The effective diameter of a lens or mirror. The ratio of the effective diameter to the focal length is called the relative aperture, which is commonly known as the aperture, especially in photographic usage. The reciprocal of the relative aperture is called the focal ratio. The numerical value of the focal ratio is known as the f-number of a lens. For example, a camera lens with a 40 mm focal length and a 10 mm aperture has a relative aperture of 0.25 and a focal ratio of 4. Its f-number would be f/4, often written f4.
The light-gathering power of a telescope depends on the area of the lens, i.e. it is related to the square of the aperture. However, the larger the relative aperture the greater the aberrations. In microscopy large-aperture objectives (corrected for aberrations) are preferred, since they reduce the blurring caused by diffraction of light waves.
The diameter of the main lens or mirror of a telescope, or, in radio astronomy, of the collecting dish. For Schmidt cameras, however, it is the diameter of the corrector plate. See also Unfilled Aperture.
1. (window) The portion of a data record which is selected for specifying operators for use on the data set, e.g. operators such as autocorrelation functions and filters are applied by apertures on seismic records.
2. In a mollusc (Mollusca) shell, the opening through which the soft parts of the animal emerge. It is often a simple, circular opening, but is modified in some genera. In gastropods (Gastropoda), where the aperture is circular or elliptical it is said to be ‘holostomatous’ or ‘entire’; where it is notched (to accommodate a siphon) it is said to be ‘siphonostomatous’. In cephalopods (Cephalopoda), the aperture may be indented or notched at the ventral margin by a hyponomic sinus, which houses the hyponome. Some compressed ammonites (Ammonoidea) possess a pair of lateral shell extensions (lappets) on either side of the aperture.
Aperture
- inverse Compton effect
- inverse Compton scattering
- inverse correlation
- inverse elasticity rule
- inverse element
- inverse feedback
- inverse filter
- inverse Fourier transform
- inverse function
- inverse functions
- inverse function theorem
- inverse gain
- inverse Gaussian distribution
- inverse homomorphic image
- inverse hyperbolic function
- inverse image
- inverse Lagrangian problem
- inverse limiter
- inversely proportional
- inverse matrix
- inverse methods
- inverse myotatic response
- inverse normal distribution
- inverse of a complex number
- inverse P Cygni profile