SteveR wrote:
That is a rather perfunctory explanation as to WHY a lens is fast. A lens of f2.8 or wider (i.e. f1.8, f1.4, etc.) is considered "fast" because a lens will focus at it's widest aperture. The more light the lens takes in when focusing, the faster it can focus. After focusing, the lens will shut down to the aperture selected to make the shot.
I was hoping the person would go to google and see all of the optional answers to his/her questions rather than to go into a detailed and personal view description of their questions. Research is better than a multitude of opinions, in my opinion.