Thaks Richard.

Quote: Like Tom I would be interested if there are any specific optical reasons why someone should buy the fastest lens that is available when what they want to shoot is covered by a lens that is perhaps a stop slower, given that the quality of both optics is the same ie. lens coating and glass?

That really is the subject.  I see little specific optical reasons, mostly user opinion and that is fine and good...but who compares the lenses?  Apparently some have and use the "ligher to carry" but "slower lens" and find that to do the job just fine.   

I asked the question simply because "fastest lens" is the general recommendation with little reason exactly why!   A local man who teaches digital photography and does fine work told me once the f/4 would do just fine in almost all instances. He saw my lens and was pleased with it.  He was also the only one not to immediately recommend the fastest lens right off the bat.   It all comes down to results.  

We hear hearsay and perhaps repeated "urban legends" or pride of ownership of the most costly lens...who knows but all that is off subject as to terchnical reasons "why".   Even scholars repeat the mistakes of those they use for resources...

(If my f/4 does the job just fine, why should I spend three times as much for another lens when I cannot afford it in the first place? That would be false economy and money spent which could possibly be spent on another prime.)

Thanks for the comments.          Tom.