Schwarzschild effect
The Schwarzschild effect is a phenomenon that occurs during exposure in (chemical) photography .
The reciprocity law of Bunsen and Roscoe (1862) states that mathematically equally large products of exposure time and intensity result in the same blackening. If, for example, the aperture is closed by one value, but the exposure time is doubled, the same blackening of the film material should result.
The astronomer and physicist Karl Schwarzschild discovered in 1899 that the sensitivity of a photographic layer decreased exponentially for exposures of more than one second (the time depends on the film material used) , i.e. the expected density did not match the blackness achieved. The pictures were underexposed - with the same dose of light, but longer times. To compensate for this effect, the images should be exposed longer. For this purpose, the manufacturers offer data sheets from which the corresponding exposure times can be found.
Color films are particularly affected by the Schwarzschild effect, as the different emulsion layers for the individual primary colors show this effect to different degrees and this can lead to color casts. These occur from about 1/30 s. For exposure times over 1 s, artificial light color films are recommended, for which no longer exposure times are required up to 5 s due to the Schwarzschild effect.
This effect can be avoided by using special film material, so-called hypersensitized films . These films have been thoroughly dried and treated with hydrogen and do not lose, or only hardly lose, their sensitivity even after long exposure.
Even with very short exposure times (around 1/1000 of a second, depending on the film material used), a similar effect occurs, known as the short-term effect. Schwarzschild and short-term effects are graphically displayed in the characteristic light quantity curves, which are unique for different photo materials .
Such a graph shows for the exposure levels (logarithmic exposure time) plotted along the horizontal axis in an average interval of typically 0.001 to 1 s horizontal progression with the function value plotted upwards of constant 1, but outside this range, curved, increasing values than those to be used in practice Extension factors for the exposure time. The two curved branches can also be shown separately on different scales in greater detail.
This effect does not occur in digital photography because the CCD or CMOS image sensors used do not lose their sensitivity. However, other effects are noticeable in long-term recordings with little light, for example dark noise .
See also
literature
- Gerd Koshofer: Agfachrome Professional Films, Recording Technology, in particular page 79 ff., As a print from Agfa-Gevaert AG, Leverkusen-Bayerwerk 1974
- PHOTOGRAPHIE magazine issue 11/1988, page 46
Individual evidence
- ↑ Gerd Koshofer: Agfachrome Professional Films, Recording Technology, as a print by Agfa-Gevaert AG, Leverkusen-Bayerwerk 1974, p. 79 ff.