greetings!
very interesting, I tend to use the other end of the aperture value to create a deep depth of field - then again i rarely use my wideangle for anything other than landscapes - unless there are ppl in it.
The camera ken duncan uses - linhof technorama 617s has a lens from f5.6-64 (technorama Apo-symmar 5.6/180). The other lenses available on this system are f5.6-45. He shoots everything up in this high area, and also on fuji veliva - ISO 50. - hence the need for a tripod - he uses a ball head, again i think its a linhof.
My old 15mm ais lens is f3.5, but it blurs the edges of the photo unless its aperture is over f8 - f11 seems to be its sweet spot. I assume the fisheye has this same drawback?
Most lenses have a max f value of f22. f2.8 is the fastest i have on 20mm ais, but i would rarely shoot down this end.
All depends on your application really. For landscapes id use larger f stops.
keep on learning!
JD