Fast lenses are lenses of large aperture (width), or low f-stop. (F stop is the ratio between the width of the lens you are using and the focal length.) Fast lenses are big and heavy, but you need them. Fast lenses let you do shallow depth of field. That means you can focus on a person and leave the background, and also foreground, blurry. A large fraction of good photography uses this trick to define the composition of a shot. Small digital P&S can’t do this very well, even though their lenses are sometimes quite fast, because shorter lenses don’t have as narrow a depth of field.
Fast lenses can be stopped down to be like slow lenses, and get more depth of field when you want it. However, usually a stopped down big fast lens is better than a wide-open smaller, slower lens. Most lens problems occur at the edges of a lens, and if you stop down, you cover that up —digital SLR lens
Fast lenses can shoot in less light. They’re great, but they cost money and are big and heavy. But most shooters won’t work with less.
