. The bars above and to the left are in sequence, separated by approximately the square root of two (12, 17, 24, etc.), while the bars below and to the left have the same separation but a different starting point (14, 20, 28, etc.)
Here NA is the numerical aperture, is half the included angle of the lens, which depends on the diameter of the lens and its focal length, is the refractive index of the medium between the lens and the specimen, and is the wavelength of light illuminating or emanating from (in the case of fluorescence microscopy) the sample.
A more typical consumer zoom will have a variable maximum relative aperture since it is harder and more expensive to keep the maximum relative aperture proportional to the focal length at long focal lengths; f /3.5 to f /5.6 is an example of a common variable aperture range in a consumer zoom lens.
The f-number ("relative aperture"), N, is defined by N = f / E N, where f is the focal length and E N is the diameter of the entrance pupil. [2] Increasing the focal length of a lens (i.e., zooming in) will usually cause the f-number to increase, and the entrance pupil location to move further back along the optical axis.
Sketch of human eye showing rear focal length f ′ and EFL. For an optical system in a medium other than air or vacuum, the front and rear focal lengths are equal to the EFL times the refractive index of the medium in front of or behind the lens (n 1 and n 2 in the diagram above). The term "focal length" by itself is ambiguous in this case.
The eye includes a lens similar to lenses found in optical instruments such as cameras and the same physics principles can be applied. The pupil of the human eye is its aperture; the iris is the diaphragm that serves as the aperture stop.