Semi-transparent “real” shadows in Photoshop

[This is an ancient draft note I forgot to publish. Had to dig it out for a current task so figured it was time to press go.]

Struck upon a good way to isolate a semi-transparent, real shadow in Photoshop CC when trying to remove the background of an image. This can be appropriate for things such as package or portfolio shots.

This technique seems to work well so long as the shadow is against a fairly even background. An example of where I’ve found this useful is with some scans of booklet spreads. A false shadow wouldn’t look right because it wouldn’t have the variable character of the original shadow.

Draw path around object throwing the shadow (omitting the shadow).
Copy/paste object on to new layer. cmd + J
Select layer w/ full image.
Use Color Range to select the shadows. Switch the dropdown to “Shadows”. The fuzziness and range will really depend on your image, so start in the middle for each and it out a few times to see what works best. Usually fuzziness = 100% and range = 70 works well for me.
With the shadows selected, create a new empty layer above the full image layer.
In the new empty layer, fill the selection with black or a similar dark colour.
Turn off the full image layer visibility to remove the background, and adjust the opacity of the shadow layer. I found that 80% works well.

The downside of this technique is that the replicated shadow ends up being a flat colour and is missing the natural hue nuances, but overall this worked well for my purposes.