mfogiel: Really lovely examples you have there.
Back when I shot with nothing but SLRs, I had my own "SFFL rule" (slow film, fast lenses). For a good while, none of my lenses, primes or zooms, were slower than f/2.8, and I generally avoided any and all films over ISO 100, with good reason at the time. This went on well after film grain in ISO 400 color film (neg film at least) had largely been brought under control by Fuji and, a little later on, Kodak. The images, to a point, benefited: my back, alas, did not.
Then, six years ago, I experienced something of a sea-change, dumped the SLRs and went full fathom five into RFs. By that time, high-speed films such as Fuji Pro 800 showed amazing advances in grain reduction and overall image quality (it's not just about grain), and ISO 400 color film, both neg and slide, were good enough to be an everyday staple for a lot of my work. Combine that with the welcome absence of SLR-borne mirror-slap vibration, and, suddenly, even without f/1.2-1.4 "night vision" glass, I felt as if I'd picked up an extra stop in overall terms. My slowest lenses are still f/2.8, but my fastest lens, a 50 f/2 M-Hexanon, seems super-fast compared to, say, the 85 f/1.4 Nikkor (manual focus) on my long-ago F3. (Not knocking that combo: I took a lot of killer 'chromes with it, but that was then...)
F/2.8 usually does the job; f/2 simply shines for me, even in the dark.
- Barrett