Originally Posted by moscato_images
I had a 4 light setup the other day for portraiture. (mail,fill,hair,BG) The issue I have ran into both times I have tried a 4 light setup is the BG light not showing up at all. Reguardless if it's turned on full power with a bare bulb or completely off. The BG looks exactly the same in both instances. I have the exposure of the hair, fill and main right where they need to be.
So, what's up with the BG light? I'm shooting in a very small room and I'm figuring that even with my main and fill down almost to the lowest settings, it's still too much power. Would I be correct?
You're most likely seeing spill onto the background from your other lights. With a bigger room you can put enough distance between your model and the background so that your main, fill, and hair lights don't light the BG too (The inverse-square law is your friend, here). The best way to tell for sure is to use a flash-enabled spot meter and meter directly off of the background as you add each of your lights. Move the model out of the way and start by metering the BG light reflected off of the BG. Add each light one at a time, take a meter reading, and see what happens to your exposure. You'll then know which light is affecting your lighting solution.
If you just want to know if your BG light is having any effect, take a meter reading of the BG with all 4 lights, then turn off the BG light and meter again. If the exposure doesn't change then you know that your BG light isn't doing anything for you.
If you don't have a flash-enabled spot meter, you can take an incident flash meter reading right at the background. Use the same procedure. Hold your meter against the background at same height as you would if you were metering your model. Start with the BG light and then add the others one at a time to see which ones are lighting your background.
Grids (for your bare heads), fabric grids (for your softboxes), and flags can help you here by keeping light out of the areas where it's not wanted.