Whether you’re painting a portrait, still life, landscape or anything else, it’s important to work the entire canvas. Too often I see painters taking one area of interest to them all the way to finish, before working the rest of the canvas. To be blunt, that really isn’t PAINTING, it’s rendering.
Painting is playing one area of your canvas against the other, letting it tell you what elements in the composition need to be darker or lighter, warmer or cooler, contain more detail, be handled more loosely, etc.
Juxtaposition is a fickle mistress. You work diligently on an isolated area until you’ve adjusted the value of a particular dark to perfection, only to have your hard work negated, once you stroke in the surrounding values, which of course, changes everything. The flesh tones you’ve achieved on the live model you’re painting down the center of your, otherwise, pristine white canvas are the best you’ve ever painted, but when you add the cool background surrounding her, she’s suddenly suffering from a severe sunburn.
This is due to the dynamic range, or selected range of values from light to dark or colors from warm to cool, in your subject. No given subject contains all values or all colors in the spectrum. Each subject encompasses a subset of the entire spectrum. A color or value is right only in comparison to the other colors and values in the particular dynamic range of your subject. That dark gray is perfect sitting next to the pale yellow of the dress in your painting, but choose a different color/value for the dress, say a darker yellow ochre and now the gray is too light. Every color or value is effected by the colors or values surrounding it. This is why a color may look perfect isolated on your mixing palette, but looks wrong when applied to your, in progress, painting.
When you develop the entire canvas, working dark to light, you avoid this problem. Utilizing this approach, colors/values are chosen based on the whole of your painting.
You also want to avoid developing detail in any isolated area of your canvas, while you ignore the rest. I like to compare the developing of detail on a canvas to a sculptor working a block of marble. The sculptor begins with large chisels, knocking off gross chunks of the block, roughing out the overall shape of her subject. Then, slowly working towards a more refined finish, she employs smaller and smaller tools, as she goes.
I approach painting the same way. In the beginning, utilizing large brushes, broad strokes across the entire canvas, roughing in the masses, then slowly refining the whole, as I progress, relying on smaller and smaller brushes, as I go. This approach makes clear to me, the areas screaming for detail and those that can left in rougher states. You want more detail in the areas of focus in your work, less everywhere else.
I’ve found this historically time-tested, all encompassing approach to be the most efficient, consistently achieving the best results. If you’re not already working this way, give it a shot. It’s just paint, what have you got to lose!