Return to

If you would like to
make a donation to
help fund this site,
click below!
Understanding Reciprocity & Reciprocity Failure

Various Aperture Size & Shutter Speed Combinations Yield the Same Exposure = Reciprocity
As you may have guessed, there are numerous different - but equivalent - combinations of shutter speed and f-stop that will produce exactly the same exposure. This is the concept called "reciprocity." For example: a small aperture such as f/16, combined with a shutter speed of 1/30, will produce the same exposure as the larger f/11 aperture at 1/60; or f/8 at 1/125.. Each combination doubles the light entering the aperture compared to the previous setting, but cuts the time it is allowed to enter in half. Still, in each case the same total amount of light exposes the film.

Reciprocity Failure
However, at very long exposure times, the reciprocity concept fails to hold true. The conventional relationship between aperture and shutter speed breaks down, and it is typically called "reciprocity failure." As shutter speeds get very slow (open for more than four-five seconds), the chemical sensitivity of the film loses its reciprocal relationship (actually, the same thing is true at extremely short exposures, but it is rarely talked about since most mechanical cameras are incapable of speeds approaching 1/10,000 second or less).

The non-technical explanation for this is that when the shutter is opened for longer than it's "optimal" range, the film speed decreases. In that case, the film responds much more slowly than usual, resulting in underexposure. Reciprocity failure (also known as the Schwarzschild effect) is a decrease in light sensitivity (speed) with increased length of exposure. The adjustment needs to made in the shutter speed, often adding even more time to the exposure. Generally, the adjustments needed to compensate for reciprocity failure are included on the data sheet that comes inside the box of film. But for those whose appetite has been whetted, here are some additional online sources:

It is by no means complete, but the items I am working on so far are listed below (note: the line items in blue are live page links; those items in black are still under construction):
If you would like a specific repair or instructional item to be considered on this site, simply go to my Mamiya 35mm Forum! and make your suggestion known. I will add it to the list of things to do. However, please don't hold your breath waiting, because it is likely to be some time before I can get around to documenting all the things that might need explanations, and I would hate to be responsible for even small portions of the population turning blue! Also, because mechanical and technical skills vary widely from person to person, no liability for results or damage is either expressed or implied in the use of these notations.
© 2000-2010  R.L. Herron    All Rights Reserved.  Legal