From a physics perspective ISL relates to any physical law stating that a specified physical quantity or strenght is inversely proportional to the square of the distance from the source of that physcial quantity.
In plain English, as flash-to-subject distance doubles the light reaching the subject is only 25% of the original light that left the flash. For example, you move your subject from 5 to 10 feet away from the light source, four times the amount of light is needed to get the same exposure at 10 feet that you get at 5.
Here are some calculations courtesy of Adorama TV.
As you see from above at 1' away from the light source you have full power of the light. When this light reaches 8' we only have 1/64 of the amount of light. The further away the subject is from the light source the less dramtic the light will be or the less falloff we'll have from the light source.
If we look at taking images of a white card at varying distances from our light source we can see the effect of falloff more clearly.
At one foot away the card looks like this and if this were a person we would have a high contrast portrait.
At 16 feet away the falloff is not so noticeable and therefore we have a more evenly distributed light. This would allow us to take a portrait where some of the background would be visible and not blown out.
The following chart shows how falloff progresses at various distances.
Understanding the effect falloff has on our images enables to explore different photpgraphic opportunities. The nearer our subject is to the light we are able to isolate them and get a high contrast look. The further away more background is captured and we have a softer look.
Falloff also has an effect on exposure and we may need to either move the subject to achieve a correct exposure or alter our aperture.
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