I accidentally happened to come over a big bad and strange phrase, “Reciprocity Failure”. Before this, the jargon of photography never feared me. But now I can say that it’s not that bad how it reads! Yes, I am going to explain that. You need not be afraid though. Because I have already simplified this kind of nerdy stuff! So let’s plunge!!
So let us divide the phrase into two. ‘Reciprocity’, and ‘Failure’. Hence Understanding this phrase becomes as easy as combining both the meanings! The two most appropriate meanings of this word are, ‘Mutual action and reaction’ from Webster's Revised Unabridged Dictionary, © 1996, 1998 MICRA, Inc. & ‘a relation of mutual dependence or action or influence’ from WordNet® 3.0, © 2006 by Princeton University. This mutual thing happens between our aperture and shutter speed. When this relations or rule fails, there happens a failure!
For a photographer, it is quite a common sense that a combination of a shutter speed of 1/125th of a second with an aperture of f8, which is referred to as an exposure setting of '1/125th at f8', will give the same exposure to the film or digital sensor as 1/250th at f5.6, which is the same as 1/500th at f4 etc. This relation or phenomena between aperture and shutter speed is termed as reciprocity. So in theoretical terms, it refers to the inverse relationship between the intensity and duration of light that determines exposure of light-sensitive material. Within a normal exposure range, for example, the reciprocity law states that exposure = intensity × time. Therefore, the same exposure can result from reducing duration and increasing light intensity, and vice versa. So in practical life, what we actually do is using this law of reciprocity. This again refers to the relationship whereby the total light energy, proportional to the product of the light intensity and exposure time (controlled by aperture and shutter speed, respectively), determines the effective exposure. We use these different combinations to cater our special needs of focusing and depth of field.
Now here is a game, this law does not hold true in extreme conditions; like in long exposures or too fast exposure like 1/10000 seconds. I don’t know who is in that hurry to click this much fast. For most photographic materials, reciprocity is valid with good accuracy over a range of values of exposure duration, but becomes increasingly inaccurate as we depart from this range. Only in such conditions, certainly not in our daily life (oh, what a relief!), this reciprocity does not hold. As the light level decreases out of the reciprocity range, the increase in duration required to produce an exposure becomes higher than the formula (exposure = intensity × time) states. This breakdown in the relationship between aperture and shutter speed is known as reciprocity failure.
So you interested in getting deeper! Some physics/chemistry jargon comes into play. I have already simplified everything to my best. Still, if you stuck upon some word. Just google it! So here it goes…
Each grain must absorb a certain number of photons for the light-driven reaction to occur and the latent image to form on the film. In particular, a few dozen photons are required for the surface of the silver halide crystal to result from absorption to its developable rendering. At low light levels, i.e. few photons per unit time, photons impose upon each grain relatively infrequently; if the four required photons arrive over a long enough intervals, the partial change due to the first one or two are not stable enough to survive before enough photons arrive to make a permanent latent image center. Reciprocity failure may also affect the tonal range of a photographic scene when at the limit of exposure, resulting in burnt highlights while losing detail in the shadows.
This was all for film. But digital is neither a different story. On one hand film prints are affected by graininess, on the other hand, digital photographers may experience excessive noise, even at lower ISOs. That is all. It is not that frightening as you must have thought before is it?
I owe the information to the links that appear on the first page when I googled the term!
