When I teach lighting I always teach the 1:3 lighting ratio. After I show how you set up the main light and the fill I show them some ways to change the background quickly using gels.
Now I will turn the main light off and then turn on just the fill light.
Now for the main light it was measured for ƒ/5.6. I didn’t change the exposure on the camera I just shot the fill light at ƒ/4 to show it is darker than the main and where the direction of the light is coming and how it affects the model’s face.
Then I combine the two lights.
Then we talk about how she is in front of the white background but it looks like a light gray.
I put two lights on the background and then measure the light so that it is about 1stop brighter than the main light. So the background here is ƒ/8.
Now when I add the gels like this red or the blue above we take a light reading of the background. We want the value to be 2–stops darker than the main light. So here the background is measuring ƒ/2.8.
One more thing you will notice is you need to move the person away from the background when using white for a background.
Now I demonstrate this also using a black background and to get the color to look like this you need to be sure the background is then 2–stops brighter than the main light. So if this red background was really black with the gel on it the reading would be then ƒ/11 which is 2–stops brighter than the ƒ/5.6 of the main light.