Feathering is a phenomenon related to direction, which affects quality, and to some extent quantity. At a basic level it refers to angle of light, but being more specific, we’re talking about angling a light so that we create certain effects on our subjects, with respect to how the intensity of the light falls off from the centre of the light source to the edges
We might imagine that the light emitted from something like a softbox is perfectly even from edge to edge, but if we were to be able to measure the fall of light as it’s projected, we would see that in fact the amount of light is stronger at the centre, and then it falls off towards the edges.
Stop for a minute, and think about this, and it becomes obvious. Put yourself mentally in front of the light, and if you stand dead-centre, you’ll see the whole face of the light square on. As you move to one side, you’ll see less and less of the light, and consequently there will be less falling on you, and you’ll be “darker”. You can see how if this imaginary person was to start in the middle, most of the light will travel directly towards them, and then as they move to either side, some of the light will have to travel much further. Of course, the further it has to travel, the more the quantity drops.
It’s obvious if I take a beauty dish and point it right at you. Bright, isn’t it? Now, as I angle it away, you’ll be able to see less and less of the light. Although I’ve moved the light, it’s the same effect as you standing on the edges of the beam it’s casting. The fall off of intensity of light is gradual, and will vary depending on what sort of modifier you’re using. Generally speaking, the softer the light source, the more gradual the transition from full intensity of the light down to the darker edges. This transition is the “feathering” effect.
The effect of feathering is accentuated by the use of grids, which we’ll cover in more depth when we talk about modifiers, but very briefly here, we’ll insert a grid into this beauty dish, and watch what happens. You get the same effect, but it happens much more suddenly. The honeycomb shapes in the grid have focused the light down to a degree, and narrowed down it’s spread. It’s still the same size - therefore still the same quality, but we’ve tightened the beam if you like. This has the effect of making any fall off of light more dramatic. Returning to our top down diagram we can see how the grid has channelled the light down into just the light being cast directly out the front of the beauty dish, almost none is spilling off to the sides anymore.
In practice, it allows you to carefully control where your light is falling. You can use feathering to light your subject with a soft, flattering light, but keep that light off the background in order to keep it darker, or light it separately.
This image of golfer Luke Donald was lit from the right of camera with a beauty dish, so still a fairly soft, flattering light, but feathering it in such a way that it hits him and not the back wall. I then added a 2nd light to put some light on the back wall - the fact that the light was feathered off the wall and just on Luke meant that I was able to light them as 2 separate objects.
You can also use it to create a dramatic change in tone from highlight to shadow across a small area - such as a face. This image was lit with the exact light I demonstrated to you with - a beauty dish with a grid on, angled very carefully to keep the top of Joel’s face in highlight, and let his neck drop off to shadow. Joel is seated in the chair, and if you look closely, you can see that the beauty dish is not pointing straight at him - it’s slightly angled off. I’ve feathered it so that rather than the middle of his face being slap bang in the sweet spot at the middle of the light, he’s closer to the edge, therefore you see more of the falloff.
So, feathering is basically a fancy name for angling the light, and it obviously functions as a part of direction of light, but of course it has an effect on quantity, and to some extent quality too. All that from just turning the light round a few degrees!