Please dont beat me up too bad, but I have a newbie question:
I'll admit most of this thread is WAAAAY over my head, but I'm studying DSLRs for my next big purchase, and I pick out little things to try to understand as I see them. How can a lens be fast???

Can anyone explain that in dummy terms?
Just a warning: there's a bunch of logic to follow along (ie. if a, then b, then c, etc). But in the end,
larger aperture lenses = smaller f-number = faster lenses.
You probably already know this:
the smaller the f-number, the larger the aperture. Also, the aperture is the opening of the lens.
When you have a large aperture, you let more light into the camera. Letting more light into the camera allows you to use faster shutter speeds. That's why large-aperture lenses are also known as "fast lenses." Sports photographers specifically look for fast lenses.
Here are some made-up numbers. Let's say your kid's playing soccer outside, and it's late afternoon / early evening. Let's say that the scene in front of you requires a shutter speed of 1/30 and an aperture of f/5.6. That's a pretty slow shutter speed for sports. If your lens' maximum aperture is f/5.6, then you're gonna be cursed with blurry images during the game, unless you increase the ISO. Thus, a lens with a maximum aperture of f/5.6 would be considered a "slow lens".
If you've been reading about exposure, you'll know that there are equivalent settings you can use to achieve the same exposure. If you use a larger aperture (letting more light into the camera), you can use a faster shutter speed to achieve the same exposure. Conversely, if you use a smaller aperture (letting less light into the camera), you'll have to use a slower shutter speed to achieve the same exposure.
So in the above made-up example, equivalent exposures could be achieved with the following settings:
Code:
Shutter Speed: Aperture:
1/30 f/5.6
1/60 f/4
1/125 f/2.8
1/250 f/2
1/500 f/1.4
As you can see, if you have a lens that has a large maximum aperture, you can use faster shutter speeds. So the 50mm f/1.4 lens would be considered a "fast lens" because you can use larger apertures and, thus, faster shutter speeds.
Like I said earlier, there's a lot of logic involved. Hope that all made some sense.