Each image in Photoshop is made up of three separate colour channels, red, green and blue, and each of these is shown as a black and white image in the Channels Window, together with the combined colour image as RGB (Red, Green and Blue). This can be particularly effective in selecting fine detail such as hair.
Channels can be used to create masks where there is a good brightness difference between white and black in one of the channels, and for a colour image the three channels can look quite different.
While in Channels click the mask icon and it creates an Alpha Channel, which is a luminosity mask above 50% grey. You can invert this alpha channel but deselect the selection first, otherwise only the selected area is inverted. 01, Phlearn duplicates the best mask first and uses this.
The simplest mask selects everything over 50% grey and paints it as black within a white layer mask. Go to Channels and CMD + CLICK on any channel, then it will form this selection. Click on the mask (???) to add the selection. If you create an adjustment layer, then the mask will come with the selection added automatically.
CMD + SHIFT + CLICK to select highlights above 75% grey. To select the darker 50% or 25%, first select the bright one, then Invert the selection (CMD + SHIFT + I). 01,
This selection from Channels remains active when you switch back to Layers, and will be visible as a selection when you click on an image layer. If you create a Mask, then the selection will be automatically applied to the mask.
Remember that Channels show the current image details. You may find that disabling some layer masks or temporarily emphasising some aspects of the image can help you produce a better mask.
Using a Channel as a Layer Mask
Never edit the original channel as it will permanently destroy that part of your original image.
Always make a duplicate copy of a Channel first and edit the copy
To duplicate a channel drag its Layer Icon down to the Create New Channel Icon, or highlight it and click the Circle in the bottom of the toolbar.
To turn a channel into a selection click CMD + CLICK on the channel, or click the ring of dots at the bottom of the Channels Window. This always selects the lighter areas. Invert the selection with SHIFT + CMD + I.