[Kunzite]

Why f/2?
They do have a D FA large aperture standard single focus lens on the roadmap.

[Nicolas06]

Look carefully at, do some research upon the Pentax 50mm f1.7. In almost all versions going back into the film era, it has been regular rated as the best overall ~50mm lens made by Pentax. Only 1/2 stop slower (how often do you expect to be using f1.4? how often do you need less DOF or 1/2 step higher shutter speed?) and significantly less expensive, maybe 1/4 the price of the f1.4. High speed lenses were once all the rage, the faster the better: f1.4, f1.2! f1.1!!! f0.95!!!!!!!), but you pay a great deal to get 1/2 to one stop more, and that stop almost always has the worst IQ.

Both the DA*55, FA50 f/1.4, FA50 f1/.7 and DA50 f/1.8 are using a variation of the old 50mm lenses design. Soft wide open, insanely sharp closed down. Even the FAltd share this aspect (but at different focal length).

Some modern design are much sharper wide open like the sigma Art... If f/2.8 is fast enough for you the DFA50 macro will blow away all the other for sharpness. There no contest.

What may be indeed more important than pure sharpness, is the actual lens rendering. How pleasing it is, the bokeh it has and so on.

Lenstip reviewed the FA50 f/1.4 as significantly better at f/2 than the FA77. But I got both lenses and I can say I never liked the FA50 and I love the FA77. It worked better for me at the same apperture. Maybe it wasn't better on the bench, but the photo looked much better. (And funilly it is sharper than the sigma art once closed down).

I think when you buy a 50mm lens for 1500€ at f/1.4, one of the FAltds or a DA*55 that 2 time the FA50 f/1.4 and 4 time the FA50 f/1.7 you may have purely apperture and sharpness in mind. But what you really get first and foremost is the lens that take actually better looking pictures.

And that's what really count. Not that the FA50 f/1.7 is not great. It is, really. But there more than appertures and border sharpness to expensive lenses.

[bxf]

I had to go through the same question some years back. Since I was buying a fast lens primarily for use in limited light conditions, I put much weight on the wide-open performance of the lenses, and this is where the Sigma is superior to the DA*55, according to reviews I read at the time. The DA*55 seems lacking wide-open, and although it's superior to the SIgma at some settings, the Sigma is plenty good at those settings anyway.