An accomplished painter knows how to manage edges effectively.
Edges occur wherever shapes meet.
They can be hard or soft, lost or found.
Hard edges are clean and crisp and draw attention. They can be the center of interest as shown by the hard edges in the face in this portrait drawing by Ingres.
The "less important" edges of the clothing and chair at the bottom are soft and do not draw the eye away from the face.
Soft edges can help integrate a background and foreground objects. Where does the hair end and the background begin in this portrait by Ingres?
Edges get lost when adjacent shapes are similar in color or value, i.e., the boundary between the coat collar and the portfolio "get lost" in this portrait by Ingres.
Other edges of the collar and portfolio can be "found" again when it becomes dissimilar in color or value. Can you see how the manipulation of the edges here is a huge compositional element that makes this painting so successful?
My apologies to Vermeer for writing on his painting of "The Milkmaid." But in each of Vermeer's paintings, you can so clearly see how his hard edges "spell it all out." and his soft edges merely - but beautifully - "suggest."