In a nutshell, film reciprocity is the inverse relationship between shutter speed and aperture. For example, if you have a shutter speed of 1/250th of a second at f/5.6, then reciprocally, a shutter speed of 1/125th of a second at f/8 is the same exposure, just a smaller aperture. It's how we can control our depth of field by sacrificing shutter speed to get the f-stop we desire.

At normal shutter speeds, this inverse relationship works well, but when shutter speeds begin to get longer - generally more than 1 second - then the film reciprocity fails, thus "Reciprocity Failure".

In digital photography, this is not an issue that needs to be considered, but in film, the emulsion sensitivity begins to fail at "X" time and the film requires a factor of multiplication to do the same work over that period of time. Using this P-Factor in an equation, we can calculate with some degree of accuracy how much longer we need to keep the shutter open to get the desired exposure.

For example, in Ilford's HP5+, we're told (by Ilford) that the P-Factor for that film is 1.31 after 1 second in exposure time. That means we can calculate the time needed to get the correct exposure by taking the shutter speed (i.e. 2 seconds) and multiplying it by the power of 1.31 to get the actual exposure time necessary. In our example, it would be 2.5 seconds. Further complications arise when we have to factor in additional stops for filters. So our 2 second exposure with an ND-8 filter adds 3 stops to our equation turning that 2 seconds into 15 seconds. Now our calculation tells us that a 15 second exposure should actually be 34.7 seconds - over 2x the metered and filtered exposure time.

Not all film manufacturers do a good job of telling us what their film's P-Factor is, so some data provided in this app is sourced from online conversations of educated guesses based on real-world experimentation. Film is also rarely an exact science, so use this as a guide to get in the ballpark. Best practices would be to bracket your long exposure shots by +/- 1 stop so when you develop your film, you have a backup over/underexposed frame to work with.