 BetterPhoto Member |
Why are Lenses Referred to as 'Faster'? Sometimes I hear people say, a lens that is f/5.6 or faster? I thought that the f/5.6 just refers to the max aperture of the lens... how does "speed" factor into the equation?
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Ken Pang |
A lens is referred to as faster, if the maximum aperture is larger. I'd be taking a guess, but it's called faster, due to the fact that the exposure time can be much shorter with larger apertures. For example, given any particular lighting situation, the exposure needed for a f/5.6 lens is four times that of a f/2.8. So if exposure is normally 4 seconds on f/5.6, it would be 1 second on a f/2.8 - hence, faster.
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Mark A. Braxton |
That is one reason Ken. But the major reason is that your autofocus uses light just like your exposure meter does. So the more light you let in the faster the lens can focus. For sports action, for instance. The larger aperture will not only give you a faster shutter speed but, also it can focus on your subject faster.
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John A. Lind |
Ken guessed right! The reason is very historical for exactly the example Ken gave. A faster lens with larger maximum aperture diameter allows a faster exposure time. It has nothing to do with focusing the lens. The jargon of "fast" and "slow" applied to lenses predates auto-focus systems by almost a century. It dates from the late 1800's at a time when shutters didn't exist (you used a lens cap), large glass plates were used for film, exposures were easily 10 seconds and longer, lens apertures were called "Waterhouse stops" (thin metal plates with different diameter holes drilled in them), and the thought of an "auto-focus" camera would have been the stuff of a Jules Verne novel. -- John
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