Fast Lenses—Shutter Speeds / Background Blur (Bokeh)
A “fast” lens has a large maximum “f” number (small numerically) eg f/1.2, f/1.4, f/1.8, f/2. A large aperture allows higher shutter speeds because it delivers a much brighter image.
A really fast lens is one like the Leica 50mm f/0.95 Noctilux, reviewed as part of my Leica M9 review in DAP.
Blurring the background is a big advantage of a fast lens. It allows the subject to stand out. See this page.
Fast shutter speeds
For the image below, a typical consumer zoom lens would have required 1/5 second at ~f/5 instead of the 1/60 second made possible by f/1.4. That’s one major advantage of a fast lens.
Shooting this scene under heavy foliage late in the day, the “fast” f/1.4 maximum aperture of the Zeiss ZE 85mm f/1.4 Planar came in handy at (1/60 sec).
Depth of field
Of course, there is no free lunch. Using a lens with a f/1.4 aperture at f/1.4 presents certain challenges in getting accurate focus, not to mention subjects moving at the same time. I shot a 2nd frame where I nailed the focus on the gander’s eye at left, and with the head turned just the best angle into the light—but the other ducks did not line up as directed.
Boy, that eagle would enjoy one of those fat city ducks...!