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Focus on Focus

There's more to getting your camera in focus than meets the eye. But what is focus?

Focus is the act of bending light through a lens onto a plane, which clearly represents the objects on the other side of that lens. In the natural world, muscles in your eyeball perform this by causing the thickness of your lens to change, in order to project an image on your retina. In your camcorder, it's done by a series of glass or plastic lenses, whose distance from one another is changed by a rather complex collection of threaded tubes that move - some forward and others backward - to keep images sharp throughout the range of your zoom.

The distance that these lenses need to be from one another in order to produce a sharp image on your camera's CCD is well known, and in the film industry there is no looking "through the lens" to adjust focus; rather, focus is set with the aid of a tape measure and a set of tables. There are people, called "focus pullers," whose entire job - and they make careers from it - is to sit at the front of the camera and turn a knob from one preset point to another, to keep the objects in the lens in focus. They do this by looking at the distance of the subject to the lens and by using scales on the camera. Camcorders are much more conve…