3D depth for quick thumbnails

When it comes to sketching thumbnails and practising values, a 'depth pass' from a 3D renderer is probably the most practical way of seeing the change in value based on the distance of the objects (dark values close to the viewer and light values are far away).

I set up a quick scene with a few cubes and a character (for scale reference) in Blender, but you can do the same thing with some basic geometry in any software you want.  The advantage of using Blender is that you can use Eevee and visualise the 'depth map' in real-time.