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The ABC of Photography – Inverse square law

This law particularly relates to the use of studio lights or flash and says that if an object is twice a particular distance from a point source of light, it will receive a quarter of the illumination. For example, if your subject is two meters away, and you increase it to four meters, the resulting fall-off means you’ll need four times the amount of light to keep the same exposure settings. Alternatively, you’ll have to increase the exposure by two stops.