So what does fast mean?
Fast, when talking about a lens, actually refers to the greatest amount of light the lens will let in.
Think of it this way…
If your camera needs more light to hit the digital sensor in order to make a correctly exposed image, then it has three choices:
1) It can open up your aperture and let more light in through the lens…
2) It can slow down your shutter speed and let the light that’s coming in from your lens sit on the digital sensor for a longer amount of time…
3) It can speed up the “film speed” or ISO on digital cameras.
So if your camera measures the light in the room and thinks it needs an aperture reading of 1.4 but your lens only opens to 5.6, it’ll slow your shutter speed to get the shot. And a slow shutter speed — if you’re hand holding your camera — will give you a blurry image.
Most photographers consider a fast lens to be one that will open up to at least f-2.8.
That’s a lot more light than what I can get with my (broken) f3.5 lens. (Remember, 1.0 is the largest aperture setting you can get. So 2.8 will open up a lot more than 3.5 — the smaller the f-stop number, the larger the opening on the lens.)
And an even faster lens would be one that can open up to f-1.4. (To get f-1.4, though, you’d have to give up the zoom feature and use a “fixed focal length” lens. You won’t be able to zoom but a lens like that would let you hand hold the camera even in a dim restaurant.
Consider getting a 24-70mm 2.8 lens. That’s a lens with range from moderate wide angle to moderate telephoto (great for street shooting in Paris) and a larger maximum aperture (the f-stop, or the 2.8 number) for shooting in dim restaurants and cafes.
Now, is that “fast” lens an absolute necessity for getting great shots in a restaurant or café?
No, not necessarily. Something faster than what I already have would be a nice addition to my toolkit for portraits and shooting indoors. But I don’t “need” it.
Here, for example, are a few other ways you can get better shots in low-light — without buying a faster lens…
- Use a tripod, which would allow you to take the picture with a slow shutter speed without worrying too much about camera shake. (If you don’t have a tripod, you can try bracing my camera against something sturdy like a table, wall, or light post.)
- Experiment with different ISO settings that will allow light to sit on the digital sensor longer without slowing the shutter speed.