how sharp is sharp enough?

It looks like the focus is spot on. Not sure, but the image may appear to be soft due to:

- The 50mm f/1.4 lens at f/1.4. This is a really decent lens at f/1.4, but it is definitey sharper stopped down a bit. Try your test again at f/2.8 and see if there is a difference.

- The image that you posted is very underexposed, which gives an impression that the image quality is poor when you first look at it. It's still a tad soft looking when you correct the exposure using levels (shadows and highlights), however much better.

- Using in-camera sharpening versus applying USM in PS may make a bit of a difference, so may be worth a try.

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