When considering the values of an image, we divide them into two camps, light and shadow, that which is directly receiving light and that which is not. Halftones are part of the light family, but are receiving less direct light because the surface of the object is starting to angle away from the direct light. A shadow does not implicitly imply a complete lack of light, as it might be lightened to varying degrees by indirect or reflected light. With these seemingly obvious basics we can then grasp of the subtler little details of lighting and shadows that create the illusion of our visual reality.
For a limited time and for a limited number of people, I am offering a free critique of one artwork to help start pointing you in the right direction. Become a member on my site to take advantage of this offer. Details of how the critique will be done, will be emailed to you after you register.