Camera Obscura.

A camera obscura is, in its simplest form, a box with a hole in it! The hole allows light through, projecting an image onto the opposite inside of the box. If that side were made of ground glass the image can be seen without the inconvenience of being inside the box! The clarity (or sharpness) of the projected image is dictated by the size of the hole. The smaller the hole - the sharper the image. Unfortunately a small hole lets in a small amount of light making the image dim, but whilst widening the hole brightens the image it also dramatically reduces its sharpness.

You can easily make a primitive camera obscura to see these things for yourself. Get a cardboard box, the size is relatively unimportant but I wouldn't try the one the washing machine was delivered in! One about the size of a computer software box will be fine. Tape up the box so that it cannot open. If you are a youngster, get an adult to help you please. Cut out one of the short sides with a sharp knife, leaving an edge all the way round. Now tape a piece of tracing paper or grease proof paper over this hole, try to keep it flat - this is the viewing screen. On the side opposite the screen make a hole with a sharp pencil about 3mm across.

See simple diagram

Now, standing indoors point the camera obscura at a bright window, there should be no strong light behind you. Hopefully you will see an image of the window projected onto the screen. It will be blurred and not very bright, but remember this is just a box with a hole in it! If you hide under a jacket or blanket to prevent much light falling onto the screen from behind, the image will be easier to see, and it will become instantly obvious why early photographers employed the same method of working. You will notice also that the image is inverted. When you have satisfied yourself with this, widen the hole in the front to the full width of the pencil, the image will appear brighter, but less sharp. Continue to widen the hole and watch the result each time, very soon the image becomes so blurred it is impossible to distinguish any detail. To overcome this problem, a lens of suitable type could be placed in the hole to focus the image onto the screen. Since you are unlikely to have a lens, this is where this experiment ends.

There is a camera based on this principle - the Pin Hole Camera.

Return to potted history.