[jfrancis]

Light tends to fall off faster near the source than it does at a distance. Photographers usually cultivate this shallow 'depth of light' by placing the lights as close as possible outside of the frame.



Strictly speaking the Inverse Square Law is not an inherent propert of light, but a property of expanding spheres.

When light does not expand spherically (as in a collimated beam) or when the source cannot be approximated by a point, then light seems to 'violate' the Inverse Square Law.

[jfrancis]

So the idea isn't just that light from a point source falls off according to the ISL. Pretty much everybody knows that.

The idea is that as a consequence of the ISL distant lights are potentially boring (unless uniformity is what you need) and all the sexy falloff is in the close lights.

[RonB]

Very cool info, thanks for posting that jfrancis. I never thought about the expanding sphere's description. Makes it easier to understand, as do the diagrams.

Cheers, Ron

[jfrancis]

Another look at 'depth of light,' which is not so much about the falloff of the light per se, but how to control the ratio of the falloff between two objects, often a figure and a background.

[sampson]

very interesting and useful, thanks.

[jfrancis]

I've been interested in this lately. I've known about inverse square since I was a kid, and yet I still find it trippy that because of the ratios involved, to make the wall go dark, the right thing to do is bring the light in much closer to everything.

[Richard]

Another thing I learnt today, thanks mate! Nice explanation and example! Now just wish my gf had a figure like that!

[jfrancis]

I made a little video

Depth of Light
http://www.youtube.com/watch?v=tX-z9-R1Juo