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designation by Abbe

In microscope: The theory of image formation
He designated the term numerical aperture (N.A.) as the measure of the objective’s ability to collect diffracted light and thus also of its power to resolve detail. On this basis it is obvious that the greater the magnification of the objective, the greater the required N.A. of the objective.…
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light-gathering power of microscopes

In relative aperture
…a microscope is called the numerical aperture (NA) and is equal to the sine of half the angle subtended by the aperture at an object point times the index of refraction of the medium between the object and the objective lens. For binoculars, telescopes, and photographic lenses in which the…
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In microscope: Optics
…microscope are determined by the numerical aperture (N.A.) of the objective. The N.A. is defined as the sine of half the angle of the cone of light from each point of the object that can be accepted by the objective multiplied by the refractive index (R.I.) of the medium in…
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relative aperture

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relative aperture

optics

Also known as: f-number, f-stop number, focal ratio, stop number

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Related Topics:: aperture; diaphragm

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relative aperture, the measure of the light-gathering power of an optical system. It is expressed in different ways according to the instrument involved. The relative aperture for a microscope is called the numerical aperture (NA) and is equal to the sine of half the angle subtended by the aperture at an object point times the index of refraction of the medium between the object and the objective lens. For binoculars, telescopes, and photographic lenses in which the object may be distant, the relative aperture is taken as the ratio of focal length of the objective to the diameter of the entrance pupil. The relative aperture of a camera lens is sometimes expressed as a simple ratio—e.g., 1:4.5—or more commonly as its f-number, f/4.5. In either case, a lens of 180-millimetre focal length set at this relative aperture would have a pupil diameter (effectively, the lens diaphragm opening) of 40 mm.

numerical aperture

Learn about this topic in these articles:

designation by Abbe

light-gathering power of microscopes

relative aperture