Reciprocity and Reciprocity Failure – an Explanation
Reciprocity is the interchange of shutter speed and aperture. This rather posh word just means 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 reciprocity works well, up to a point, but film users will find that, when you have very long exposure times, several seconds or so, the reciprocity breaks down and extra exposure time is necessary to compensate. This is known as reciprocity failure.
Reciprocity Failure (film users only)
All color films suffer from reciprocity failure when exposed for longer than a few seconds. This results in a color shift as the three layers of the film respond to a different degree. The exact color will depend on the brand of film you are using. The color cast can often be corrected at the printing stage so is not a major problem unless you are using slide film. The only real way to avoid the problem is to use a faster shutter speed, which will mean also using a larger aperture and sacrificing some depth of field. If you don’t understand the connection between aperture and depth of field read my article on shutter speeds and apertures.
Reciprocity failure will also be a problem with black and white film but in this case all that is required to put things right is to make an even longer exposure. How much extra you need to give is hard to tell, the best solution is to make several exposures at different times. After all your subject is not going to run away, if you are hoping to get a good shot of it with a two or three second exposure, it has to be something that is not going to move.
All you really need to be aware of is that if you are shooting night scenes for instance at very small apertures, you would be well advised to shoot a series of photos at varying exposures, this is called ‘bracketing’ and is a very useful technique even in these days of digital cameras and instant replay.
Good news for digital camera users
Reciprocity failure is not a problem with digital cameras, however noise can be.
Bad news for digital camera users
Long exposures on digital cameras can produce visible noise, which looks a bit like the grain you see in a fast film image. This noise or grain is usually most noticeable in plain areas of the picture. See my article on ISO Film Speed for some pictures and more information.