Pinhole Exposure Information
I am sometimes asked how I determine the exposure for my pinhole photographs, taken with my Zero Image camera. First, I use a hand-held light meter to get a reading. I then use a conversion wheel on the back of the camera to determine the time for the camera's aperture of f/138. For instance, if I get a reading of f/8 and 1/2 second, then at f/138 the time would be about two minutes (f/8 to f/138 is about 8 stops less light, and 1/2 second to 2 minutes is 8 stops more light). Then I use a table to compensate for reciprocity failure (the fact that, beyond a second or so, film will be underexposed if you use the predicted time - the amount of underexposure depending on the type of film). After trial and error I recommend the following settings for TMax 400 film:
Thus, with my example of a predicted exposure of 2 minutes, I would actually expose the film for 9 minutes to avoid underexposure. While I not infrequently lose track of time and let the exposure go on too long (I am, afterall, enjoying a meal out while I take these pictures), I have yet to ruin a negative due to overexposure. For instance, if the time should be 10 minutes and I let it go for 15 minutes, I am only overexposing by about 1/2 stop, which is not much of a problem. Underexposure (and losing all detail in the shadows) is more of a risk, so err on the side of longer exposure.
Delta films are said to respond similarly to TMax films. Tri-X, on the other hand, has more severe reciprocity failure with very long exposures. For instance, while a 4 minute exposure becomes 20 minutes with TMax 400, it would become 40 minutes with Tri-X. Similarly, a 10 minute exposure becomes 1 hour with TMax but 2 hours with Tri-X. Pinhole photographer DelioTO has a graph comparing a variety of films at http://www.flickr.com/photos/11058938@N06/6135581125/sizes/l/in/photostream/ .
Outdoors, with TMax 400, the exposure is usually a second or more, so I haven't had to deal with fractions of seconds, which would be impossible with this camera (with its manually opened and shut aperture). Also, whether the exposure is short or long, any camera movement while opening/shutting the pinhole opening can blur the image (even when a very long exposure is necessary, there are often lamps, candles or other bright objects which will present as lines or wiggles if the camera shakes). My Zero Image does not have an attachment for a cable release (which more expensive models do have) so I always place my palm in front of the pinhole during opening and closing, and remove my hand after the camera is still. Also, I never hand-hold it for any exposure, because even one second this way can result in a blurry mess. I carry a mini-tripod with me, but usually use a table, counter, mailbox, or whatever flat surface is handy to keep the camera still.
Also consider the effect of long exposures on color, if you use color film. The three colors of the film will shift to different degrees with long exposures. I have just been using black and white film, so have not had to deal with this issue.
For outdoor pinhole photographs, such as the one taken at the Museum of Modern Art in NYC on the left, I usually will not bother metering, but will open the shutter for a second or two if it is sunny, and 5-10 seconds if it is overcast. For indoor images such as the self portrait on the right, I always meter since, even after doing this for ten years, I can have difficulty telling by eye how much light is present (our pupils adjust to the light so rooms of varying brightness may appear similar).