The Inverse Square Law Of Light - BYU Photo

BYU Photo's Nate Edwards demonstrates the Inverse Square Law and how you can use it to level up your studio lighting. By using the physics of light, Nate will teach you how to master light falloff for both portraits and group photos.
Photographed with the Canon R5 and Profoto D2 and Pro 10 Studio Lights.
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[@Eduardo_Aguirre]

The best video about “The Inverse Square Law” by far, period. And I watched quite a few.

[@byuphoto]

Thanks Eduardo! We hope it becomes a good resource for others.

[@zavoina]

Truly one of the better explanations I've seen.

[@byuphoto]

thank you! Hope it was helpful!

[@TeddyCavachon]

The simpler way to use the ISL with camera lighting is to take a string and attach it to the light and then tie knots at 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 feet. As you move the light one knot further from the subject being illuminated the intensity will decrease by one f/stop. If FILL over camera at 11ft then placing the KEY light off axis at 8ft will make KEY 2x brighter than 1x FILL a 2:1 incident or 2-KEY+1-FILL : 1- FILL 3:1 reflected ratio which will render a full range tone from black to white with most digital cameras.

[@pc5447]

This has to be the best illustration I’ve seen Thanks!

[@byuphoto]

Thanks for watching!

[@gustavolopez5403]

I’ve seen other videos explaining the law but didn’t grasp the theory quite well. Until now. Thank you!!

[@byuphoto]

So glad it was helpful!

[@ChuckSeayII]

Great tutorial! Very well done explanation on the how, where, and why to position your light and subject and how light reacts across distances! I have heard this for years but this video is the one that clears it up for me! I'm a very slow learner LOL! Thank you so much for sharing this! It's a true gift! Now I know I will do so much better lighting stuff!

[@byuphoto]

So glad this video was helpful Chuck! Thanks for watching!

[@SteveMitchell-bt8ez]

I feel like you might have mentioned that those numbers don't have any similarity to the corresponding f stops you would use.

[@ChritsianBucic]

By far the best one, and straight to the point!

[@byuphoto]

Thanks for watching!

[@DMATcyt]

Wow..best explanation I have seen/ heard! Definitely subscribing. The test of true comprehension is when you are able to teach it to someone else, your tutorial has enabled me to accomplish this. Xie xie ni

[@byuphoto]

So glad it was understood and helpful. Best of luck!

[@johncalhoun4806]

I wish this was posted a couple days earlier because these valuable tips and explanations would have helped in a session I was trying to improve using off-camera flash. But all’s not lost - knowing what my problem and challenges were allowed me to quickly find the correct solution, which is all contained in this video! Knowing where I went wrong now, won’t make those mistakes again! Thanks BYU Photo!

[@byuphoto]

Sorry it wasn't posted a couple days earlier, but at least now you knew which questions to ask in figuring things out. Glad the video was helpful!

[@dance2jam]

I've seen many basic discussions of this, but yours was to the point and well subsidized by visual examples. I've only been using a strobe for 4 months, but I read up on this and put it into practice nearly immediately (testing it out visually for myself). The one thing I haven't seen anyone do yet, is talk about the limitations of the inverse square law and exceptions of when it doesn't really apply. Question: "Larger small groups" - what is this? and how would using a small light source further away (harder light) be better than a larger source closer, modified, and feathered? Thanks for an excellent video and taking the time to produce it.

[@byuphoto]

great question. the reason i say a small light source farther away over a large light source closer for group photos is because of the inverse square law. a large light source close will not light a group of people evenly. it is all relative to the size of your light source. the larger your light source, the further back you will need to place it to get a more even falloff. the 1'-10' illustration is just an example, not exact. the distance of falloff changes relative to the size of the light source. the smaller the light source, the shorter the distance of falloff. the larger the light source, the further the distance of falloff. you can for sure feather the light as you mentioned. this is just an easier way to get a more even light across a larger area (like for a group photo). I hope that makes sense as i wrote it out. it does in my head haha

[@dance2jam]

@@byuphoto LOL - Yes, I followed what you were saying. Got it.

