Most of my art teachers wanted me to add more contrast to my work. In graphic design the standard workflow for images adds some contrast, vibrancy and sharpness. A digital image is somewhat flat without being sharpened but vibrancy and contrast are more in the eye of the beholder. Sharpening is a technical requirement due to the printing process. Stochastic printing has reduced this requirement so you might not notice it with a home inkjet printer.
I’ve noticed that focusing my attention on slight contrast differences in the midtones often make the painting more lifelike. I think that a human’s ability to see tonal differences in nature is quite different than in a photo. Our eyes have the ability to see very fine shades of green but less so with blue and red. This may be a characteristic of our eyes or it may have more to do with our brain processing the information. Regardless I’ve noticed that the realistic look of a painting is often enhanced by careful attention to variation in the midtones. This doesn’t mean that the painting will look flat. Bright highlights and dark areas still need to be added but midtone variations are essential.
I first block-in the shadows and highlights. Then I start working in the midtones by adding detail that I’ve missed with the first iteration. Midtone details often impart a realistic feel to the painting. This may only be noticeable when I look quickly at the painting or go and have lunch before looking at it again. At this point adding bright highlights and/or dark shadows will usually make the painting pop; if not I start over with blocking in.
Now any details I add with highlights and shadows usually enhance the realistic look. I pick a location and add detail until I can no longer see anywhere to add detail or I am really tired of it. I’m usually thinking about a new project at this point.