I remember learning about feathering a long time ago.
But it took me a long time to employ feathering.
I don't know why, but a lot of times I read things and understand it conceptually and then fail to implement it. I think I've got a thick skull...
A buddy of mine asked me on Facebook about softboxes and how come the effect is never natural, the light always seems too harsh...
I suggested feathering as a short-term solution to a rather complicated and situational problem. There have been situations where I've found that my softboxes weren't the right application/lighting for the shot I was trying to create. For example the following image:
I had a hell of a hard time setting this one up correctly and in retrospect I would have lit it differently. I guess the shot came out well but I remember working with it in Photoshop and seeing the reflection of the umbrella in the window and overall feeling that the light was still too harsh. If I were to redo this shot, I'd just use a reflector... probably my 4x6' Scrim Jim ;)
Here's the thing, as soft as softboxes are, they'll never compare to the softness of ambient light. Ambient light is softer because it bounces around and is "everywhere". No matter how large your softbox is, it's still directional... therefore it'll cast shadows and comparatively speaking it'll look "harsher". The best solution is to keep that softbox close to your subject for the softest light.
With that analysis, feathering isn't necessarily the best answer...
But this reminds me of another image where feathering might have helped. See the following:
Here's Catherine lit with an umbrella and some ambient light. Again, a reflector would have been a better choice. And again, feathering isn't completely the answer but rather... "powder". Harshness is sometimes exacerbated by oil and keeping shine to a minimum will reduce specular highlights.
So when is feathering a good idea? Feathering is always a good idea as long as you have the luxury of not spilling to the background. Feathering keeps the light soft and provides natural fall-off. Feathering is also good at preventing hot-spots such as the oily places on Catherine's face because it doesn't burn the highlights. Again, powder would have done better but feathering is a great all-around solution to keeping light soft and not concentrated on any particular spot.
Feathering your light will help you yield immediate results. Try this: Instead of pointing the softbox directly at the subject, direct the center of the light 1 foot in front of the subject's neck. So not only is it lower, but the concentration of the light will be in front of the subject. The result should be softer light that gently falls off the subject (assuming the light is angled from side-to-side).
Oh and remember to keep the softbox close to your subject and also to keep oil/shine to a minimum :)