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UFO

18 Mai 2012 09:04



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@nobody cu cat F/d este mai mic cu atat "luminozitatea instrumentului este mai mare

For either a lens or a mirror, an important parameter is the ratio of focal length to diameter. This is called the f ratio. If we term the focal length of the lens or mirror, f, and its diameter, d, then the f ratio is just f/d. This ratio quantifies the brightness of the image produced by the lens or mirror and the size of the image produced. A small f ratio, such as f/3, for example, gives a brighter image than a large f ratio like f/15. In other words, an f/15 lens or mirror spreads the image out over a larger angular region than a f/3 lens or mirror does, hence the image brightness decreases.

de aici http://www.physics.upenn.edu/balloon/telescope.html