As we know, there are three variables which will determine the exposure of a photograph - the shutter speed (which we can easily control) the ISO (which we know depending on the 'negative' we're using - film/digital back/photo paper) and the aperture (which we need to calculate). This calculation is simple:
- Measure in millimetres the focal length (distance from the hole to the paper/film/sensor). If this is curved i.e. varying focal length then I suggest you take an average. For my cake tin it's about 135mm.
- Measure the diameter of the pinhole in mm. Suggest you use a precise metal ruler with half mill increments to get a good estimate, and hold it up to the light to see the hole clearly. I estimated mine at 0.75mm.
- Divide the focal length by the diameter, and this is your f number or aperture. So my camera has an aperture of f180. (135/0.75 = 180)
My camera uses photo paper - which generally has an ISO somewhere between 2 and 10. I was using Ilford multigrade and spent a bit of time testing that, and found it to have a sensitivity of about 4. If film it will be whatever the film is: 100, 400 etc. and obviously digital you can alter freely. So now you know two of your variables, you can use those figures in your light meter and the available light will determine the third - shutter speed.
It gets a bit tricky because most light meters do not go up to such high aperture numbers, so you may have to take a reading and then 'step up' along your f stops, doubling the time as you go to, until you reach your aperture number. For example if I get a reading of 1 sec at f22: f32 is then 2 sec; f45 is 4 sec; f64 is 8 sec, f90 is 15 sec, f128 is 30 sec; f180 is 1 minute, and hence a 1 min exposure.
An example of an aperture table can be found at the back of the 'Pinhole Pictures' handout (also found in the library) and this link explains the whole process nicely too:
Also a video on making a scanner camera - http://blog.makezine.com/archive/2008/09/weekend_project_scanner_c.html
(cheesey, American and delightful)
Here's a pinhole picture I made with a four-way camera - holes arranged squarely, and paper coiled round in a circle in the middle. The negative came out like this:
It's then a simple case of contact printing that negative (with a test strip of course) to get your positive picture. Bear in mind that it's a good idea to have your enlarger fairly bright for this to get light through the paper, and you will need glass to keep the negative pressed flat and the image in focus.
Another one I made this time with eight pinholes.