What is Reciprocity Failure in Photography?
Reciprocity failure in photography is the fall off of light on camera meters for exposures longer then about 2 seconds because the film took longer to expose properly. The camera meters don’t figure a long enough exposure time for long shutter exposures. This was a larger problem with film, but it is still something to watch out for with digital cameras.
For example, the built in camera meter tells you 8 seconds for a shutter speed at night. You use 8 seconds but it is underexposed. This is reciprocity failure. Just use a longer exposure time, for example 12-16 seconds. The longer the exposure that is needed the less accurate the built in camera meter will be, and the more exposure time you actually need. You can’t trust the camera meter and your sensor for long shutter exposure times, it isn’t as accurate.