aperture

(noun)

The diameter of the aperture that restricts the width of the light path through the whole system. For a telescope, this is the diameter of the objective lens (e.g., a telescope may have a 100 cm aperture).

Related Terms

Examples of aperture in the following topics:

  • The Camera

    • The two major features of a lens are focal length and aperture.
    • The focal length determines the magnification of the image, and the aperture controls the light intensity.
    • The shutter is what opens and closes to allow light through the aperture.
    • For a larger aperture, the f-number is generally small for a quick shutter speed.
    • For a smaller aperture, the f-number is larger, allowing for a slower shutter speed.

  • The Rayleigh Criterion

    • (a) shows a light passing through a small circular aperture.
    • This effect can be seen with light passing through small apertures or larger apertures.
    • θ - angle the objects are separated by, in radian λ - wavelength of light D - aperture diameter. shows this concept visually.
    • (a) This is a graph of intensity of the diffraction pattern for a circular aperture.
    • (a) Monochromatic light passed through a small circular aperture produces this diffraction pattern.

  • Limits of Resolution and Circular Aperatures

    • For telescopes with circular apertures, the size of the smallest feature in an image that is diffraction limited is the size of the Airy disc, as shown in .
    • As one decreases the size of the aperture in a lens, diffraction increases and the ring features from diffraction become more prominent.
    • The denominator $nsin \theta$ is called the numerical aperture and can reach about 1.4 in modern optics, hence the Abbe limit is roughly d=λ/2.

  • Resolution of the Human Eye

    • The pupil of the human eye is its aperture.
    • The iris is the diaphragm that serves as the aperture stop.
    • Refraction in the cornea causes the effective aperture (the entrance pupil) to differ slightly from the physical pupil diameter.

  • Diffraction

    • Intensity pattern formed on a screen by diffraction from a square aperture.

  • The Human Eye

    • The human eye is had an aperture, just like a camera.
    • The pupil serves this function, and the iris is the aperture stop.

  • The Heisenberg Uncertainty Principle

    • If a large aperture is used for the microscope, the electron's location can be well resolved (see Rayleigh criterion); but by the principle of conservation of momentum, the transverse momentum of the incoming photon and hence the new momentum of the electron resolves poorly.
    • If a small aperture is used, the accuracy of both resolutions is the other way around.

  • An Astronomical Aside: Magnitudes

    • For a star one generally can extrapolate the flux that one observes in the sky to the total flux, but the intensity from a galaxy or other extended source generally falls off gradually so one defines a magnitude within a certain aperture or down to a limiting intensity (surface brightness).

  • Huygens' Principle

    • Diffraction effects are the deviations from rectilinear propagation that occurs when light encounters edges, screens and apertures.