So…edges! This is the important place in which two values meet each other in a painting. With this in mind, you can see in any painting that there are a multitude of edges. Most are very "soft" and the hues seem to blend into each other gently. This is easy on the eye. It's like the artist is saying, "there's nothing to see here; move along." (of course, in an Obi Wan Kenobi voice) The planes of a barn, the copse of trees, the petals of a rose all benefit from soft, blurred edges in an impressionistic representational painting.
There are edges so soft that they are called "lost edges." You might see this in the side of a house that disappears into the shadows or the edges of trees that blend into one another. This treatment gives a painting another dose of perspective, since the lost edges indicate distance and depth.
Here's another exercise for you: pick an object, a vase or lamp, something large enough to really focus on. Look at it steadily but begin to add the awareness of the surroundings. Without looking away from the vase you will see that the objects around it are out of focus…blurred. Soft edges. If you now shift your focus to something near the vase, that object now becomes clearer and the vase you had been looking at has the soft edges.
The clear object, the focal point, is best represented in a painting with the crispest edges, often only on one side or in one small area. These are called "hard edges." You will rarely see a hard edge on a soft subject like a face or baby or tomato. That would make the soft thing look like it was brittle or malformed. Some artists never use a completely hard edge; everything in their paintings are in varied degrees of softness. But a well-placed hard edge on a focal point does helps to tell the story of what the painting is about. It attracts the eye and says, "look here!" to the viewer.
When I paint now, thinking about edges, values and temperature are on my mind and, often, on my tongue as I talk myself through an afternoon's session. And, yes, I do talk to myself…a lot. A very well respected art teacher says it's a good thing!