Photoshop now comes with lots of “extensions” you can install – either paid or for free – you get to them from this menu item
That opens up a web browser – you need to be logged in to your Adobe account if you’re not on Creative Cloud this may not work.
So if type in “luminosity mask” like I’ve done in the screen shot – it’ll do a search… the results are like this:-
The one I’ve got is – Luminosity Masking Panel by Greg Benz
You click on that and can install it – I can’t do to screenshot it as it’s already installed, but here’s the extension info page.
Once you’ve got it installed – to use it you need to click the following menu item:-
And a small box will open up with just a few buttons – you click on “create masks” to start.
then open your CHANNELS panel to find about 20 new channels all named “lights”, “darks” and “midtones”.
To select a dark area, hold down your CTRL key and click on one of the dark channels – you’ll get that mask selected as you can just about see here
Now you can open up any Adjustment layer you like and the mask will be automatically applied – so if you wanted to make the “darks” darker, you can use curves/exposure/levels, for example. Look at the Curves 1 layer, the mask is a black and white representation of the selection. So if you do anything with the layer, the effect will only happen to the white parts, leaving the black alone.
That’s how to install and use it.
For example, if you wanted to lighten the mid-tones a bring out some detail, then you can control-click on the mid-tone channel, create a curved line and then pull the curve up in the middle of a little bit. You’ll see just the mids changing – the lights and darks stay the same.
It’s really useful and something you can’t really do so well with Lightroom.