Reciprocity failure. You may have heard the term thrown around in the world of film photography and dismissed it as unimportant or sounding too complex to bother.
And in the everyday world of shutter speeds of a second or less, you can probably get away with this dismissal.
But if you’re looking to enter the world of long exposure photography on film, reciprocity failure is really important to understand, and not as complicated as it might sound.
What is Reciprocity Failure?
So, let’s talk about what reciprocity failure is first.
If you’ve been in the world of manual film cameras for any length of time, the word “reciprocity” refers to a concept you’ll already be familiar with on a practical level. That is, you’ll understand that making a properly exposed image depends on a relationship between two variables: shutter speed and aperture (leaving ISO to the side for simplicity’s sake).
You’ll also have some idea of the way in which shutter speed and aperture work together in an inverse or reciprocal relationship. For example, if a scene requires a shutter speed of 1/250 of a second at f/5.6 for a proper exposure, a shutter speed of 1/125 of a second at f/8 will produce the same exposure value. This is what is referred to as the law of reciprocity, and it’s a law that holds up pretty well for photos taken at normal shutter speeds.
Reciprocity failure is what happens when, at longer exposures (generally shutter speeds of 1 second or more), the law of reciprocity (you guessed it!) fails!
So why does this failure occur with longer shutter speeds?
Well, unlike digital photographers, to be a film photographer is to be a chemist of sorts. A film’s emulsion is made up of a layer of silver halide chemicals that react when exposed to light. So, whenever a film photographer exposes an image, their shutter is setting in motion a series of real chemical reactions that eventually will give you your negative.
If you remember from high school chemistry, when a chemical reaction takes place, the chemicals in said reaction begin to be used up and lose their potency in the course of the reaction. This is no different when it comes to film.
Over the course of a long exposure, the chemicals in the film emulsion lose their potency and become less sensitive to light as a result. The longer the exposure, the less potent the emulsion becomes over time, and, therefore, the more compensation is needed in order to achieve a proper exposure.
Film that had a particular sensitivity to light at the very start of the exposure will be less sensitive to light after one second, and less sensitive than that after 30 seconds, and still less sensitive than that after 3 minutes, and so on and so on.
Reciprocity Failure in Long Exposure Film Photography
With that all explained, what does reciprocity mean practically for your long exposure photography?
Essentially, it means that for exposures of 1 second or more you are going to have to make certain compensations in order to adjust to the film’s loss of sensitivity through the exposure.
In other words, if you don’t add more time to your exposure, your image is going to come out under exposed.
The most obvious compensation is that you will need to add more exposure time to properly expose the image than your light meter reading suggests. If your light meter is giving you a reading of 1 second, you might need to expose for 2 seconds. For longer exposures of 1 minute or more, you might need to expose for 2 minutes.
Reciprocity Failure and Different Film Types
The concept is simple enough, but it becomes a bit tricky when applying it to specific film types. The problem is that each film type is affected by reciprocity failure differently, and, therefore, each require different time compensations when calculating exposure.
Fuji Acros 100, for example, was one of my favorite films for long exposures because it was not affected by reciprocity failure until exposure times reached 2 minutes or more.
Ilford HP5 Plus, on the other hand, requires an additional 1 minute and 20 seconds of exposure on a metered exposure of 15 seconds, for 1 minute and 35 seconds of total exposure.
Compensating for Reciprocity Failure During Development
The other potentially necessary compensation for reciprocity failure is done during development.
Given the extra exposure time, images with really bright highlights have a tendency to blow out in the image, leaving you with no detail in those areas. Lessening the development time (also known as “pulling” film in development) can serve to lessen the increased contrast that comes with long exposures and keep detail in the highlights that might be lost otherwise.
While reciprocity failure always requires you to compensate by lengthening exposure time, I’ve found that compensation in development is only really necessary in images with a really wide range of tones.
So how do you calculate reciprocity failure for specific films?
The first (and probably most accurate) way is to consult the data sheets given by the film manufacturers themselves.
On Ilford’s website, for example, they assign each film a numeric value (P) that when plugged into an equation will give you an adjusted exposure time that accounts for reciprocity failure.
Personally, I use a lot of Ilford HP5 Plus, so I printed a chart with adjusted exposure times using this data.
Unfortunately, it’s not always easy to find this information on the film manufacturer’s website, and, let’s be honest, doing math is a pain. Luckily, there are also apps available on your smartphone that make calculating compensated exposure times for reciprocity failure really convenient.
I’ve found there are some small discrepancies in the times it gives for Ilford films compared to the data sheet information from Ilford’s website, but it’s a difference that hasn’t really affected my exposures. One of the advantages of reciprocity failure is that it actually allows for a greater margin of error when it comes to exposure time, especially in exposures over a minute.
Given that, an exposure time that’s “in the ballpark” in terms of accuracy is often good enough. Also, as the film gets less sensitive to light throughout the exposure, it also becomes more resistant to overexposure, so if I’m not sure of an exposure time I usually try to err on the side of more exposure time rather than less.