50 1.7 is sharper, but the 50 1.4 is better.
Wrap your head around that one
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PS - sharpness isn't very useful unless you are photographing newsprint.
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Enough 'tude
. The 1.4 is pretty sharp wide open, and while it is not AS sharp as the 1.7, it has a nicer rendering (softer backgrounds), nicer colour, and *IS* sharper than the 1.7 when you stop it down to f5.6.
The only thing the 1.4 is worse at is controlling flare and CA. My solution is to use a hood and stop pixel peeping.
Really though, since the 1.4 and 1.7 commonly sell for the same price these days, just get the 1.4. The 1.7 is only better if you ONLY shoot at f1.7 and need very high sharpness in low light. The 1.4 is better all around (and I also prefer it for low-light, since it gives you 50% more light).