Last week’s blog post about incident light metering apps was not only relevant for film photographers to be honest, but also for digital. This time I want to have another look at apps helping you with the right exposure but it’s going to be more relevant for film. I’m talking about reciprocity failure. (If you think r e c i p r o c i t y is an awe-inspiring word you should know that the German translation Schwarzschild-Effekt is even cooler!)
So what is it? In short, it means that film does not behave the same at all exposure times. At longer exposure times (usually longer than one second) films require even more exposure than calculated by your ASA/f-stop combination. So, if your light meter tells you to expose for, say, 3 seconds in reality it might be 3 seconds plus another x seconds.
Now, there is a physical law behind this, but unfortunately the effect is not the same for every film. Some films are more forgiving than others and have been optimized for long exposures. With other films though, the Schwarzschild-Effekt (I couldn’t resist) kicks in much earlier. So, what can you do?
Film manufacturers provide the details on the film data sheets that you can download for every film type (well, most film types), so that you can calculate the necessary adjustments. In the field, however, you might not have every data sheet handy and it could be inconvenient to consult them for your shot. Thanks to the Gods of Photography there’s an app for that!
Quite a few apps to be precise:
The most basic of apps is Reciprocity Failure Rate. It is for a specific film type and when I say “a” type of film I mean it. This app is made exclusively for Ilford’s classic HP5 b/w emulsion. So far so good, but it also has a rather weird business model. Instead of paying for the app, you purchase a number of “calculations” or “uses”. Fortunately you get the first 10 uses for free. If you’re exclusively shooting HP5 and do long exposures very, very rarely this might be the only app you need. For everyone else there are better options.
Reciprocity Timer supports Ilford films, Fuji Velvia and Provia, as well as Kodak’s Ektar and Tri-X, plus a few others. You can adjust for bellows extension (for large format photography), focal length of your lens, and filters. Finally it also has a timer that does the counting for you if you have exposure times of several seconds. It’s neither cheap nor expensive for an app at 1,79 EUR and has not been updated since late 2011 [EDIT: As per developer’s comment below the app will get an update pretty soon]. But it looks like the most complete solution out there and does what it says on the tin (and it does it well).
Film Timer comes cheaper at 0,89 EUR but also has less features. It, too, includes a timer but no settings for filters etc, and the list of films is limited to a few Kodak and Ilford b/w films. It does, however, offer an interface for adding your own film reciprocity data so in a way it could be more versatile. The app also has not been updated or added to since 2011.
As reciprocity failure is a phenomenon that can also effect digital cameras to a certain degree there are some apps out there that do not take into account different film types but simply calculate the exposure compensation based on the math alone. Examples are ReciprocityCalc and Reciprocity. I asked the latter’s developers on their Facebook page whether it could be used to adjust for film reciprocity failure. They kindly replied thus:
There is a place to enter the data for how your film reacts to reciprocity failure, and it will take that into account for long exposures. Currently, you can only have data for one type of film at a time.
So, you have to decide for yourself whether that is enough. I guess it depends on whether you mostly shoot film, digital or both to decide whether one of the more generic apps is sufficient for your needs.