The fact is, we spend all of our time with most of our view out of focus, but we tend to paint as if everything is sharp and hard-edged. That's because whatever we are about to paint gets our best attention, which means it is brought into sharp focus. No wonder so many paintings suffer from too many hard edges. The gods are toying with us.
Most of us are old enough to remember when cameras required that we decide what was going to be in focus. That is a lot like what we need to do with our realist paintings. One way or another, we need to separate shapes from each other to suggest that there is space between them. Contrasting hard and soft edges is a very effective way to do this, if we remember that what we see when we focus on a particular shape or area is not necessarily how we should paint it.
This placement of hard and soft edges is obvious in its purpose, and relatively traditional. The foreground is in focus and the background is not. The result is easy to understand. This is not to say that every painting should be organized like this. Just because a shape is in the foreground doesn't mean it should be in focus. You have to make a decision based on your main purpose.
The profile edges of each of the major shapes in this scene are all hard, but most of what happens within the shapes is soft-edged. This makes it possible to see the shapes as one thing while suggesting that the surfaces are lively and complex. Hard edges describe separation, like the mill buildings silhouetted against the sky, and distinct transitions, like where planes change which direction they face. Soft edges describe areas that are attached to each other, and subtle transitions, like where a rounded shape turns toward the light.
Here are a couple of images that display a clear separation between fore ground and background. See what happens when you make hard and soft edges to focus the viewer's attention.