How does a camera lens work?

Answer

A camera lens uses refraction to focus light on the film, or in a digital camera on the CCD or other light-sensitive array.

When a ray of light passes from a less dense to a more dense medium (such as from air to glass) it slows down. If it strikes the glass surface at an angle, it is also bent a little, and this is called refraction. When it passes back into air, it speeds up again, and is again refracted if the surface is at an angle.

This makes it possible to design a curved piece of glass that will focus a parallel beam of light (arriving perpendicular to the lens, that is along its axis) to a point. The ideal surfaces are parabolas (or you can use one flat surface and one parabola, this is a plano-convex lens). When you use a magnifying glass to focus the sun's rays and set paper on fire, that's the effect you are using.

The interesting thing happens when the light rays are parallel to each other but not parallel to the axis through the centre of the lens. Just take it on trust for a moment, this same lens will also focus these, but to a point above, below or beside the focal point for rays along the axis, and all these points of focus of parallel rays will form a plane, called the focal plane of the lens. So, you put the film at this focal plane, and you've now used the lens to concentrate the light on the film, and form an image.

That will only work for objects far enough away that the light rays are roughly parallel to each other. For closer objects, you need to move the film a little closer to the lens. That's what happens when you focus a camera (or autofocus does this for you). The bigger the lens, the more concentrated the light, but the more critical focussing becomes. So, camera lenses have a second control, the iris or f-stop, that changes the size of the lens by masking the outer bits of it. It's a compromise between getting lots of light and making the focus more forgiving. If you use a very small lens, lots of things will be in focus. That's called depth of field.

Whew! Still there?

Camera lenses are normally made of four or more bits of glass or plastic, because this ideal one-piece lens doesn't work for two reasons. Firstly, the amount of bend depends on the colour. This is called chromatic aberration. Secondly, in practice it's hard to make parabolic lenses, but far easier to make spherical ones, which are close to ideal in the middle but get fuzzier as the lens gets bigger. This is called spherical aberration. Both of these can be corrected by using compound lenses, that is lenses made of more than one element - but never perfectly.

Next, we can look at zoom lenses, or retrofocus lenses (a way of designing lenses to make them more compact). But that's probably enough for now.

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