Fast refers to the size of the lens’ maximum aperture, and it’s a relative term in more ways than one. The ƒ-number designates the diameter of the lens’ maximum aperture opening relative to the lens’ focal length (i.e., ƒ/2 means the aperture diameter is one-half the focal length; ƒ/4 means it’s one-quarter the focal length), and fast depends on the focal length, as well. Sigma’s 200-500mm ƒ/2.8 APO EX DG zoom is the fastest 500mm lens available (and comes with a dedicated 2X teleconverter that makes it a 400-1000mm ƒ/5.6, the fastest 1000mm lens available). But ƒ/2.8 isn’t at all fast for a 50mm lens; ƒ/1.4 would be fast for a 50mm, and there are even faster ones. For this article, we’re considering the collection of elite lenses that are faster than ƒ/1.4, regardless of focal length.
Today, there are a number of lenses faster than ƒ/1.4. In fact, several are faster than ƒ/1.0—their effective aperture diameter is wider than the lens’ focal length. Note that a lens’ focal length and its physical length aren’t the same thing. The focal length is the distance between the optical center of the lens and the image plane when the lens is focused at infinity. For telephoto lenses, the optical center is actually in front of the front lens element. The physical length is, of course, the length of the lens from filter threads to mount. Wide-angle lenses are generally physically longer than their focal lengths, while telephotos, by definition, are shorter physically than their focal lengths.
Fast lenses offer several advantages. For one, they let you shoot in dimmer light than slower ones because they let in more light. More light transmittance means you can use a faster shutter speed at a given ISO setting, or a lower ISO setting at a given shutter speed, in the same conditions. Combine the high-ISO capabilities of today’s DSLRs and mirrorless cameras, in-lens or sensor-shift image stabilization, and a fast lens, and you can shoot handheld in amazingly dim conditions.
Faster lenses provide brighter viewfinder images for composing and manual focusing with DSLRs, assuming your DSLR’s viewfinder can handle the lens speed (some DSLR viewfinders don’t actually appear brighter with lenses brighter than ƒ/1.2 or so). That can be very beneficial when shooting in dimmer lighting, especially important, as depth of field is extremely limited.