The official dictionary definition of reciprocal is “something that is equivalent to something else; counterpart; complement.” Reciprocity is a form of the word reciprocal and means “a relation of mutual dependence, action or influence.”

If you think of it in terms of a relationship it is a bit easier to understand.  Here is reciprocity used in a sentence to help:

The two companies have a relationship based on reciprocity as they joined together to fight their multi-million dollar competitor.

So how do we translate that to photography?  Rather simply actually.  In photography reciprocity refers to the relationship between the aperture (f-stop) and shutter speed.  Each f-stop has a corresponding or reciprocal shutter speed that will result in a correctly exposed image.  Essentially, the smaller the aperture opening (larger the f-number) the longer the shutter speed needs to be to get the optimal amount of light to the film or digital sensor.  Conversely, the larger the aperture (smaller the f-number) the shorter the shutter speed needs to be.  Simply put, as one goes down the other goes up, they are inversely related.  The chart below demonstrates this relationship between aperture and shutter speed.  As your aperture moves closer to the “large opening” end your shutter speed will move towards the “short exposure” end.  As your aperture moves closer to the “small opening” end, your shutter speed will move towards the “long exposure” end.

Diagram explaining reciprocity in photography

The reciprocal part of the relationship means that in order to maintain an accurate exposure, for each stop up or down you make in aperture (each change you make in f-stop) you need to make an equal number of stop changes for shutter speed and vice versa.

You may have heard of the term reciprocity failure as well.  This is a term related to film photography and occurs when the film fails to expose correctly with very long or very short shutter speeds.  This is because film is designed to work within specified shutter speeds.  The result of reciprocity failure is an incorrectly exposed image.  In digital photography however, reciprocity failure is not an issue.  The digital sensor in DSLR cameras does not have the same limitations of film so you can focus on the reciprocal relationship of the aperture to the shutter speed.



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