Camera principles

The word photography means drawing (or writing) with light. It's a good description because every time you take a photograph you are really allowing light from the subject to draw its own picture on the sensor or film. But just how does this 'automatic drawing' take place? Have you ever been lying in bed in the morning watching patterns formed on walls or ceiling by sunlight coming through gaps in the curtains? Sometimes the shadowy shapes of trees and buildings can be made out, especially if the curtains are dark with only one narrow space between them. If you can use a room with a window small enough, cover the window completely with black paper or opaque kitchen foil. Pierce a small clean hole through the blackout with a ball-point pen. Provided the daylight is bright and sunny you should be able to see the dim outlines of the scene outside projected on a piece of thin paper held about 30 cm (1 ft) from the hole (Figure 7.1). Various shapes should be visible although everything will be upside down.

Figure 7.1 A small hole in a window blackout forms a dim image of the sunlit tree on the tracing paper.

This arrangement for making images is called a camera obscura, meaning 'darkened chamber'. It has been known for centuries, and all sorts of portable camera obscuras about the size of shoe boxes were made which also allowed people to trace over the image, and so help them draw scenes. Figure 7.2 shows a camera obscura you can make yourself out of an old cardboard cylinder and tracing paper. The image is upside down because light always travels

Figure 7.2 Home-made camera obscura. (Paint its inside surface matt black for best results.)

in straight lines. Light from the top of the window passing through the small hole reaches the bottom of the image on the paper viewing screen.

Enlarging the hole makes the image brighter but much more blurred. However, you can greatly improve clarity and brightness by using a magnifying glass instead of just an empty hole. A magnifier is a piece of glass polished so that its edges are thinner than its center. This forms a converging lens, which is able to give a brighter and more detailed image of the scene, provided it is the correct distance from the screen. Try fitting a lens of this kind to the hole in your camera obscura. You will find that you now need some way of altering the distance between lens and screen ('focusing') until the best position is found to give a clearly defined image. All properly made camera lenses are made up of several lenses together in a single housing. In this way, the faults, or 'aberrations', of individual lens elements and be cancelled out to give clearer, 'sharper' images.

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