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P i n h o l e D r e a m s

Technique

My pinhole camera

I made it using cardboard, glue and black painting. A black masking tape must be used for masking the edge of the box. It is very important that the camera is light-tight.

The camera use a photo paper which can be easily loaded in a dark room. I planned to use roll film. But I began with photo paper to make the first try. The results were so nice that I decided to continue with paper.

The dimensions of the image is about 6x12 cm. So I obtain panoramic pictures. The focal length is 6 cm and the view angle is about 130°, a rather large value.

Making the pinhole

Cut a square of aluminium plate. Then, using a sewing needle, make a hole in the center of the piece. With the point of the needle, you can clean the other side of the hole. Remove the blurs with a sand paper.

When the hole is finished, glue the aluminium piece on a square of black paper with a small window on the center (see the picture in the right).

Controlling the size of the pinhole

You can use a flat bed scanner to control the diameter of the hole. I use the transparency mode of the scanner with 3200 dpi resolution. You can then measure the diameter of the hole with any image editing software.

The first time the diameter of the pinhole was 0.5 mm. Then, I make another one using only the point of the needle. Finally, I obtained a 0.3 mm diameter which is the optimal value for a 60 mm focal length.

Exposure

Firstly, it is necessary to know the apperture of the pinhole. It is the ratio of focal length over the hole diameter. In the case of my camera, the focal length is 60 mm and the pinhole diameter is 0.3 mm. Then, the apperture is f/200.

Secondly, make a measurment of the exposure time for a f/16 apperture. Then the pinhole exposure time is about (200/16)² = 156 times the measurment. For example if you obtain 1/2 second at f/16 the exposure time must be 78 seconds.

With bw paper the iso rating is about 6 and the exposure times are often large.

Due to reciprocity problems, the longer you expose the paper, the less sensitive it gets. You must increase the exposure time by a factor which is given here.

For example, if you calculate a exposure time equal to 20 mn, you obtain a real exposure time equal to 2 hours !

(c) JFD 2005