The sunny f/16 rule is very helpful when you are trying to figure out exposure. Here's an example...
If you use an ISO of 100 and the scene you want to photograph is in sun the exposure is 1 over the ISO at f/16. In this case, we can round of 100 to a standard 1/125 and get the exposure of f/16 at 1/125th of a second. The problem with pinhole cameras is that you don't have f/16 to work with. The f/stop is usually something like f/180. To figure out the correct exposure with ISO 100 and a pinhole of f/180 you need to make yourself a little chart like the one on the left.
Each exposure on the chart is equivalent to the other. So...if you are using a pinhole camera with a fixed f/stop of f/180, you're exposure will be 1 second. Another problem sometimes pops up if you are using film. The problem is called the reciprocity factor. That means that with long exposure, you usually have to add some exposure. You might have to shoot at about 2 seconds, in my example. You need to refer to the film for the exact modification for your film. As far as I know, there is no reciprocity effect for digital camera exposure.
