Inverse square law
This law applies in particular to the employment of studio lights or flash, and it states that an item will get one-fourth of the illumination from a point source of light if it is located twice as far away from that source as the minimum distance specified. For instance, if your subject is two meters away, and you increase the distance to four meters, the ensuing fall-off indicates that you will require four times the quantity of light in order to maintain the same exposure settings. Alternately, you might try increasing the exposure by two stops to achieve the desired effect.
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