Really, this might be the dumbest question I have presented on the forum. Littlejock's thread on finding a tele-zoom is the reason for this question.

Local sports photogs and wildlife photogs, those who use long lenses, all tell me to get the fastest lens I can afford.  So the question is:

Does an f/2.8 tele at f/4 take a cleaner pic than an f/4 lens at f/4?  Sure, dumb question. But honestly, most images I see from these folks are not taken at f/2.8 but at higher aperture (smaller opening, larger number) .  The pics are clean and nice...is that from the capibility of the lens?

Is the f/4 lens straining optics at f/4 and the f/2.8 lens is not? Would the f/2.8 have better optics(and larger glass) and that somehow make the recommendation of "get the fastest lens you can" valid?  Is fast glass only valid if you shoot at wide aperture?

I am really curious about this from those who do have the really fine glass on their cameras.

Thanks for any input.  I am totally at a loss as to what responses might be.   There has to be a reason for recommending the fastest glass.

    TomD'Art.