The focal length is a basic optical property of any lens, and the most important one to a photographer. The simple way to think of the focal length is to think of it as a numerical value, expressed in millimetres, which represents how much of a given scene (the coverage area) a lens can take in.

Focal lengths for Canon SLR lenses range from ultra-wide (14mm) to incredibly long telephoto (600mm and 1200mm). The typical range of an affordable lens or lenses is from 28mm to 105mm or so.

Why, then, these strange values in millimetres? Why not indicate the angle of view taken in by each lens instead? Well, partly for historical reasons and partly for practical reasons. The technical description of focal lengths is rooted in the mathematics of optics - it’s the distance between the focal plane and the rear nodal point of the lens, given infinity focus. How this bit of Martian translates to the field of view depends on the size of the imaging area being used, which can be different between 35mm film cameras, APS cameras and some digital cameras. And as for practical reasons, the focal length of a lens is an innate property of the lens, but the actual coverage area of the scene depends on the size of the image format used.

So note that crucial point - all the examples I gave above are for a 35mm film camera or full-frame EOS SLR only. If you were to use a 28mm lens on an APS camera or a digital camera with an image sensor smaller than that of 35mm film (ie: most digital cameras sold today) then you would have a much narrower field of vision than if you were to use the same lens on a 35mm film camera. For more details see the section on focal length multipliers/cropping factors.

The same focal length system is used to describe lenses for other types of cameras as well, such as medium format cameras. But the area taken in by a lens on a medium format camera will be totally different from that of a lens of the same focal length on a 35mm camera, because the area covered by medium format film is considerably larger.