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Technical Talk Part 11
Making a pinhole camera.
One of the interesting things about optics (well interesting to me, but then I need to get out more) is that a hole alone can form an image, without needing a lens. This effect is used in the pinhole camera. Because of the way the pinhole works, blocking all but a very narrow beam of light from each point on the subject, all parts of the image are equally sharp and the camera needs no focusing. Unfortunately, this is not as sharp as we would expect from a lens, but still useful. So useful that the print that won the Gold Medal in one RPS International Print Exhibition was taken using a pinhole camera.

Making a pinhole camera is fairly easy (there's a kit available, if you can find it). Firstly, decide on the apparent "focal length" of the pinhole you want to make. Pinholes can be made with "focal lengths" of anything you like, but the quality of the image will suffer at the extremes. Try starting with 100mm. Get an old aluminium drink can and cut a section out about the size of a 35mm frame (have the box of sticky plasters standing by). Holding the piece of can on a scrap of wood, tap it with a round punch or old ball-point pen to make a dent. Smooth down the raised side of the dent with fine wet or dry paper until it starts to form a hole in the centre. Carefully pierce the hole with the point of a fine needle and smooth down the burr. The optimum size of the pinhole can be calculated by finding the square root of the "focal length", then dividing by 25. For 100mm this would give Sqrt 100=10 10/25=0.4mm. Although there are ways of measuring the size of the pinhole, it's difficult to do it accurately and probably easiest at first to measure the needle as best you can and pierce the hole carefully. The size of the pinhole is not critical, but the closer it is to optimum, and the rounder it is, the better the image quality will be.

Then make a light-tight box that's black inside. Black card stuck together with masking tape works well. Make a lid to allow loading and unloading. The size will depend on the "focal length" of the pinhole and the size of the film used to record the image. Because the image is not that sharp, enlarging it is not always a good idea. This means that a large negative that can be contact printed is usually a good thing. A cheaper alternative to sheet film is RC printing paper. Although not sensitive to red light, with variable contrast types varying the contrast over the neg depending on the colours in the subject, it still works surprisingly well. Try making the film or paper curve towards the pinhole, easily done by making the width of the box less than the width of the sheet of film or paper (which can be cut down), so it has to be curved to get it in. Stick card stops on the walls of the box to keep the film in place. This can give a more panoramic effect and helps to stop the neg getting less exposure and less sharp towards the corners. Cut a hole in the front of the box and tape the pinhole to the inside. Hinge a piece of black card over the hole to act as a shutter.

The f number of the pinhole can be calculated because it's the "focal length" divided by the diameter of the hole, both of which we know roughly. For 100mm this would give 100/0.4=f250. This means that pinhole exposures will be long, unless the light is very bright. Experiment to get the best results. The speed of RC papers vary, but start at 400 ISO. Process the film or paper as normal.

© Barry Leighton FRPS