Flash to subject distance – Digital Photography Tip of the Week

This week’s tip is about controlling the brightness of your background when using your on camera flash.

Controlling the brightness of your background relies on a little technical knowledge, the inverse square law. The brightness of the flash output is relative to the inverse square of the distance from the flash from the subject. Sounds complicated, but to sum it up simply, the brightness of your flash at a given distance is reduced by to 1/4 it’s brightness at twice the distance (the inverse square).

Using this, when the flash is your primary light source of your image, you can utilize this knowledge to help create some separation between your subject and the background. By moving your subject further from the background (still assuming correct exposure for your flash) the background will get darker. This technique can be used to completely darken the background as you may sometimes see with macro photography with flash, or make your subject stand out from the background.

Subtle use of this technique is required though. If the exposure of the foreground and background is too dramatic, your subject may look pasted on to the background.

Until next time, happy shooting.

Comments 1

  • For those of us that are mathmatically challenged…a hint:

    If you look at the f stops on your lens barrel (we’re talking old fashioned film cameras now) and think of them in terms of feet, you will know distance at which you available light changes by half (one stop). Do the math–1.4 feet to 2 feet= 1/2 the light or one stop.
    5.6 feet to 8 feet= 1/2 the light or one stop. 8 feet to 16 feet is 1/4 the light or 2 stops.

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