It's refreshing to be exposed to a piece of information that radically changes my way of thinking about something. Especially when that something is as mundane as the distance of a source of light from a subject.
We're familiar with the notion of depth-of-field when it comes to focus, but now think about exactly this concept, but applied to illumination instead. We know that light falls off in intensity as we get further from the source. In fact, elementary physics tells us that the amount of light falling on the subject will be inversely proportional to the square of its distance from the light source. Fine. That's easy-peasy. So if we want to double the intensity (1-stop up), we need to reduce the distance to the light source by a factor of 0.7071 (1/sqrt(2)). Conversely, if we want to halve the intensity (1-stop down), we need to increase the distance by a factor of 1.4142 (sqrt(2)). That's trivial. Now here's where it gets interesting:
We're normally illuminating more than just one plane. Typically the scene consists of a bunch of objects scattered about at varying distances from the light source, so if we expose correctly for the middle-distance objects, the ones closest to (furthest from) the light source will be over(under)exposed. But by how much? If we define +-1-stop of light as an acceptable exposure deviation, then from our calculations above we know that objects within the 0.707x to 1.414x (where x is the distance of the correctly exposed object from the light source) box are "acceptably" exposed.
The cool thing is that this gives us a useful creative knob to tweak. The diagram above shows that by moving the light source further away (and cranking up the light power to compensate), we can increase this box of "acceptable" exposure. Similarly, if we want the zone of acceptable exposure to fall off quickly, we just have to move the flash in really close (and decrease its power). The object in the middle-zone gets exactly the same illumination each time, but the gradient of illumination can be as smooth or as sharp as you like.
A practical application of this is when doing portrait shots against a background. By controlling the depth of illumination, we can easily control the relative illumination of the subject versus the background simply by changing the distance of the light source from the scene.
In a nutshell, the following 2 rules should help:
- Given a correctly metered subject, you'll know that 30% inside and 40% outside its distance from the light source, all objects are within a 1-stop illumination deviation.
- Conversely, if you've got a scene of depth x, then to illuminate it all within a 1-stop deviation of the central part, the light source has to be at distance at least x from the near edge of the scene.