When prints block up the shadows, but there clearly are details to be found. Tony Kuyper suggested that a luminosity mask would help. Follow the link for more details.
- Create a Darks-1 luminosity mask of the unsharpened image.
- Sharpen the image using the normal sharpening method.
- Create a Levels (or Curves) adjustment layer at the top of the layer stack above the sharpening layer(s).
- Set the adjustment layer’s blend mode to Screen.
- Apply the Darks-1 luminosity mask created in Step 1 as a layer mask to the adjustment layer.
- Lower the layer’s opacity to between 25 and 50 percent.”
Tony Kuyper writing for Good Light Journal: