What is a 'fast' lens?

When people talk about a fast lens, they are talking about how much light the lens can let in for a given amount of exposure time. The two images at right were both taken with an exposure time of 1/8 second. As you can see, one of them is much brighter than the other.

The 'brightness' or 'speed' of a lens is given as its maximum aperture, usually written as 'F/' followed by a number. The lower the number, the faster the lens. The images at right were taken with an F/3.5 lens (the darker image) and an F/2.8 lens (the brighter image).

The way a camera lens is made brighter, or allows more light in, is to use a larger piece of glass. This has two consequences - first, grinding a larger piece of glass to the exact specs required of a camera lens costs more, and thus faster lenses tend to cost more than slower ones. Second, a larger piece of glass (or more accurately, several larger pieces of glass used to construct a lens) weigh more. So while a faster lens may be better for letting light in, it will also be larger. In the case of a longer lens, like a 300mm you might use to photograph distant objects, this can result in dramatically heavier lenses (a very fast 300mm F/2 weighed in at over 15 pounds - as compared to about one or two pounds for a typical 70-300mm f/5.6).

Generally speaking, a 'prime' lens, a fixed focal length lens, will be faster than a zoom lens at any particular price point. Canon has an excellent example of this in their 50mm F/1.8 lens, which can be had for under $100 US dollars. As above, the lower number means the lens allows more light in for a given exposure length. At right, the Tamron F/2.8, shown at its maximum zoom of 50mm, is now the darker exposure, while the Canon prime 50mm, with its maximum aperture of F/1.8, allows in much more light.

It's important to note this overview omits a lot - for example, not all lenses are created equally, and some will have much better resolution than another lens that looks equal on paper. Even the same lens produced at different times will have different qualities (and, of course, manufacturing goofs can make individual lenses better or worse). In general, a fast lens is not a requirement, and in some cases (bright light, for example) is totally unnecessary. If you've started with the kit lens or lenses that came with your DSLR, consider starting with your camera manufacturer's cheap prime. Not only is it a very different experience shooting with a prime, but it will also help you get a feel for why you might, or might not, want to start investing in faster, more expensive lenses.

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