People who are familiar with flash photography know the inverse square law, which states that illumination from any light source falls off with distance. For a point light source, the intensity of light is inversely proportional to the square of the distance from the source.
This is a law based on physics principles, specifically in this case, energy conservation. As energy radiates from a point source, it spreads out to an area that is proportional to the square of the distance from the source. Hence, the radiation passing through any unit area is inversely proportional to the square of the distance from the source.
Like any physics law, there is a specific set of conditions the inverse square law can be applied. One of the key phase here is “point light source”, which typically means something infinitesimally small in mathematical term.
The flash head cannot be a point light source, right? Well, the bare flash head is quite small. It can be a good approximation if the subject is far away from the flash. If you put the flash behind a softbox or umbrella, the light source is essentially the entire surface of the softbox or umbrella. We typically do not use the softbox or umbrella very far away from the subject. So you cannot consider the softbox or umbrella as a point source. Will the inverse square law fail then?
Yes and No. If you blindly apply the inverse square law using the distance between the subject and the softbox (or umbrella), the law fails especially when the distance to the subject is short compared to the dimension of the softbox. However if you think the surface of the softbox as a collection of tiny point light sources, the law continue to apply and the intensity of the light at the subject is result of being illuminated by the collection of point light sources that have various distances from the subject.
Someone tried to bust the inverse square law myth and failed. The tests contained numerous errors, such as measuring the light intensity off the axis, using the pixel values as intensities, ignoring the gamma correction done by the camera, etc. When the experiment was done right, the myth was un-busted.
Keywords: Flash photography, Inverse Square Law, Physics