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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).
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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.