I'm taking self-portraits using Yongnuo speedlights as my lighting. I would like to light the background for better separation. I tried using a bare flash at 1⁄32 power at 85mm, but the falloff is too hard.
My background is a muslin hand-painted background. I'm trying to achieve the traditional portrait look.
What modifiers should I look at? Would a snoot help, and which direction to I position the light and snoot? Or would putting an 18" octagon soft box behind the subject achieve the effect?
Don't think of the background light as just a background light. Play with it.
If you double the distance you will diminish the fall-off.
You will need to double the output, and probably put a card so you don't spill light to your subject.
But play with the light, cut a cardboard in different shapes.
From different angles
Put a diffuser before the cardboard, remove the diffuser, move the distance to the screen, to the cardboard, the angle.
You can also construct a snoot just wrapping cardboard around your flash head.
Take a time to shoot not your portrait, but to shoot just backgrounds.
After you play with background's illumination probably you need to separate a bit more the subject from the background, because the light of the main subject will contaminate the background. Again, play with the distances.
I'm adding this part. Lighting the background will not add "separation" on its own. Playing with the illumination on the background will give you an ambient, hopefully it will add some mood or a story to your portrait.
But to "separate it" you need to play with the illumination between your model and the background. A hair light; a shadow-light vs light-shadow scheme perhaps; or narrower DOF.
You need softer "diffuse" light. Easiest is to shoot near a white wall and/or ceiling and bounce the flash at higher power and wider angle off the surface. For more control buy an umbrella and mount, a reflector and something to hold it, or step up to a real soft-box.
Great tutorials for shooting with speedlights at http://strobist.com.
