What is Reciprocity Failure?

 Anonymous

"Lower Manhattan Glowing" - by Shawn Hoke, via FPP Flickr Pool

Whether you’re using a lower speed film in daylight, trying to maximize your depth of field in a landscape, or just setting up the camera for an exposure at night, sooner or later you’re going to start pushing the limits of your film’s light gathering ability. As light becomes more scarce, the silver halide grains residing in your film will be less uniformly struck by photons, causing a steep drop in density after a few seconds of needed exposure. This exponentially diminishing response to low light levels is more popularly known as a film’s reciprocity failure. Not all films are created equal, and some will respond better than others, but here’s what you can typically expect:
B&W Film - exposures in excess 1-2 seconds will result in reduced density, yielding very thin, if not non-existent shadows.
Color Negative Film - exposures in excess of 20 seconds will result in color-shifting, as dye layers of the film will absorb light unevenly over the prolonged exposure.
Slide Film - exposures in excess of 5 seconds have color shifts similar to color negative. In high saturation films such as Fuji Velvia, this shifting is much more extreme.

"We Saw It Happen" - by Kevin Joes, via FPP Flickr Pool

So how can we avoid the “Effffed!” moment after getting our images developed? Compensation, compensation, compensation! In B&W films, you compensate by adding more exposure time, and developing slightly less time. In color emulsions, you add slightly more time, and apply color filtration to compensate for color shifts. But to what degree to we make these changes? Luckily, chemical engineers and other well trained film professionals have rigorously tested films before bringing them to market. Almost every major film out there gives us a general use data sheet, telling us what working exposures we can expect from a given metered scene. Take, for example, this lovely chart (page 2) for Kodak’s classic B&W film, Tri-X.
Dissecting B&W exposure compensation a little more, why is it necessary to reduce development time if we’re already increasing the exposure time? The simple answer is to save highlights. During that longer exposure, photons hitting the film emulsion in the highlight region will give the silver halides plenty of light. To make sure there’s still some detail in the final image’s highlights, you reduce the development time; this will give the shadows adequate time to develop while taming just how dense the highlights get. In other words, we’re taking a very high contrast exposure and applying a low contrast development to obtain a “normal” negative. For more information and much more on exposure and development, I highly recommend Ansel Adams’ The Negative.

"Patiently Waiting" - by Jeff Soderquist, via FPP Flickr Pool

If anything here is starting to sound too technical, have no fear. Remember that a vast majority of the pictures you’ll be taking won’t even need to consider reciprocity failure. But if you regularly shoot at night or with a large format camera, however, you may want to get to know your film a little better with a five minute “Da Google” search. A couple other ways to “cheat” reciprocity failure is to shoot with a film stock that has very low reciprocity failure or responds well to color shifting. In the B&W world, the hot film for long exposures the past couple of years has been Fuji Across 100. For exposures 2 minutes or less (that’s right, 2 minutes!), no compensation is needed. For anything after that only needs ½ stop adjustment or 1.5 times the indicated exposure time. For color film shooters a, now harder to find, popular solution was to shoot slower speed tungsten balanced slide film such as Ektachrome 64T for a really long time, as the shifts would lean more towards daylight, no filtering required! You can see a great example below:

Puerto Madero, Buenos Aires, Argentina - by Rafakoy, via FPP Flickr Pool

So now that you’re in “the know” on reciprocity failure, get out there any time of day and shoot. Stop all the way down in broad daylight, shoot with ridiculously slow film in low light, and even play around with color shifts. Just use that trusty tripod, remember the pros and cons of your film, stay super positive, and have fun! And when you get those tasty film images developed, be sure to head over to the Film Photography Project’s Flickr Group, and post ‘em there too.
Happy shooting and long live film.