OK, here's something that will illustrate what is going on (not that anyone here needs to
)
Copy and paste the grey bra lady into Photoshop, open a Curves layer and move the lower point to the right to reset the black point to a proper level.
Open another Curves layer above the first one, and you'll now see the histogram stretches right across the tonal range. So mess it up by moving the bottom left point up about one and a half grid squares, or until the image looked like it did originally.
Now, open up a third Curves layer and you'll see that the histogram is similar to the one in the first Curves layer. Note that the histogram is now a bit spiky, and probably has some holes in it. That's because - and everyone is way ahead of me here
- you stretched a compromised dynamic range out to fit all points from 0 to 256, but there wasn't enough information in the image to do this without a little bit of damage.
So, if you have not lost the will to live by now, repeat the entire process after going
Image > Mode > 16 bit. It's better, but not perfect because you started with a compromised image, but if you ever doubted the benefit of opening RAW files in 16 bit when you think you are going to do some heavy lifting, this should make you think again. Heavy lifting means B&W because all three channels have the same information so you have a drastically reduced bit depth, or OTT use of Topaz or Nik or Perfect Suite etc.
OK, I'm sending myself to sleep now...