Reciprocity (photography)
The Schwarzschild effect is a phenomenon that occurs in photography the exposure in the (chemical).
The reciprocity law of Bunsen and Roscoe (1862 ) states that arise mathematically equal products of exposure time and intensity of the same darkness. For example, if the aperture is closed by a value, but doubles the exposure time to the same optical density of the film material should arise.
The astronomer and physicist Karl Schwarzschild discovered in 1899 that the sensitivity of a photographic film at exposures over one second ( the time depends on the used footage ) decreased exponentially, so the expected darkness did not agree with the achieved darkness. The recordings were - for the same light dose but longer times - underexposed. To compensate for this effect, the recordings are to be exposed longer. These provide the manufacturer data sheets, where the corresponding exposure times can be removed.
Color films are particularly affected by the Schwarzschild effect, since the different emulsion layers may show different degrees for each primary color this effect and thus will lead to color casts. This already occurs from about 1/30 s. For exposures longer than 1 s artificial light color films are recommended where there is no exposure time extensions are necessary because of the Schwarzschild effect to 5 s.
This effect can be avoided by using a special film material, the so-called hyper -sensitized films. These films have been excessively dried and treated with hydrogen and lose himself, or only barely in sensitivity at long exposure.
Even at very short exposure times (such as less than 1 /1000 second, depending on the film material used ), a similar effect occurs, the short-term effect is called. Schwarzschild and short-term effects are graphically displayed in the photo for different materials unique characteristic amount of light curves.
Such a graph is for the horizontal axis plotted exposure level (logarithmic exposure time) in a central interval of typically 0.001 to 1 s horizontal curve in the upward applied function value of constant 1, but curved outside this range increasing values higher than in the practice to wendene extension factors for the exposure time. In more detail, the two curved branches are shown separately in different scales.
In digital imaging, this effect does not occur, due to the CCD or CMOS image sensors used do not lose sensitivity. However, familiarize yourself with the long exposures in low light other effects noticeable, for example, the dark noise.