Understanding Edges in Oil Painting, By Don Finkeldei: The degree of hardness or softness in transitions between boundaries of different  colors and values.

First, some observations about edges and the difference between using your eye to observe a scene, capturing the scene on camera, and using edges in your painting:

1. When your eye focuses on a real life object in nature, it automatically softens a lot of edges outside the focal area.  That doesn't happen in a painting.  You have to construct the edges very carefully when painting.
   
   
2. If you use a camera to take a picture of a real life image, keep in mind that the lens does not do what the eye does.  A camera's lense does not soften edges radially outside the focal area (where your you're pointing the center of the camera).  This reason is one of among several why you should paint on location rather than from a photograph.
   
   
3. Contrast between complementary colors makes your mind see a harder edge than the contrast between close colors.
   
   
4. The eye perceives a harder edge at boundaries between higher value contrast and a softer edge at boundaries between lower value contrast.
   
   
5. Objects like Trees and Clouds have mostly soft edges at their boundaries.  Generally, there's no sharp boundary in clouds because they sort of "Fuzz" out at the edges naturally.  Trees generally have "Fuzzed edges" because the leaves/limbs don't have a definite boundary, but consist of leaves and twigs that gradually and non-homogeneously "fuzz" out.
   
   
6. Round objects have very different edge transitions than flat plane objects.  Usually, round objects have very soft edges at most it's boundaries. It's a hard edge only where value or light contrasts are at a maximum.
   
   
7. A photograph has much less contrast sensitivity than the eye sees in real life.

In a painting, you will want to keep in mind "what type of edge do I need" for every stroke you place on the canvas (There's several things you should keep in mind, the type of edge you need is only one of them).  You'll have to construct the softness or hardness of edges with your brush, fingers, palette knife or any effective means.  Why?  Because the eye looking at a painting won't automatically create the edges like it would in real life.  Also, you will generally want to decide what you want the viewer to focus on and not allow their eye focus on whatever they want.  That's one of the main differences between a picture and a painting.

Edge manipulation in a painting is a valuable tool and can help you lead the viewer's eye around the painting, taking them to the focal point, making certain areas more important, making other areas less important and preventing the eye from wondering out of the painting. 

There are three broad classifications of edges.  Hard, soft and lost edges.  Lost edges can greatly enhance a painting if done correctly.  A lost edge can be describes as a visible edge that becomes indistinguishable and lost when this edge enters an area of equal values.  It may re-appear when values contrast more. The eye has a tendency to fill in the lost edge and allows you to work colors from one form into neighboring forms of equal value.  It loosens up a painting and makes it visually dynamic and interesting.

Of course, a painting that that has soft edges all over is not very interesting.  Conversely, a painting with mostly hard edges is not very interesting either.  Normally, hard edges in the focal area will help a painting.  Not too many.  A few well placed hard edges is generally better than a lot in the focal area).  A few hard edges elsewhere, placed in areas that will help you lead the eye around the painting is also a good practice.  Sometimes, a few very well placed hard edges in a sky or at the boundaries of trees and other objects can help.

A filbert brush is designed to help make a soft edge.  It has rounded corners.  A Flat brush is designed to help make a hard edge.  I also use my fingers to smudge edges.  A Palette knife is a useful tool for making very hard edges.

How do you judge the softness or hardness of an edge looking at a real life scene?  Squint a bit.  Don't squint at your painting, squint at the life scene.  Harder edges will be more distinguishable and softer edges less distinguishable when squinting.  Know that boundaries between complementary colors have a harder edge, boundaries between higher value contrast have a harder edge.  Know that round objects have softer transitions than sharp edged objects.  Think about the object --- does it transition at the edge like clouds or trees?  Know that in shadows the value contrast isn't as great so edges are generally softer.