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Tips & Tricks

The Inverse Square Law of Light – Why It’s Important for Photographers

By Paul Faecks on July 4th 2014

Inverse square law sounds ridiculously complicated, doesn’t it? Luckily, it’s not as complicated as it sounds and as a photographer, you should know what it is and how to use it. Karl Taylor, a commercial photographer from the UK, shows you everything you need to know about the inverse square law and how to use it to your advantage in a short understandable video. (This is Part 1 of the video).

[REWIND: HOW TO POSITION A BEAUTY DISH FOR PORTRAITS]

…any physical law stating that some physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity.

That’s the exact wording of the inverse square law, but in this post, we’ll just take a look at what that scientific term specifically means for photographers.

The inverse square law actually determines two separate characteristics of the light:

  • The fall-off in relation to the distance
  • The power in relation to the distance

The setup

Karl does a nice experiment to show what the inverse square law means. He set up a camera on a tripod that is filming him and his model. At a 45° angle, a light is pointed at them.

inverse square law close

When the light is relatively close to Karl Taylor and his model, you notice a big difference in exposure between the two. Karl is way darker because of the greater fall-off at the lower distance.

inverse square law far

If you move the light further away, the difference in exposure becomes smaller, and as you can see, Karl and his model are almost equally exposed although the light is closer to his model. This is especially important if you light large groups of people, because you want to expose everyone properly.

The light is also getting harsher because is is becoming a smaller light source relative to the subject.

As mentioned this is only part one of the video. Now that you know the inverse square law of light, how can this help in your photography?

[via Karl Taylor, images via screencaps]

About

Paul Faecks is a portrait- and fine art photographer, based in Berlin. If you want to check out his latest work, you can do so by following him on Instagram or by liking his Facebook Page

Q&A Discussions

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  1. Rafael Steffen

    I love his work and all the passion he puts on do deliver great content.

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  2. Ian Moss

    The “more distance, slower fall-off” is simply a matter of ratios, but perhaps the concept of the ISL would have been better demonstrated using lighting equipment that actually demonstrates the effect, rather than, as Stan points out, equipment that complicates the issues raised.

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  3. MARTIN MIANO

    mind freak …….great article

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  4. Tyler Friesen

    great post!

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  5. Servando Miramontes

    I love science, physics, and chemistry… Photography is just perfect

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  6. Anthony McFarlane

    Im a big fan of Karl. Thanks for posting.

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  7. Stephen Hunt

    I guess the concept is explained here of further the light is from the person the smaller the falloff will be (relative to the subject), but I’d rather an explanation of what the inverse square law actually represents would be handy. Maybe in the next episode?

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    • Stan Rogers

      Getting all mathy and theoretical on the issue wouldn’t be particularly helpful, since the inverse square law is only “real” with point sources. With a light source of any substantial size (a softbox, umbrella, diffuser panel, or even a large parabolic reflector with the light source at or near the focus) figuring out the fall-off mathematically means integrating over the surface of the light source. Not only does the fall-off act more linearly as the source becomes softer (that is, as it gets larger in relation to the subject(s)), with a sufficiently large source sufficiently close to the subject, the “fall-off” can be negative. A vague answer may be less intellectually satisfying, but it’s far more useful.

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    • Stephen Hunt

      I guess, Stan that what I would have liked to see is him moving the light source back about, say 1 metre and showing that the light drops off 2 stops of light. He did mention Fabian having to ‘keep up’ with the light drop off but didn’t explain how the light intensity is actually affected. Which is really what the Inverse Square law is about.

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    • Stan Rogers

      Thing is he was using the Para 88 for the demonstration, which doesn’t fall off as the square of the distance at close ranges (under 3 metres or so). That leaves a choice: use a smaller reflector (such as the P70, or even a bare Pico head or his MobiLED kit) at the expense of making the close-lit pictures look horrible, or trying to demonstrate the difference at lighting distances that aren’t going to result in dramatic fall-off in a frame. Again, the math is (or the maths are, for those who prefer it) a useful foundation, but they rarely work out in studio unless you are using small, hard lights that aren’t nearly collimated (as an optical spot would be at most practical ranges) . Or do the REAL math, which is a whole lot more complicated than “inverse square” — even the “for dummies” version, the layman’s explanation, gets into head-exploding territory for the math-challenged. The “more distance, slower fall-off” generality is, again, more useful unless you’re talking strictly about bare lights, since expecting anything else to play the inverse square game is going to result in a lot of frustration.

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