The inverse square law of light holds true for a point source of light, according to the definition in physics, but in photography we don’t really use the point source of light since it is too harsh. Instead we use fixed light sources and flash lights in photography as the point source of light. Now the inverse law, which says that the intensity of light decreases by the square of the increased distance, explains the difference in exposure of objects that are closer to the light source and the ones that are away.

In the case of flash photography, your flash works as the point source of light and if your camera is on an auto mode, then the camera will handle the exposure. However, just in case you are shooting in the manual mode, you will have to do the calculations. In case your bounce off your light from the wall then your work becomes a little easier, since you won’t get harsh shadows and light would rather fill up your frame rather than just targeted at one point.

If you do not wish to bounce the light and also shoot on manual mode, then you need to make sure that you adjust your exposure settings accordingly. This is because flash is a spontaneous source of light, and you won’t really get the time to make alterations. The nearer the light source is to the object, the higher its intensity and the farther away it is the lower the intensity, forcing you to open the aperture. With high powered flash, like the Canon Speedlite 580EX, you might start out with a great light intensity, but still have to adjust the aperture, or shutter speed, to make up for differences in distance. To be precise, if you move at a distance that is double the original distance, then you will have to open your aperture by 2 stops or lower the shutter speed by 2 stops. Similarly, if you move to half the distance, then you will have to close the aperture by 2 stops or increase shutter speed by two stops.