Originally Posted by sotorious
Where is the specs do i look for it to be fast? i cant see it in the specs where it says aperture numbers.
Also those bigggg white lenses are those fast? the ones that are anywhere from 2k to 10k and above.
They're in the name itself. Take for example, the kit lens:
Canon EF-S 18-55mm f/3.5-5.6 IS
f/3.5-5.6 is the maximum aperture. This means that the aperture will change at different focal lengths, with the maximum aperture being f/3.5 at the wider focal lengths and f/5.6 at the narrower focal lengths, with the aperture being in between at the middle focal lengths. This is standard for a zoom lens.
Now another lens:
Canon EF-S 17-55mm f/2.8 IS USM
f/2.8 is the maximum aperture here. This means that there's a constant f/2.8 aperture regardless of the focal length used for this lens. f/2.8 is considered fast for a zoom lens, especially at the telephoto focal lengths.
Canon EF 50mm f/1.8
f/1.8 is the maximum aperture here. Unlike the other two, this is a prime lens, not a zoom lens. As a whole, prime lenses usually are faster than their zoom counterparts. So even though f/2.8 is considered fast for a zoom lens, it's normal to see prime lenses at f/1.8 or f/2, with some going to f/1.4 or even f/1.2.
As for those "white" telephoto lenses you mentioned, that's Canon's L-series telephoto. L-series lenses have full weather sealing (except for the 70-200 non-IS models), superior optics, and are very fast lenses. With those telephoto lenses, you can find stuff like 70-200 f/2.8, or even a 300mm f/2.8 lens.
However, if you're thinking about buying those lenses, I'd think carefully about it first. I'm not trying to sound like a jerk, but these questions are all things an experienced photographer should already know, and it's the experienced photographer that would get the most out of those white lenses. Without the proper know-how to handle L-lenses, there's probably not going to be a difference between them or a normal telephoto lens like the much cheaper 55-250mm.