Posterizing With Adj Layers
Photoshop CS6 Tutorial
How to Posterize with Adjustment Layers in Photoshop
To posterize an image, start with a continuous tone photograph. The process converts your image to a less complex image with fewer shapes and colors.
Open a photograph in Photoshop (File > Open).
Duplicate the image (Image > Duplicate) and close the original.
With your image on the Background layer, add a Black & White adjustment layer. Adjust the sliders in the Properties panel to enhance the grayscale image contrast.
Add a Posterize layer above the Black & White adjustment layer and adjust the slider in the Properties panel.
To create a layer with all your work to this point, select the top layer (layer three) and add a new blank layer (click the Create New Layer icon or press Cmd/Opt/Shift/n (PC-Ctrl/Alt/Shift/n).
To fill your layer with the contents of the lower layers, select the empty layer and press Cmd/Opt/Shift/e (PC-Ctrl/Alt/Shift/e).
Duplicate the Background layer (drag the layer to the Create New Layer icon or Cmd/j (PC-Ctrl/ j). Drag the new copy so it is just below the top layer.
Set the top (grayscale) layer Mode to Luminosity and adjust the Opacity if necessary.