Here is the problem. Our vision adjusts so well to light and darkness, and compensates so effectively for poor lighting conditions, that it fools us. In the images above, a light is being moved back from a target. In the left-most image the light is shining brightly on a single square and is dumping about 100% of its light on that square. In the center image the light has been moved back some distance and is now falling on an area about the size of four squares. And in the image on the right the light has been moved back even further and it is showing on about nine squares.
Our intuition and our vision will tell us that if the image on the left is about 1 square wide and gets 100% of the light, then the center image, which is about 2 squares wide, then it must be getting about 50% of the light, and the image on the right, which is about 3 squares across, must be getting about 33% of the light. But our vision and our intuition would be wrong!
In the center image, the same amount of light that was falling on one square is now being shared by four squares. So each square is getting about ¼ of the light. The light on any single square isn’t half as bright as the first example – it is 25% as bright!
If we move the light back further so that it covers nine squares (image on the right), our intuition is that the light is maybe 1/3rd as bright as it was when it covered only one square. Wrong again! The same amount of light is now shared by nine squares, so each square gets only 1/9th the light. It's not 33% as bright -- its about 11% as bright.
Fortunately, it works the other way too. If our light is positioned far away, as in the image on the right, we can get the light a lot brighter than we might imagine, just by moving the light closer to the subject.
This means that you can work with a lot smaller and less powerful lights if you push them much closer to your subject. So if you can't get a good exposure, move the lights closer. And if necessary, re-frame the scene by moving the camera closer or using the zoom.
It also means that its a lot harder than you think it will be to light a large area.
We are used to thinking about changing the brightness of a light using a dimmer switch. We don’t usually move a light to change its brightness. So it takes some experimentation and some practice to get a feel for how it works.
You can use the auto exposure (iris number) on your camera to teach you about the distance of lights. Try lighting a basketball or a vase in a close-up shot. Then move the lights and watch what happens to the iris number.
In this way, you can use your camera to teach you how it sees the light.
