While I understand what the numbers mean, I have one nagging question that keeps me awake at night. It comes into comparison of fast lenses for indoor sports. I've been looking at f1.8 and f2 and f2.8 lenses, all of which seem to be fast. However, my question is when I step these lenses down. If I step an f2 lens down to f2.8, does it let through the same amount of light as an f2.8 lens wide open? Or would the f2 lens still be faster at f2.8 than the f2.8 lens. My assumption would they would be the same as far as speed, but again I keep thinking I might be wrong???