Changing the background with a simple gel for portraits

1:3 lighting ratio – Nikon D5, Nikon 85mm ƒ/1.8G, ISO 100, ƒ/5.6, 1/200

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 before I add the gel I shoot this photo where the subject in on a white background. I will shoot with just the main light, the fill light and then put both of the lights on with no background light so that the students can see individually what each light is doing.
Main Light only

Now I will turn the main light off and then turn on just the fill light.

Fill Light only

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.

Main and Fill light together

Then we talk about how she is in front of the white background but it looks like a light gray.

Main, Fill & Background Light

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.

This is the histogram without the background light. The furthest right on the histogram you can see that the value is good amount away from the far right.
This is the one where I have the background light set at 1-stop brighter than the fill. Notice here you can see most of the histogram is the same, but the far right is on the far right. This is showing how the white value is recorded. If you are not butting up on the right then there will be a little gray or often a tinge of blue when you print out the photo in the background.
Lighting Examples for teaching SOP1

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.