Canon® EOS Digital Photography Photo Workshop

6.2. FAST VERSUS SLOW LENSES

A fast lens is a lens that provides a large aperture — meaning less than f/3.0. Why is it called fast, then? It is because a larger aperture provides more light for the digital sensor, meaning you can use a faster shutter speed in your images where there's less light, as in 6-3. Consequently you can shoot faster, not that the lens itself does anything more quickly. It also means you can achieve better narrow depth-of-field images because you have a wide aperture.

Fast lenses, especially zooms, are generally more expensive and are of higher optical quality. Lower-quality, slower zoom lenses commonly have a wider, variable aperture that narrows as the focal length increases. For example, Canon's EF 70-300mm lens ranges from f/4 to f/5.6 depending upon your focal length; at 300mm, the lens physically does not offer an aperture of f/4, and you won't be able to even manually select that f-stop on the camera.

Figure 6-3. A stop-action image of men's sabre fencing taken at the 2007 World Fencing Championships in St. Petersburg, Russia. A fast lens is a critical tool for taking this type of photograph. Taken with a 1D Mark IIn, EF 70-200mm f/2.8L lens, ISO 800, 1/1000 second at f/2.8. ©Serge Timacheff

Variable apertures with settings dependent on focal length are often confusing. If you set a variable aperture lens, such as 70-300mm, at 70mm and set your ...