Leave it to the world of photography to confuse size and speed. In lens speak, the term “fast glass” refers to lenses with large apertures. The aperture is the opening of a lens. Its size is expressed as a number that shows the ratio of the opening to the lens's focal length. This number is referred to as an f/number, f/stop, focal ratio, f/ratio, or relative aperture.
How fast is “fast?” Or, how big of an aperture opening gives me truly fast glass?
In “professional” zoom lenses, the aperture of f/2.8 is generally regarded as fast. When it comes to prime lenses, depending on your level of lens snobbery, what is truly fast starts between f/2.0 and f/1.4 with many “professional” lenses featuring f/1.4 maximum apertures. Faster-than-f/1.4 lenses are the exotics of the 35mm format optical world.
We call these large-aperture lenses “fast” because they allow cameras to take photos at relatively fast shutter speeds for a given amount of ambient light. A fast lens might make it possible to take photos handheld in low light. Faster shutter speeds offer greater options for freezing action and less chance of camera shake, both of which can cause blur in your images, no matter how bright the scene. A large aperture means that you can photograph with very shallow depth of field.
Why do you want a fast lens? If you ever do off-tripod low-light photography, maybe at a concert or night club, you will want a lens that can open wide to maximize light-gathering. Wedding photographers often find themselves in less-than-ideal lighting scenarios at the church or reception and need larger apertures, too. Street photographers working at dusk or dawn may benefit from more light striking the sensor or film. Finally, sports photographers working to freeze action and isolate subjects will appreciate large apertures.
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