All the super-fast lenses are only that fast in the very center of the image. If you go just slightly to the corners wide open you realize that a 1.4 rated lens for example is mostly a 2.8 lens or so, except in the very center as I've said... All due to heavy vignetting. I'd give a thousand bucks more if the FF corners of modern super-fast lenses would be a bit brighter.
I do landscape-astrophotography. When you brighten up darker corners, you also raise the image-noise in that area to a certain degree. Or you can stop down a super fast lens to f2 or f2.8 to even out the incoming light and reduce vignetting, but then f2 and beyond isn't really "fast" to me anymore.
For portraits, this doesn't matter as some people like the effect of "framing" the subject and the fact that you are not shooting in the dark using high ISO. At daytime, vignetting is absolutely no problem because when you want to correct it, it doesn't affect the IQ at all and you won't notice it.
Other thing what's important to me is coma. That's also something you won't notice at daytime. But in the dark this can be very annoying esp. if you have city lights or stars in your frame. Still, most modern primes don't have appropriate coma correction like the EF 35 IS USM for example.
So, I wish, at least in the L-series, that Canon is giving me as good coma correction wide open as the Samyang lenses and a little less vignetting.
You're right of course. Easy to get carried away with the technical aspects of lenses (or cameras) and lose sight of the fact that in the end, what counts is whether you like the images. Still, colour fringing is not helpful. That said, I mostly use the S50 indoors / in low light these days, and I don't *think* the colour fringing is too apparent in those circumstances so I don't think it is that big a deal for me ... although I'll certainly be keeping a closer eye on it now!
