Threshold lines
I'll be turning a full color painting of big George Washington into 3 distinct shades of black.
Create a new document (atleast 300dpi if possible), and paste in Big G to the size you want. You may lose detail making it larger, but that's fine...
Create two new layers with the same image. One way to create a duplicate of an existing layer is to drag that layer to the new layer icon.
Hide the new layers (click on the eye to the left of it). Select the bottom layer and let's find a threshold to start with: Image > Adjust > Threshold. This will be the base black so slide the bar left or right to get the foundation you're looking for.
Now select the layer above the bottom one and click the eye again to reveal it. Before we find another threshold for this, change the opacity of the level to 50%. This will soon enable the standard threshold black to become half of itself as a medium shade of grey. Slide the threshold bar to reveal more of the image... whatever you deem as the midpoint of the image.
Last, reveal the last, top layer, and change the opacity of this level to 25%. Now pick the last threshold to your liking... obviously reveal more to complete the three shades of black.
Now we have a guide to help understand the lay of both prominent and obscure facial features. It's a powerful tool, so do with it what you will!
Nothing is a substitute for life drawing or honing those skills. This is just another tool to assist.
*Quick addendum: before you run the threshold, if you put a Gaussian blur on the original image then your final lines/shades will be rounded and a bit more stylized. Also if you wish to add more levels, just create more layers and gradually decrease the opacity correspondingly.
