More in TechTalk 5.
© Barry Leighton FRPS
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| Technical Talk Part 4 | ||||||||
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| Two terms that often cause confusion are Reciprocity and Latitude. They affect all films and papers, but are more important when exposing film and are the cause of many a negative that's difficult to print and transparency that ends up in the bin. This month I shall outline reciprocity, please hold on to your excitement for a month until I talk about latitude. As you know, the exposure any film or paper receives is dependant both on the intensity of the light falling on the material and the time it falls on it for. Simply, reciprocity or the Law of Reciprocity, as it is sometimes called, is the rather convenient situation where the same exposure can be given by halving the time and doubling the brightness, or doubling the time and halving the brightness, of the light that falls on the emulsion. For example, we can expose a film at 1/1000th of a second at f2, 1/500th at f2.8, 1/250th at f4, even an 1/8th at f22, and get negatives all of the same density. This is the important point of reciprocity, if you half one of the two variables and double the other, you get the same result. This works fine for the vast majority of exposures we make, but there are limits to the range over which a particular emulsion can be manufactured to stick to the Law of Reciprocity. Outside of this range we get Reciprocity Law Failure. In practice, since ultra short exposures need special equipment, this only happens when we try to make very long exposures in very dim light, for example of a night scene. If we just follow the reading of the light meter, whether built-in to the camera or hand-held, we can be led to belive that an exposure of, say several seconds or even several minutes, is enough. Note I used the term light meter and not exposure meter because it only measures the amount of light present. It does not know about Reciprocity Law Failure (it doesn't know much at all actually, like what you intend the final print to look like or how you will develop the film, but that doesn't seem to stop some photographers leaving their decisions up to it). Once we get outside the range of the film, usually about 1 second, then the actual exposure required increases rapidly. Accurate characteristics for any film can be obtained from the manufacturers, but as a general rule we need to double the indicated exposure in the range from 2 to 5 seconds, then treble from 6 to 10 seconds and quadruple from 11 to 20 seconds. After that, the amount you need to multiply the exposure by increases ever more rapidly, until the film is effectively in the dark and will not record anything. Try it for yourself. Expose a film of a scene that is lit very dimly, chose an aperture setting that starts the exposure at an indicated 2 seconds and double the exposure time for each frame for three or four frames. Then repeat the exposures, but close down to start at 20 seconds. Keep an accurate note of what you do. Develop and print the film and see which you think looks the best, paying particular attention to the amount of detail in the shadows. You may be surprised that the same moonlit scene can go from looking like a moonlit night to looking like daylight. Try the same exercise on the scene in daylight, get the reciprocity failure right, with a long enough exposure, and you can get it to look like the moonlit night. The only difference between moonlight and daylight is the brightness. It just doesn't look that way, due to the failure of our eyes in dim light. So with long exposures, when reciprocity failure could be a problem, bracket your exposures.
More in TechTalk 5. © Barry Leighton FRPS |
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