The inexpensive kit lenses used bundled with level SLRs have maximum apertures ranging from f/3.8 to f/5.6 and some point-and-shoots only have an f/6.3 maximum aperture! That won't be of much help in a dimly-lit room.
Just as with sports cars, computer processors, bullet trains, and Internet connections, being fast is great for camera lenses, too. It's much easier to take photographs in low light with an f/2 or f/2.8 than with an f/4.5 or f/5.6 lens mostly because it's easier to look through your camera at these larger aperture settings but also because you can shoot at a smaller than wide-open aperture and still maintain a crisp image.
Low-light highlight: This Chevrolet HHR is suspended from a crane over the show floor at the Denver International Auto Show. The f/2 maximum aperture of the Zuiko Digital ED 14-35mm f/2 was a big help when working under the low light levels found at these kinds of venues. Exposure was 1/100 second at f/4 and ISO 650, with the lens at the 14mm setting. ©2008 Joe Farace
The growing popularity of Live View functions on digital SLRs may make this last point moot but so far only Olympus has gotten it right, Nikon is really trying, and Canon needs to up its game. As long as some shooters prefer to look through the lenses, fast lenses will be in demand.
Let the light in: Canon's EF 85mm f/1.2L II USM offers the widest aperture of any lens in Canon's EF family and is a useful combination of focal length, depth-of-field control, and low light performance. If you want one, you'll pay for that lack of compromise thanks to its two grand price tag.
Zoom lenses for SLRs come in two varieties: fast or slow. Many slower zoom lenses have a floating maximum f/stop where that the maximum aperture changes within the zoom range of the lens. A 24-105mm f/3.5-5.6 zoom lens is a good example. At the widest focal length of 24mm the maximum aperture is the f/3.5. As the lens is zoomed toward the telephoto end, that maximum aperture shifts to f/5.6. Fortunately there are fast zoon lenses available but again expect to pay extra for the privilege of ownership.
Peak performer: The Zuiko Digital ED 14-35mm f/2 lens uses two ED (Extra-low Dispersion) glass elements to minimize chromatic aberration and contribute to its superb optical performance. At $2,299, the ED 14-35mm f/2 SWD features rugged, high-quality construction and is dust and drip-proof.
Joe Farace is co-author of "Better Digital Available Light Photography" along with Pulitzer Prize winning photographer Barry Staver. It is published by Focal Press and is available in all the best bookstores.