An Honest Confession

photo credit: db*Photography

A recent poll result revealed that we have an almost equal mix of photographers who have been shooting film all along, and those who have recently started. While I’m one of the former, I wasn’t always aware of all the technical behind-the-scenes mumbo-jumbo. And when I started to become aware of the reciprocal relationship between f-stop and shutter speed I found it hard to grasp straight away. (I began with an Olympus XA; an aperture priority rangefinder.) And so this is where this post comes into play. Whether you’ve been shooting for years, or only a few days, it never hurts to examine the theory behind the way things work as they do. Even if it is just revision.

Substance

Reciprocity law states that as you increase the intensity of light reaching the film you also need to decrease the speed it reaches the film by the equivalent amount*.

photo credit: David Blackwell (back to black)

Translated, this just means that as the light hitting the film gets brighter, your exposures must get shorter in order to maintain the correct exposure. Breaking it down further, reciprocity basically means that two things are in a state of mutual dependence. In our case, the film is in a state of dependence with light, in order to create an exposure. Referencing the reciprocity law again, we can now say for certain that if it is true that more light = shorter exposure, then it must be equally true that less light = longer exposure. This much is self explanatory, but it is also the point at where things start to get complicated.

When Everything Falls

If you look at the table above (you may need to go to the original page for a better view), you’ll notice that there are three columns. The first column is for exposure times when using an aperture of f8. The second column is a translation of these times for an aperture of f220 (apertures of this size are common in pinhole photography). The third column is a list of corrected times for Tmax 100. You’ll notice that when the light levels fall (in other words, when the shutter time increases), the numbers on the 3rd column become much greater than the numbers in the first or second column. For example: Let’s assume that for a correct exposure your light meter suggests a time of 2 seconds at f8 when using Kodak Tmax 100. This sort of metering is common on street lamp lit scenes. If you use a pinhole with an aperture of f220, then your exposure time becomes 26 minutes and 43 seconds. Now remembering the reciprocal relationship that exists between f-stop and shutter, we know that 2 seconds at f8 = 26 minutes and 43 seconds.

However, film being a man made component means that it isn’t perfect. Since manufacturers at this point in time have limitations placed on them by the chemical construction of film. The materials will only absorb so much light before starting to fail; bearing in mind that this is a simple statement of a much more complex chemical reaction, the simple resolution is to increase the time to compensate for that weak point in the chemical reaction. In the case of the chart and our example above, we need to increase our exposure time by almost three times to an exposure time of 1 hour 34 minutes to gain a well exposed image. Why? Read on.

The Perfect Night

Each manufacturer includes information about the film in the box. Each manufacturer also includes the same information available as .pdf downloads on the net. Considering you’re reading this article, I feel it is safe to assume you have access to an internet connection.

Usually this information is referred to as a datasheet. Using Kodak Tmax 100 again as an example, we can see that on page 5 of the datasheet for this film a table is given for adjustment depending on the circumstances. However this particular table only goes up to 100 seconds. Where does this leave us? Generally I just double the exposure time for readings of under 5 minutes, triple for up to an hour, then take a stab in the dark for times over an hour. This doesn’t always work, as you can see above right. The photo was taken using an LSI Diana F+ and Kodak Portra 400NC. Contrary to the title for this section, it was far from the perfect night. Lightning was striking nearby and it was pouring down with rain. I’d forgotten any sort of timing device, so I just opened the shutter for as long as I felt was necessary. I’ve posted the raw scan, and you can see that the colours are starting to appear in the sky, but despite the shutter being open for several minutes when a few seconds would do (according to the light meter), the image has failed to record much detail. (I actually quite like the shot and have printed it at 30cmx30cm with the black level adjusted and it looks great.)

Outro

photo credit: Tomas Webb

Basically as far as reciprocity failure is concerned, in terms of long exposures it pays to check the manufacturers datasheet before you head out. If you stick with one type of film you’ll eventually learn how it performs, although you may need to bracket exposures for a few rolls just to make sure you get the shot you’re after!

Additionally, when using colour film (particularly slide film) long exposures tend to create colour shifts. If you’re aware of this before hand, you can harness the creativity if offers as seen in the example on the left. I also cross-processed that roll to deepen the blue that I knew would occur.

Still left with questions and/or still confused? Ask for clarification in the comments below!

*Those of you who are metal fans out there should recognise the headings from Haste the Day albums….Just thought I might have a little fun.