Normally when we observe someone's eyes or some particular object, we take in the whole while we put our attention on that particular part. We notice with clarity whatever we focus on. The rest of the surroundings are slightly out of focus. Look at someone's eyes while you are talking to them and notice how fuzzy their shoulders are. Now, look down at one of the shoulders and notice how the face turns into a blur. The human eye sees one thing in focus at a time.
If you paint one area or point in focus and leave the rest of the painting slightly out of focus, you will find the resulting painting looks more realistic than if you were to paint in everything the way you see it when looking directly at each part. If you paint all the edges hard and crisp because they appear that way when you look at each part of the subject patter, you will not end up with a painting that looks as real as you might expect. Instead, it will look almost surreal, or too real.
There is only one part of the subject matter that we can look at with that kind of clarity at any given time. Decide what this painting of yours is all about, make that your focal area or point, and remember to paint the rest of the painting with less clarity and focus. When painting, we are creating illusions, not real things. We are painting on a two-dimensional, flat surface—but creating illusions of three-dimensions. We are creating the idea of reality, not reality itself. Tricks with line, tone and color create convincing illusions. But illusions are not the same thing as the real thing.
Everything in a painting should not be painted with equal attention. The more carefully developed areas will stand out when surrounded by areas that are less developed. Soft edges enhance the harder ones. Build relationships among the parts that enhance the effectiveness of the whole. If you were looking at two similar objects that were at different distances from one another, it would serve your painting better to paint the object in the distance with less clarity, softer edges and fewer tones than the object that is closest to you—regardless of the fact that you can see them both clearly.