7.1 HOW A LENS WORKS

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Surprisingly, lenses are not actually needed to take a picture. You can make a camera out of a shoe box with a small hole in one end. Known as a pinhole camera, this primitive device can actually focus an image and record it on film. To make a photograph, the box is loaded in the dark with a light-sensitive film or paper and the pinhole is covered with opaque tape. Peeling the tape back (much like a shutter) to uncover the pinhole (much like a lens aperture) begins the exposure, recovering the pinhole ends it. The exposed film or paper can be removed in a darkroom and the image developed.

In a pinhole camera, the light waves from the object converge on the pinhole and focus the image upside down on the film.

The Pinhole Visions web site provides information on pinhole photography, and the work of pinhole photographers.

Light is bent when it passes between substances having different densities. You can see this if you look at an object that is both in and out of water; for example a spoon in a glass half full of water looks bent at the point where it enters the water. Obviously, the spoon isn’t bent; the light reflecting from the spoon is, as it passes from the dense water to the less dense air. The same effect occurs when light passes from the air through a piece of glass. If the glass is curved correctly, as it is in a camera lens, it can bend the light in such a way that an image of the scene in front of the lens is focused behind it.

The focal length of any lens is the distance between the optical center of the lens and the point at which it focuses an image. When you use a magnifying glass to focus the light from the sun onto a piece of paper, the area illuminated by the beam will become larger or smaller as you change the distance between the magnifying glass and paper. At the point where the bright circle of light is smallest (and where it might set the paper on fire), the simple lens that constitutes the magnifying glass is in focus. The distance between the magnifying glass and the paper is the lens’ focal length.

Lens focal lengths are based on the physical characteristics of the lens so they are absolute values. However, a given focal length lens may be a wide angle lens on one camera and a telephoto lens on another. This is because descriptions such as "wide-angle" or "normal" depend on the size of the film or image sensor being used. As these get smaller, a given focal length lens magnifies more. There are currently a number of differently sized image sensors used in digital cameras. For that reason, different focal lengths are needed to give the same image coverage on different cameras. Because of the confusion this causes, most digital camera companies give the actual focal length of their lenses and then an equivalent focal length were the lenses to be adapted to a 35mm camera. For example, a camera may list its lens as 7.5mm (equivalent to 50mm on 35mm camera). Because digital equivalents vary widely, we often use the more familiar 35mm focal lengths in this book.

In the upper left corner are shown some image sensor sizes. The larger rectangle is the size of a frame of 35mm film.