An excerpt from From pinhole to print – Inspiration, instructions and insights in less than an hour. F-stops and what to consider when taking photographs with a pinhole camera.
Your camera is ready, loaded with photographic paper and you have found the perfect scene. You have got a light tight shutter, but how long do you open it?
There is a perfect exposure time and it depends on your camera, the film you are using, the size of your pinhole, and the amount of light available. If you don’t expose it long enough your negative will be too light and if you leave it too long your negative will turn too dark.
There are two basic methods you can use to determine exposure – use a light meter and do a calculation, or test your way to good results. You will find that it is probably the latter or a combination of these two that will work for you.
Keep notes of your exposure times and the results you get. Learn from your experiences.
F-stops and the pinhole
If you know the size of the pinhole and the focal length you can work out the f-stop of your camera. The f-stop will help you work out the right exposure time.
Doing the maths
The formula for calculating the f-stop of your camera is the focal length divided by the size of diameter of the pinhole.
The focal length / diameter of the pinhole = the f-stop
F-stops have a ‘standard sequence’. If you have an ordinary camera, you can see the f-numbers on the lens.
The f-stop sequence
Ordinary lenses rarely go beyond f/22. The sequence is: