Light is the first Element of creating Form in classic art. True gradation and Value differences is the second step.
1. The first step in creating your Grisaille painting is to work in Adobe Photoshop CS6 on a simple fruit photograph to make your high key, middle key and low key value examples. Start by opening your fruit photograph in Photoshop and changing the mode to gray-scale, then the size to 4 x 3 inches.
2. Pull down image to adjust and Posterize, don't go any higher in levels than what looks simple to paint!