There;s been a lot of discussion over the years about 'fast' lenses. We hear
" really need a fast lens for this situation where I don't have flash option... etc etc etc.
Think-why are lenses made at the f number selected?
Well it is a trade-off; size, weight, speed, resolution (line per) , method of focus (yes) etc.
Why would anyone ever take a shot wide open except for maybe catching a theif on a dark night?
If you make a 2.0 lens it SHOULD be better at 2.8 than a 2.8 is at 2.8 UNLESS you design it for more resolution.
Yes, Leica tends to have workable lenses at 2.8 to 5.6 whereas I don't think I would use an MF lens less than 5.6 most times. Just no DOF (Just so many "I got the nose hairs in focus, and all else is just bokeh" shots I can stand )
For one thing, the faster lens is easier to focus since you do that at full aperture, the AE stops down. And yes there are the emergencies. I always want 2 stops head room in a lens-if I buy a 1.4 over a 2.8 its because I want to shoot f4 instead of f5.6 or f8.
Some viewfinders just don't work over 5.6 (Brightened screens will go dark on one side on a prism at 5.6 and over)
So again, when judging lenses maybe it is best to compare 'real world" f stop- where we are likely to shoot. The comparison's of lenses at wide open is technically interesting , but that is not to say another lens doesn't 'overtake' in some quality stopped down.
I can't think of a wide open shot that I EVER used; but then again, I am not a pro. Maybe a pro knows how to usre it. Maybe the 'mood' shot at a wedding, etc. I can go for that.
Wre mere mortals can barely keep the focus NEAR the subject, much less deal with 2-3" DOF.