New biomass map to take stock of the world's carbon

The first of a series of global maps aimed at quantifying change in carbon stored as biomass across the world's forests and shrublands has been released today by ESA's Climate Change Initiative at COP25—the United Nation ...

NASA maps surface changes from California quakes

Damage from two strong earthquakes that rattled Southern California on July 4 and July 5—a magnitude 6.4 and a magnitude 7.1, respectively—can be seen from space. The epicenter of the quakes was near the city of Ridgecrest, ...

Monitoring Earth's shifting land

The monitoring of land subsidence is of vital importance for low-lying countries, but also areas which are prone to peculiar ground instability.

Image: Biomass Earth Explorer satellite

Set to fly in 2022, ESA's Biomass Earth Explorer satellite with its 12-m diameter radar antenna will pierce through woodland canopies to perform a global survey of Earth's forests – and see how they change over the course ...

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Aperture

In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are, which is of great importance for the appearance at the image plane. If an aperture is narrow, then highly collimated rays are admitted, resulting in a sharp focus at the image plane. If an aperture is wide, then uncollimated rays are admitted, resulting in a sharp focus only for rays with a certain focal length. This means that a wide aperture results in an image that is sharp around what the lens is focusing on and blurred otherwise. The aperture also determines how many of the incoming rays are actually admitted and thus how much light reaches the image plane (the narrower the aperture, the darker the image for a given exposure time).

An optical system typically has many openings, or structures that limit the ray bundles (ray bundles are also known as pencils of light). These structures may be the edge of a lens or mirror, or a ring or other fixture that holds an optical element in place, or may be a special element such as a diaphragm placed in the optical path to limit the light admitted by the system. In general, these structures are called stops, and the aperture stop is the stop that determines the ray cone angle, or equivalently the brightness, at an image point.

In some contexts, especially in photography and astronomy, aperture refers to the diameter of the aperture stop rather than the physical stop or the opening itself. For example, in a telescope the aperture stop is typically the edges of the objective lens or mirror (or of the mount that holds it). One then speaks of a telescope as having, for example, a 100 centimeter aperture. Note that the aperture stop is not necessarily the smallest stop in the system. Magnification and demagnification by lenses and other elements can cause a relatively large stop to be the aperture stop for the system.

Sometimes stops and diaphragms are called apertures, even when they are not the aperture stop of the system.

The word aperture is also used in other contexts to indicate a system which blocks off light outside a certain region. In astronomy for example, a photometric aperture around a star usually corresponds to a circular window around the image of a star within which the light intensity is summed.

This text uses material from Wikipedia, licensed under CC BY-SA