Reciprocity failure results in underexposure and a shift in color balance.
Most films respond more slowly than usual during very long exposures.
Film is made so that it will have fairly precise film speed characteristics that can be relied upon when making exposures, however the band of exposure time is narrow, especially on the long time-exposure side of the band. Most films will provide accurate exposure at speeds up to 1/8000 sec (and faster if electronic flash is used to make the exposure), but these same films seem to change their film speed characteristics when very slow shutter speeds are used.
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. The symptoms can be seen in your images. Very long exposures produce images that are too dark (underexposed) even though you may feel certain the exposure was properly calculated.
When a shutter speed is slower than the film’s band of exposure times, the film’s speed rating may change - in other words, reciprocity failure occurs, causing underexposure. Not only that, but color balance can be thrown off.
Most daylight color films are designed to provide precisely-proper exposure at shutter speeds that are faster than 1/10 sec. For all practical purposes, however, most photographers won’t see the effects of reciprocity failure until they begin to use shutter speeds of one second and longer.
UNDEREXPOSURE
When reciprocity failure occurs, additional exposure is required in order to achieve proper exposure for that film, even though your light meter may say differently. It is as if the film speed decreased. The increase in exposure can be achieved by either opening the aperture more or by using a slower shutter speed.
The effect on exposure is not as drastic as it sounds, however. It can be offset by increasing the exposure by up to 1 stop for exposures of one second to around nine seconds; 1 to 2 stops for exposures of ten seconds to those that approach the 100-second mark; and 2 to 3 stops for exposures that last longer than 100 seconds. Generally, the data sheet packed with your film will provide information on how to adjust for long exposures to offset reciprocity failure for the particular film you are using.
SHIFT IN COLOR BALANCE
The additional problem of a shift in color balance that occurs with reciprocity failure can be more troublesome, however. This occurs because each of the different color layers of film emulsion respond at different rates at slower shutter speeds - i.e. long shutter speeds beyond the film’s band of normal exposure times. The film manufacturer’s data sheet supplied with the film will generally give information on filtration that can offset color shift from reciprocity failure.
Sometimes, the color shift will work in your favor, producing an unexpected but pleasant color shift that adds interest to your image. One of the most common effects, for instance, occurs when photographing a sunset using a long exposure. The sky may look red to you when you photograph it, but will turn out as magenta on film. Sometimes, however, the color shift is not what you want in your image, and it is best to use recommended filters for predictable results.
For some shots, reciprocity failure is not a concern, since the colors are so vivid anyway that a slight shift in color balance means little difference. However, the exposure must be long enough to capture the color and brightness of the scene.
WHY DO THEY CALL IT “FAILURE?”
Because it fails the Reciprocity Law
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 film’s emulsion. Intensity is generally controlled by the aperture and time is generally controlled by the shutter speed. 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 law “fails,” however, when slow shutter speeds change the film’s apparent speed characteristics, making it seem to have a slower speed and produce color shift.
ALL FILMS DON’T REVEAL RECIPROCITY FAILURE AT THE SAME SHUTTER SPEED
We can’t supply any hard and fast quick fixes that apply to all films because the shutter speed at which an emulsion fails to maintain its normal film speed rating varies from one film to another. Furthermore, modern films are becoming more tolerant of longer shutter speeds as film design undergoes improvement. Your best bet is to stick with the manufacturer’s recommendations for compensating for reciprocity failure.
TAKE THE FAIL-SAFE APPROACH WHEN USING LONG SHUTTER SPEEDS
When shooting pictures using long shutter speeds, be sure to bracket a lot, and boldly. This is not the time to change exposure by half a stop. If your meter says you should use a shutter speed of 15 seconds and the film’s data sheet says you need a one-stop increase to 30 seconds to compensate for reciprocity failure, shoot first at 30 seconds, then bracket exposures using shutter speeds of 60 seconds, 90 seconds and even 120 seconds without changing the aperture setting. Why such extensive bracketing? Because longer exposures increase the reciprocity failure even more, making results even harder to predict. The longer your exposure, the more reciprocity failure there is.
Bracketing exposures will usually produce an acceptable image when the correct exposure is not defnitely known.
There is even a safer fail-safe approach. That is, to bracket exposures without changing shutter speed, but opening up the aperture instead. If you can adjust your aperture, then you will not be increasing the effects of reciprocity failure by using longer and longer exposures, and the effects will be less noticeable. You must be prepared, however, to accept a diminishment in depth of field when you open the aperture more. If your image needs all the depth of field it can get, then go back to extensive bracketing using a range of increasingly-longer shutter speeds.
RECIPROCITY FAILURE DOESN’T ONLY OCCUR IN NIGHT PHOTOGRAPHY
Low light photography can occur in the daytime, when reciprocity failure can also occur. How? By your using a slow speed film and a neutral density filter that reduces the overall amount of light reaching the film. If you reduce the amount of light in order to use a longer shutter speed in daytime, your slow shutter speed may exceed the film’s band of exposure time, and you will have to take reciprocity failure into account when making your exposure. If you find yourself in such a situation, and don’t want to be disappointed by underexposed images, bracket, and bracket boldly, and you will probably end up with at least one acceptable exposure.
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