A reader wrote to me to ask about light drop-off when using umbrellas as described in my ebook, 100% Reliable Flash Photography. Although the topic of drop-off isn't really covered in the book, it's a good one to discuss. The question had to do with how I was able to avoid lighting up much of the environment (especially the background) while getting plenty of light on my subject.
Because many of the examples shown in the book were created using large (approx. 43") shoot-through umbrellas and powerful flash, you'd think that the light would spread and bounce all around the room. The thing is, it does. But that doesn't mean everything in the room gets the same amount of illumination or exposure. As distance from the light to an object increases, the intensity of that light drops-off dramatically. The principle is explained in this video. And that's why, in photos like this, you don't see the subject and the background with the same amount of brightness:
Of course, even if you decrease the distance from the light to the background, it can still appear darker even though there's actually more light hitting it. How? Well, as long as you you decrease exposure, for example by using a smaller aperture (higher f-stop number), and don't move the light close enough to the background to make up the difference, you'll force the background to become under-exposed; it will look darker.
In our example below, as I move the light closer to the subject it becomes more intense on the subject. This means the subject will require a smaller aperture for a proper exposure. Again, the subject gets a brighter burst of flash, so we need a smaller aperture.
And although I'm also moving the light closer to the background at the same time, it's still too far from the subject (where the accurate exposure is happening for the aperture in use), to get a proper exposure. The background is getting under-exposed.
Take a closer look at the photos, settings, and distances shown to understand how this works.
All images were shot using the following settings:
ISO 200, Shutter Speed 1/125, Flash output 1/8 power.
Above: Aperture is f/5.6. Light is 4 ft from the subject and 8 ft from the gray seamless background.
Above: I moved the light approx. half the distance from the subject, so the aperture had to be increased to f/9 for a proper exposure on the subject. Light is 2 ft from the subject and now 6 ft from the gray seamless background. Because of the smaller aperture, the background is further under-exposed. Note that here it might look darker than in the original image.
Above: Next, I again moved the light to approx. half its previous distance from the subject. Aperture is f/13. Light is 1 ft from the subject and 5 ft from the gray seamless background. Although we have the proper exposure (aperture/ISO setting) for the light at this power and this distance from the subject, the background is 5X the distance from the subject so it goes pretty dark at these settings.
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