Svetelnost

Svetelnost is a term used in Czech and Slovak to describe the perceived brightness or the light-gathering ability of an optical system, such as a camera lens, telescope, or imaging sensor. It relates to how much light is transmitted from the scene to the image plane and thus influences exposure, contrast, and noise performance.

In photography, svetelnost is most commonly associated with the aperture of a lens. A lens with a larger maximum aperture (lower f-number) is described as having greater svetelnost, or being faster, because it allows more light to reach the sensor in a given exposure. Conversely, a lens with a smaller maximum aperture (higher f-number) has lower svetelnost. The f-number is a practical, widely used representation, although actual light transmission can differ due to coatings, reflections, and internal obstructions.

To account for real-world light transmission, professionals also use the concept of transmission, sometimes expressed as a t-stop, which adjusts the f-number to reflect the actual light reaching the sensor. A lens may have the same f-number as another but different t-stop values because of varying optical efficiency and coatings. Thus, svetelnost encompasses both the theoretical aperture and the practical transmission of light through the optical system.

Beyond photography, svetelnost can refer to brightness-related properties in imaging and astronomy, such as how bright an extended object appears per unit area (surface brightness). In technical contexts, it often intersects with photometric quantities like luminance, illuminance, and radiant flux.

See also: aperture, f-number, light transmission, t-stop, lens speed, photometry.