You may often hear the terms ‘fast’ or slow’ to describe a lens or aperture. This has nothing to do with the speed of operation, but to the size of the maximum possible apperture (usually quoted as a suffix to the name of a lens). A Lens with a maximum aperture of f/1.4, for example, is ‘faster’ than similar lens with f/2.8. Because the aperture is larger, faster shutter speeds can be used, which is especially useful in low light conditions.

Another practical benefit of a larger aperture is that the extra light that enters the lens leads to brighter viewing on the camera’s viewfinder-again, useful in low-light conditions.

Zoom lenses often quote two maximum apertures, for example, 18-70mmf/2.8-4. This means that the maximum aperture at the 18 mm end is f/2.8, while the maximum aperture at 70mm if f/4. In reality, the actual hole is the same size but the added focal length (and the maths) changes the f-number. There are several lenses available with a fixed maximum aperture that doesn’t change throughout the focal range, such as Sigma’s 70-200mm f/2.8 lens. These lenses are usually expensive though, and because of the large optics needed to perform the trick, quite heavy as well.