The Position of Light

The distance of the light source to the subject has a tremendous impact on illuminating your photograph. The relationship (called the inverse square law) of how quickly light changes (falls off) means a light source placed closer to its subject will fall off more quickly then a light source that is placed further away from its subject; the light falls off slowly. This means a light close up will have a dramatic effect where as a light placed further away will be more balanced across the photograph. An example is this is that you can change a white background black by only manipulating the light to subject distance.

The below photographs show the effects of the light distance on my angel and the background.  The angel is set on a white base with a white background approximately two feet behind the angel. The first photo, flash set five feet away from the angel, was taken to set the proper exposure of the angel. Notice the color of the background is gray. The second photograph, the flash was moved closer to the angel, one foot away.  The exposure was changed in order to correctly expose the angel but the background is nearly black. In the third photograph, the light was moved back fourteen feet away from the angel. The exposure was again changed in order to correct expose the angel. Notice the background, it is nearly white.

Tech Data: all photos 1/250
1ft flash power 1/128 F13
5ft flash 1/16 +7 F11
14ft flash power 1/1 F10

 

The reason the background changes to black is due to the flash power (flash now closer to the angel) being decreased to correctly expose the angel but it was not powerful enough to light the background. The flash to angel distance is one foot, the flash to background distance is three feet, thus background is three times as far from the flash as the angel.  The background turns black because of the relatively large difference between the flash to angel as compared to the flash to background distance.  Though everything is closer together, the light is nine times weaker by the time it hits the background due to the inverse square law.

On the white background the flash was moved further from the angel, and the power was increased to correctly expose the angel but it is also powerful enough to light the background. The flash to angel distance is fourteen feet, the flash to background distance is sixteen feet, thus background is just over one times as far from the flash as the angel. The background turns nearly white because of the relatively small difference between the flash to angel as compared to the flash to background distance.  Though the light is further away from the angel, the light is relatively the same power by the time it hits the background and there is enough light to illuminate the background, again due to the inverse square law.

The key point here is that you can move your light source closer or further to change effect and not the math behind the inverse square law. With this understanding can now begin use multiple flashes to independently light different areas of you photograph or just use one flash to illuminate an entire area. By manipulating light distance  your creative options increase dramatically.

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