One of the challenges that we face in a studio environment when shooting portraits is to make the white background of our photos appear truly white, and that is even after using a pure white backdrop. As you can imagine a true white background is essential in some cases.
Normally, what happens is that the key light simply cannot make the backdrop to appear as pure white. It either becomes gray or completely black.
I know there are some artistic effects that can be done with the gray background too, and some photographers would simply say, oh, the gray background looks so much better, but the fact is that the trick is beyond some.
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The solution to the problem
There are two main ways you can achieve this; i.e.; make your backgrounds appear true white. One of which uses the inverse square law. This article is not meant to scare any of you budding photographers or newbies getting into flash photography. I promise to keep this is as simple as I can; so don’t get scared at the mention of the inverse square law.
First the difficult method: using the inverse square law
The inverse square law simply states that the intensity of the light tends to drop by a factor of inverse of the square of the distance from the light source.
Let’s say your subject is standing 2 feet from a light source. That gives a reading of f/8 (considering that the ISO is set to 100).
That’s an f-stop of f/8 for a certain shutter speed at ISO 100, to give you a proper exposure. If you take a picture with this setting, the subject is going to be properly exposed but the background is going to be under-exposed because of the inverse square law.
In other words, the background is going to be gray to dark gray or even black depending on how close or far back it is from the subject.
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Keeping everything else unchanged, if you now increase the subject to light source distance, let’s say double it to 4 feet, then the light meter will likely give you a meter reading of f/3.5.
You could either shoot at f/3.5 or bring the power of your light up so that it reads again f/8. Take a picture. You will notice that the background is brighter than before.
This also comes directly as per the inverse square law. As the distance increases between the subject and the source of light the degree of light loss becomes less.
When you meter for the subject, you also increase the exposure for the background and, magically, you end up making an amazing white background portrait photo!
If this explanation was too complicated, have a look at this video which explains the subject to background distance:
Using a background light
In spite of the inverse square law in operation, the background may not be pure white at all times. To make it absolutely spotless white you will need additional help; i.e.; the background light.
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The background light forms usually, the fourth light in a studio photo lighting arrangement, coming after the key light, the fill light and the backlight. It is used to illuminate the background and for the purpose of putting emphasis on the immediate surroundings.
The background light should be set at the same intensity or slightly higher than the key light. That way you can really make the background bright and truly white.