My family and I were at the health & beauty section of our local Target one time and I was observing the pictures of the models -- specifically, the catchlight on their eyes to try to learn something about the lighting of the pictures. I noticed that in almost all cases, the light source (usually an umbrella) wasn't facing the subject exactly but was offset somewhat. I tried to imagine the position of the umbrellas based on the catchlight, and then it suddenly dawned on me that they were feathering the light.
Feathering means aiming a large light source tangentially to (instead of directly at) the subject. The effect is to create a smoother transition between highlight and shadow. Check out the comparison shots between feathered and non-feathered light here (note the darker shadow in the non-feathered sample shot):
Some photographers have stated that feathering works because the the light is "softer" at the edge. With all due respect I disagree. To my understanding, the feathering effect is from more light reaching the shadow area. The lower contrast (due to the lighter shadow) creates the illusion of having softer light.
FEATHERING EFFECT OF DIFFERENT LIGHT SOURCES
Light sources with different shapes will have different feathering effects. A shoot-through umbrella, because of its convex surface, will have a softer/smoother gradation (lower contrast between highlight and shadow). Conversely, a reflective umbrella with a concave surface will have a stronger gradation (stronger contrast between highlight and shadow). A flat light source like a softbox will be somewhere in between.
HOW TO FEATHER
Sometimes I get confused about how to position the light source correctly for feathering, so here are the simple steps I follow to avoid getting confused:
1. Start by aiming the light source toward the subject.
2. Visualize where the shadows will be.
3. Without otherwise repositioning the light source, rotate the light source toward the shadow area.
That's not the only way of course (you could for example reposition the light laterally) but that's the easiest way for me.
Here's a self-portrait where I used feathering.
Propet bracket and umbrella in the process :) ) which was aimed not at my face but to my right ear (the wall on camera left). Here's the diagram:
For a sample of feathering in action, see Annie Leibovitz's photoshoot with Keith Richards for Louis Vuitton, which David Hobby recently blogged about:
Besides the fact that Annie used only one light for her subject, I found it interesting that the light was consistently being feathered. The Softlighter umbrella softbox can't be seen in most of the video, but when it is visible, it is clearly being feathered because it appears to be aimed at the area in front of Keith Richards instead of at his face. Here is the final result (copyright Annie Leibovitz):