Flash distance
The science behind the spread of light explained
The light from your flashgun only reaches so far, and the further a subject is from the light source, the less brightly they will be lit. That much may be obvious, but light fall-off is more rapid than you might think, because it’s governed by something known as the ‘inverse square law’…
The inverse square law is defined as:
“A law stating that the intensity of an effect, such as illumination or gravitational force, changes in inverse proportion to the square of the distance from the source.”
This essentially means that for every doubling of the distance of your subject from the light source, it only receives one-quarter of the intensity of the light. So a subject two metres from a light source doesn’t just receive half the light of a subject one metre away, but only a quarter as much. Move them three metres away and this drops to 1/9, and at four metres it’s just 1/16 the intensity, and so on.
You can see that placing your subject any more than a few metres away from the Speedlight means the strength of the light becomes negligible. For this reason, you’ll ideally need to place your flashes as close to the subject as possible.
Note that this doesn’t affect the distance from the camera to the subject in any meaningful way – it’s only distance from the flashgun to the subject that counts.