Controlling Light Falloff

In this post we will see how to control “light falloff” – the difference in light intensity between the areas of the photo who are closer to the light source and the area who are a bit farther away – this will let you create dramatic lighting (for example, cause one side of the face of fall into darkness) or more even lighting that is very flattering in portraits and essential for product photography.

Basically, this is very simple, the intensity of light drops has the subjects gets farther from the light source (no surprises there) but the drop is not even – the light drops off very quickly near the light source but very slowly farther away.

This is because the light falls off according to something called “the inverse square law” – here’s a picture of a wall lit by a flash to demonstrate the effect:

If you look at the area between the rightmost blue line and the center blue line you can clearly see that on the right is much brighter than the center – actually it’s about 4 times brighter (or 2 stops), now look at the area between the center and leftmost blue lines, this area overs approximately the same distance but the lighting difference is much harder to see – I’ve measured it and the center is twice as bright as the left (1 stop).

If we also look at the red ranges around the blue line we can see that in that tiny distance around the rightmost blue line we lost 20% of the light intensity, at the same distance around the center line we lost only 10% of the light and at the leftmost line the light is the same for the entire range.

So what does it mean in practice? let’s take a “one of a kind” toy from Ikea and photograph it with the flash to camera right just out of frame:

We see the left side of the face is completely dark and so is the background, let’s take the exact same picture but move the flash about 2 meters (6 feet) back (and raise the flash power to compensate for the change in distance):

Now the face is well lit because the relative distance between the parts of the face is now small compered to the distance from the flash, also, the background is no longer black (because the distance between light and  to background is no longer much longer than distance from light to subject).

If you remember in previous posts we used this effect to make the background black by placing the flash very close to the subject and to make the background white by placing the flash near the background.

Now, there’s just one more thing, while the light drops in intensity between light source and subject it does not lose power between subject and camera, to show this I’ve taken two more pictures of the same toy, both pictures are taken with the same camera settings (f/8, 1/200 sec, ISO 400, 135mm) and the flash at the same position with the same power settings, the first picture is from the closest distance that fits the subject in frame:

The second picture is from another room, the farthest I could get inside my home:

And here is the first picture and a crop of the same area from the second picture, the colors are a bit different because I forgot to turn auto light balance off but the brightness is absolutely the same:

Hope you enjoyed this post, and that this will help you better control the lighting in your photos.

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