Originally Posted by The1
Now on the other hand, saying you don't need fast lenses when you have a 500mm with an F4. Sorry, but that's fast for a lens that size. It's fast enough to give you fairly razor thing DOF at distance. That would probably compare to about a 100mm at 2.8 or something as far as the DOF goes. And in this case, you shelled out just to get that extra "fastness" in that lens....
At the 500mm focal length the max f-stop becomes a matter of size and weight limits. With today's technology, f/4 is about all most of us can hand hold and, in fact, most people use a tripod for these lenses.
I think that one reason to spring for "fast" lenses in the 100mm to 400mm range is so that you can throw on a 1.4 or 2.0x TC and still AF. People aren't worried about bokeh with those lenses, because it's there easily. If the quality of the bokeh is bad, then that's an issue, but most of those lenses have creamy, smooth bokeh.