EN: Reciprocity law
Fotografie digitala si IT
To make an exposure, a photographer can, for example, choose a combination of a small aperture and a slow shutter speed or a large aperture and a fast shutter speed. Each movement of the aperture is classed as one stop – go from f/8 to f/11 and you close the aperture by one stop. Similarly by adjusting the shutter speed from 1/125sec to 1/500sec you reduce the exposure by one stop. If you adjust the aperture by a stop and counteract this by also adjusting the shutter speed by one stop you will produce the same exposure value. Therefore an exposure of f/8 at 1/60th could be changed to any of the following combinations: f/11 at 1/30th, f/16 at 1/15th, f/5.6 at 1/125th or f/4 at 1/250th. This is called the law of reciprocity (if one value increases the other will decrease proportionally and visa versa).
Refers to the relationship between a camera s shutter speed and its aperture when taking a picture or making an exposure reading. Proper exposure of a subject will have a correct aperture that corresponds to a correct shutter speed. If you change either one, the other must change reciprocally to maintain correct exposure. Most cameras have the capability to automatically adjust one or the other setting when you make a change to either one. AS the aperture decreases, the shutter speed is slowed, and vice-versa.
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When a film’s speed cannot be relied upon for proper exposure at slow shutter speed,reciprocity failure (or the Reciprocity effect ) is said to occur. Additional exposure is required in order to achieve proper exposure for that film, even though your light meter may say differently. The additional problem of a shift in color balance that occurs with reciprocity failure can be more troublesome.
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Aperture and shutter speed have a reciprocal relationship in making an exposure. Various different combinations of the two will produce the same exposure. The reciprocity law therefore means that an exposure provided by ƒ8 and 1/250 sec will gave the same results as ƒ11 and 1/125 sec., or ƒ16 and 1/60 sec., and so on. If one choice of settings provides proper exposure, then the others will as well. The reciprocity law states that Exposure = Intensity X Time. Intensity is the amount of light, and Time refers to how long that amount of light is allowed to act on the sensor or the film’s emulsion. The law fails, however, when slow shutter speeds change the film’s apparent speed characteristics, making it seem to have a slower speed and resulting in color shift.
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Most films are designedto be exposed within a certain range of exposure times-usually between 1/15 secondto 1/1000 second. When exposure times fall outside of this range-becoming eithersignificantly longer or shorter-a film s characteristics may change. Loss of effectivefilm speed, contrast changes, and (with color films) color shifts are the three commonresults. These changes are called reciprocity effect. Generally, as a quick reference,exposure beyond one second needs to compensate for this characteristic of film.
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A phenomenon that occurs when film is exposed under conditions that are not within its practical brightness range.
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Expressed by (H)=Et, where E is the light intensity, and T is time. When E or T are varied to the extreme, an unsatisfactory exposure can result.
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Reciprocity law (exposure)
Exposure = Intensity x Time. When you cut either the intensity or the time (shutter speed) in half you have only to double the other to keep your meter happy, up and down the exposure scale, except at the far ends, where the law is not enforced.
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Reciprocity law failure (mostl
The lack of reciprocity at both extremes (very long and very short exposures). Solution: increased Exposure and, if necessary, additional compensation for the reciprocity failure of the first compensation. Video/film folk normally need not fret.
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Reciprocity law states that as you increase the intensity of light reaching the film you also need to decrease the speed it reaches the film by the equivalent amount. Most films work quite happily between exposures of 1/2sec and 1/1000sec, but go beyond these extremes with a very low intensity of light and a long exposure or a very high intensity of light and a correspondingly short exposure and the law fails an exposure increase may be required when the shutter speed is beyond these limits. At these extremes the law fails. Compensation is required to adjust for this, but there is no strict rule to correct the error. Most film and paper manufacturers provide technical details on request with a rough guide to exposure adjustments. As a rough guide for an exposure of one second you would increase the speed to two seconds, or open the aperture by one stop. A speed of 10sec would need to be increased to about 50sec or open the aperture up two stops. With black & white film you only have to worry about this exposure correction, but with colour film does not only suffer from exposure problems but also colour casts. A colour film is made up of three individual colour layers, each layer suffers from reciprocity failure at different levels. On an uncorrected film the shadows may have a magenta colour cast but the highlights may suffer from a cyan cast. To correct the cast not only would a longer exposure be needed but also the inclusion of a colour correction filter of a low value, care would have to be taken in choosing the correct filter otherwise an over corrected result may appear. .
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