Normally when we observe someone's eyes or some particular object, we take in the whole while we put our attention on a 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 matter, you will not end up with a painting that looks as real as you might expect.
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 -
notice where the focal area or point is, and remember to paint the rest of the painting with less clarity and focus.
When painting, we are creating illusions of reality. We are painting on a two-dimensional, flat surface but creating illusions of three-dimensions. We can create convincing illusions. However, illusions are not the same thing the three dimensional reality. Knowing what to put in and what to leave out is a big part of making a painting work. The relationships of the parts in service to the whole enhances the effectiveness of the whole. Ideally a painting creates one whole singular powerful communication a viewer can take in all at once.