DIY Photography

Your one stop shop for everything photo-video

  • News
  • Inspiration
  • Reviews
  • Tutorials
  • DIY
  • Gear
Search

Submit A Story

The Inverse Square Law Cheat Sheet – Myth Busted

Sep 29, 2009 by Udi Tirosh 1 Comment

  • Share
  • Tweet
  • Flipboard
  • WhatsApp

The Inverse Square Law Cheat Sheet UPDATE: This Experiment is all Wrong. I should hit my head on the same wall I used to measure reflected light off. Some great comments about what went wrong, and great discussion going on – I posted the main points here.

Have you heard about the Inverse Square Law? It’s the law that says that light intensity falls the farther you move your light from your subject. It also tells you that if you move your light to be twice as far it will fall by 4 (the square of 2). if you move the light three times as far, it will fall by 9.

We all swear by that law. The only thing is this law does not apply to the way most of us use flashes.

I’m gonna explain this in a beat, but first here is my newest cheat sheet. (I love cheat sheets. If you are as senile as me, you can print them and then pop them up later and look really smart).

The Inverse Square Law Photography Cheat Sheet

You can see it really, really, really big by clicking here.

How’s This Card Working & Some Myth Busting

The right lobe of my brain always told me that there was something a bit off with the way we apply the inverse square law to strobed shooting. I mean the Inverse Square Law applies to light that expands in a sphere like shape. Usually our strobes are not spreading a sphere of light, but a beam of light. (Ok, Inv.Squ.law is too long – it’s going to be ISL from now on).

So I set out to test this. This card is made from four different scenarios – each show how light expands (and falls off) depending on the way we set our strobe.

The markers on the top represent the distance from the light source AKA Strobe (each mark is 50 cm). The words on the right tell the scenario under test.

First – the good news. When we use a stofen we get what we paid for – ISL in action. With a Stofen, the light is spreading from the flash in a very spherical way – hence when we double the distance from the flash (from 1 meter to two meters) we get aproximetelly1/4 of the light intensity.

Now, the not so good news – Whenever we set out flash on zoom we can throw ISL out of the window. The focusing of the light burst makes the light behave more like a narrow beam and less like a sphere. When we double the distance from one meter to two meters we lose just a little bit of light. When we double the distance again – it makes more sense, yet sampling dots on the 105mm line deviates far from ISL.

Actually, this is not really bad news – it means that we can get some nice power far away if we focus the beam.

Now, I’ll admit, there’s some sampling error when we are close to the strobe – please take the big version and see for yourself.

The Making Of

Luckily, I am still leaving on a construction site, this makes it easy for me to find long stretches of empty walls. The nice thing about it is that you can see the blocks – each block = 50 centimeters.

I placed the strobe on a lightstand (the 001b)  and placed my D300 on a far away tripod. Strobe was triggered with poverty wizards (AKA cactus triggers). 

I took a set of images that look very much alike – the only diff was the flash settings / modifier mount.

For the card I used the middle strip of each image

The Inverse Square Law Cheat Sheet

The blocks made it easy to mark lines 50cm apart.

For light intensity I used Photoshop‘s sampling tool.

Sharing Is Caring 

As with my other Cheat Sheet Card, I placed the card on a CC license – feel free to print it, share it, use it in class and distribute it in general.

You may even use it to tell me I am wrong

More Reading:

  • The Hard Math behind the Inverse Square Law
  •  
    Intuitive& Practical explanation of the Inverse Square Law
  • Measuring by Length – Use this one for stofen-ed strobes 
  • I love cheat Sheets – Here are my Portrait lighting, Reflector Card and Light Falloff cheat sheets

 

FIND THIS INTERESTING? SHARE IT WITH YOUR FRIENDS!

  • Share
  • Tweet
  • Flipboard
  • WhatsApp

Related posts:

Default ThumbnailThe Inverse Square Law Experiment Done Right – Myth unBusted These Two Photos Will Make You Instantly Understand How The Inverse Square Law Works Lighting Design the Easy Way – Secrets of the Inverse-Square Law of Light One way or another, this video will help you understand the Inverse Square Law

Filed Under: Inspiration Tagged With: cheat sheet, Cheat Sheets, flash

Udi Tirosh: from diyphotography.net

About Udi Tirosh

Udi Tirosh is an entrepreneur, photography inventor, journalist, educator, and writer based in Israel. With over 25 years of experience in the photo-video industry, Udi has built and sold several photography-related brands. Udi has a double degree in mass media communications and computer science.

« Wix Just Got Better (And Cheaper For DIYP Readers)
The Inverse Square Law Experiment Done Right – Myth unBusted »

Submit A Story

Get our FREE Lighting Book

DIYP lighting book cover

* download requires newsletter signup
DIYPhotography

Recent Comments

Free Resources

Advanced lighting book

Recent Posts

  • Botched selfie escalates to draining of reservoir
  • Panasonic PanaTrack is a big bendy motorised camera slider
  • Once again, tourists are getting too close to bison for photos
  • Fujifilm announces shipping delays for the new Fuji X-S20
  • International Photography Awards reveals breathtaking 2022 winners (mildly NSFW)

Udi Tirosh: from diyphotography.netUdi Tirosh is an entrepreneur, photography inventor, journalist, educator, and writer based in Israel. With over 25 years of experience in the photo-video industry, Udi has built and sold several photography-related brands. Udi has a double degree in mass media communications and computer science.

Alex Baker: from diyphotography.netAlex Baker is a portrait and lifestyle driven photographer based in Valencia, Spain. She works on a range of projects from commercial to fine art and has had work featured in publications such as The Daily Mail, Conde Nast Traveller and El Mundo, and has exhibited work across Europe

David Williams: from diyphotography.netDave Williams is an accomplished travel photographer, writer, and best-selling author from the UK. He is also a photography educator and published Aurora expert. Dave has traveled extensively in recent years, capturing stunning images from around the world in a modified van. His work has been featured in various publications and he has worked with notable brands such as Skoda, EE, Boeing, Huawei, Microsoft, BMW, Conde Nast, Electronic Arts, Discovery, BBC, The Guardian, ESPN, NBC, and many others.

John Aldred: from diyphotography.netJohn Aldred is a photographer with over 20 years of experience in the portrait and commercial worlds. He is based in Scotland and has been an early adopter - and occasional beta tester - of almost every digital imaging technology in that time. As well as his creative visual work, John uses 3D printing, electronics and programming to create his own photography and filmmaking tools and consults for a number of brands across the industry.

Dunja Djudjic: from diyphotography.netDunja Djudjic is a multi-talented artist based in Novi Sad, Serbia. With 15 years of experience as a photographer, she specializes in capturing the beauty of nature, travel, and fine art. In addition to her photography, Dunja also expresses her creativity through writing, embroidery, and jewelry making.

Copyright © DIYPhotography 2006 - 2023 | About | Contact | Advertise | Write for DIYP | Full Disclosure | Privacy Policy