From Kevin MacPherson’s Landscape Painting Inside & Out:
WHAT EDGES DO IN A PAINTING
Edges suggest the quality of atmosphere, as when distant or fog-obscured objects look softer, or close-up, brightly lit objects look sharper
Varying your edges give your viewer areas of intrigue and mystery.
Edges direct the viewer to the focal point. When you look at a scene with your eyes, only the area you’re looking at directly is in focus, everything in your peripheral vision is out of focus. Use edges in your paintings the same way.
Edges suggest the physical properties of objects. The side of a house will be harder-edged than the rounded form of a bush.
WAYS TO MANIPULATE EDGES
WITH CONTRAST: To make one edge appear softer, you can make another one harder, or vice-versa. The more contrast between shapes (contrast of value or color contrast) will give the illusion of a harder edge. Reducing the contrast will soften the effect. Adjacent shapes of similar value or color will create a softer appearance, even if the edges themselves are crisp. A very contrasting value shape or very contrasting complementary color from shape to shape will appear hard-edged due to the natural contrast, even if the edges are quite soft.
WITH THE APPLICATION OF PAINT: You can physically soften an edge with a brush or even a finger. A palette knife can create a razor-sharp edge.
WITH A TRANSITIONAL SHAPE: If you want to soften an edge, you can mix up a pool of each shape color and mix them together. Then lay a stroke in between the shapes, (this) will make the transition between them more gradual.
Chapter 3 – “The Theory of Relativity”
Here’s a great article on painting edges from ArtTutor with step-by-step pictures.