Inverse square law

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The inverse square law is a physical law which applies to the way light is transmitted. Essentially, if you increase the distance between a light source and the object it illuminates, the amount light decreases in proportion to the square of the distance, not merely in proportion to the distance.

So, if you have a 200 Watt Second studio strobe at 1 metre from the subject, it will cause the same amount of light to fall on the subject as an 800 Watt Second light at 2 metres. This is the same as a reduction of 2 EV, because each EV halves or doubles the amount of light.