 Faye Bishop |
What is Reciprocity Failure? Hi All I keep hearing about Reciprocity Failure and how it had to be taken into consideration when taking certain photos. Please would someone explain to me exactly what it is and how to correct it. I saw a photo recently and asked how it was taken and was told it required a calculator to work it all out taking in to consideration reciprocity failure. Please help.
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Jon Close |
Within a specified range of exposure times (usually 10 seconds to 1/10,000) changes in f/stop and shutter speed are equivalent. Eg. 1/125 @ f/8 gives the same exposure as 1/500 @ f/4 ==> -2 stops in shutter speed is offset by +2 stops in aperture. At extremely short or long exposure times, however, the change is not one for one as the film's light sensitivity is not linear. So for example if your meter suggests 1/2 sec. @ f/2 and you want to shoot at f/16 (a -6 stop change in aperture), you would normally just increase the exposure time +6 stops to 30-32 seconds, but that puts you beyond the film's reciprocity limit of 10 seconds. The reciprocity failure of the film may require that you add another stop or two (to 1-2 minutes) for the film to get the proper exposure. Additionally, at extremely long or short exposures the film may also require filters to keep the color in balance. The film makers publish technical papers that suggest a starting point for the amount of additional exposure time and filtration required, or the photographer will make his/her own tests or bracket exposures (take the same shoot at say f/16 and 30 sec, f/16 @ 1 min., f/16 @ 2 min.). For example, Kodak Tech. Pub. E-4024 for E100G slide film ( http://www.kodak.com/global/en/professional/support/techPubs/e4024/e4024.jhtml?id=0.3.8.8.10.3.14&lc=en ) has a section titled, Adjustments for Long and Short Exposures, that states: "No filter correction or exposure compensation is required for exposure times from 1/10,000 to 10 seconds. At exposure times of 120 seconds, add CC10R filtration. "Note: This information applies only when the films are exposed to daylight. The data are based on average emulsions rounded to the nearest 1/3 stop and assume normal, recommended processing. Use the data only as a guide. For critical applications, make tests under your conditions."
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