Long exposures are among the most exciting creative techniques at every film photographer’s disposal. However, they don’t come as easy in film as they do in digital due to the law of reciprocity failure. Jason Robert Jones covers this in one of his YouTube videos and accompanying cheat sheets for two of his go-to films. If you’ve never heard of this technicality before, we suggest you pay close attention as it has a hand in the outcome of your shots, especially if long exposures.
There are many technical elements involved in gaining a complete understanding of reciprocity failure. But, Jason’s explanation is concise enough to provide a gist of what it means, how it affects film photos, and what we can do to work around it. So, if you’re ready, let’s have a go at it.
While we can’t be expected to fully grasp this law on the initial go, research and practice will definitely help us get the hang of it. Jason first explains reciprocity refers to the inversely proportional relationship between the amount or intensity of the light, and the length of time it hits a light-sensitive medium (in this case, the film emulsion). If you know your Exposure Triangle by heart, you know that a smaller aperture requires a longer shutter speed to make a proper exposure. If you opt for a larger aperture, you need to pair it with faster shutter speed so the photograph doesn’t become overexposed. The more light that gets into your camera, the less time you need for a proper exposure.
Reciprocity failure happens when this relationship breaks down. This commonly occurs with long exposures as we tend to shoot with a small aperture to maximize the depth of field. At small apertures, when shutter speed exceeds the range the film is capable of in order to create a normal exposure, you have to make adjustments for reciprocity failure. This typically results in underexposure and/or color shifts, which means you have to add exposure time to increase the amount of photons hitting the film. So, if you plan to do a lot of night photography, landscape photography (especially when you’re stacking up ND filters), light trails, and light painting using film, you really have to account for it.
However, what do you do when you forget to compensate with a longer exposure time? You compensate by pushing the film or increasing the developing time. This is easy to do with sheet film, but if you’re shooting with roll film, you can only do this when you’re sure the entire roll was underexposed. You can easily ask the lab to do it for you.
The reciprocity adjustment only applies to film because each emulsion responds to light differently. This is unlike digital sensors, which have a linear response to long exposure times.
Jason typically shoots with Fuji Velvia and Kodak Ektar 100. If you shoot these films a lot, you’re in luck — he made a reciprocity chart for both which you can use as a cheat sheet for your next shoot. You can check out these charts and a recap of his explanation on his website. Visit his YouTube channel as well if you want more of his photography tips and tricks.
There are tons of resources if you need more to get a grip on reciprocity failure. For starters, you can check out Ignacio Alvarez’s article on how to expose film correctly at night, this article on I Still Shoot Film, and this article on Film Photography Project.