Film is a funny medium. Depending on the film, it loses sensitivity when the exposure time is long enough (usually after 1-2 seconds). This is called (low intensity) reciprocity failure (you’ll find science and math behind the link). Basically all you need to know is that you have to compensate the reciprocity failure by extending the metered exposure time if you want to get properly exposed frames. Each film has its own characteristics so the needed compensation is different for each film. Fuji Acros 100 film is one of the only films that have very small reciprocity failure and you can use exposure times up to 20-30 seconds without compensating.
Noon Pinhole camera, Kodak T-Max 100, f/207, 4-5 seconds
Here you can find a small reciprocity correction table for some of the popular black and white films. For example, if you meter that the correct exposure time is 30 seconds, with Ilford HP5+ you have to expose for 54 seconds to get good exposure, and with Kodak Tri-X 400 for 78 seconds (1:18). Google is usually your best friend when trying to finding out the film’s reciprocity failure if manufacturers’ data sheets fail to help.
I’m planning to take a few long exposure photos this weekend. At the moment I’m thinking of taking them with Yashicaflex with cable release. The pinhole camera is another option. I’ll probably use Kodak T-Max 100, Fuji Reala 100 and / or Rollei Pan 25 as the film. The latter would enable really long exposure times. Rollei Pan 25 is said to be very similar to Agfa APX 25 so APX 25′s reciprocity failure compensation could be used. Here’s a screenshot from APX 25′s data sheet.
By the way, digital camera sensors do not have a reciprocity failure.
Pinhole photography
If you are photographing with a pinhole camera which has an aperture value that is out of your light meter’s scale, you have to calculate the exposure time based on the light meter and then add time for the reciprocity failure. A simple formula for calculating the exposure time (ignoring reciprocity failure) for a pinhole camera is ( f / R )^2 x Tm = Tc where f is the camera’s aperture value, R is the light meter’s aperture value, Tm is the exposure time in seconds given by the light meter, and Tc is the corrected exposure time in seconds.
For example, my Noon Pinhole camera has an aperture value of f/207. My light meter goes up to f/64. If I measure 1/2 second at f/64, the correct (but not compensated) exposure time for the pinhole camera is (207/64)^2 x 0.5 = 5.25. If I was using Kodak T-Max 100 film, the compensated exposure time would be around 6 seconds. With Fuji Acros 100 I could use exposure time of 5.25 seconds because Acros 100 does not require compensation below 20 seconds.