[@7189k]

Well done. Finally understand the concept!

[@byuphoto]

So glad to hear! It makes all the difference.

[@graememacdonald1088]

Excellent video with really useful exemples!

[@byuphoto]

Glad it was helpful!

[@davegalegor2865]

OUTSTANDING Information.

[@byuphoto]

So glad it was helpful!

[@azppmd]

Brilliant and clear explanation. So many amazing videos from BYU Photo.

[@byuphoto]

Thank you!

[@dandaninglis]

Awesome video!! Great explanation of the concept with practical examples showing how to utilize it. Top notch!

[@byuphoto]

Thanks Dan! We hope it is a good resource for others.

[@Pattern_seeker202]

Amazing information

[@EPWillard]

Thanks. very helpful.

[@AlejandroRodriguez-wt2mk]

awesome

[@dennisjones5579]

Great video, The graphic showing the percentage of light fall off with distance was very helpful. I have 2 questions, which may be the "stupid" questions of the day. First, you estimated the changes needed either to the light source or camera to be 3.5 stops, the second time "about" 3 stops. How did you determine that? Was that more of an intuitive "knowing" based on experience or was there something else? Second, did you make the changes in this video to your light source or to your camera? Thanks, and again, well done.

[@byuphoto]

Great questions. To the first question, I made sure I knew the exposure before we started filming so I could have it in my script. But once you do it over and over, you are able to get a pretty good general feel for the amount of adjustments to make. To answer the second question, I made the adjustment on the power of the light. I typically keep my camera settings the same and just compensate with the power of light when I am shooting.

[@roncournoyer2792]

Fantastic info, always good to have a refresher on the basics once and a while. Your videos always help us bring oout the best in our shoots. Little off topic question, as we';re redoing our studio as we speak. What are you using for the backdrop and cyc wall and floor in your studio. Looks like it is one piece of material and that would be helpful to know as we build our out. Thx!

[@byuphoto]

we used Pro Cyc and they are the best. www.procyc.com/?gclid=Cj0KCQiAgqGrBhDtARIsAM5s0_nHasGqGt94zAGr_1rvZHtPVc7EGq-W68oNxdH9UD33rRCJgPRm

[@mimosz8097]

love this explaination!

[@byuphoto]

Thanks for watching!

[@ldouglass6]

Boom! Mic drop Thank you

[@rahulaldar8531]

wow nice video sir😍❤️🔥

[@byuphoto]

Thanks! Hope it was helpful!

[@judesonataisi4503]

I really understood this

[@byuphoto]

so glad to hear!

[@JoshuaBrittonAndrews]

What about the size of your modifier, how does that play in?

[@byuphoto]

great question. The smaller the modifier, the distance and amount of the light falls of is much shorter. The larger the modifier, the distance and amount of light fall off is larger. Hope that makes sense. That is why with a group photo you would need to put a larger modifier further away than you would a smaller modifier, which you could keep closer. It is all relative on the size of the light source.

[@mikeletson1962]

The sun is a perfect example of light dispersion at longer distances. It so far away that its light illuminates an entire half of the earth relatively evenly.

[@byuphoto]

yes, perfect example!

[@LupinYonderboy]

Did he lock that stand before he walked away ? hmm.. ;-)

[@byuphoto]

😂

[@MrKravmagadude]

"and what the inverse square law tells us, is that the model has fantastic long legs."

[@MrBigJonathanD]

This is certainly a better-than-average video on the Inverse Square Law and its application to photographic lighting, but it still gets ONE THING COMPLETELY WRONG. It starts by saying that 100% of the light output can be found at one foot from the light source! Why this completely arbitrary point is chosen to represent the origin of the light - instead of the point of light itself - is beyond my understanding. This concept should be taught using a range of reference points, and examples should demonstrate light intensity Increasing moving toward the point of light, and Decreasing moving away from the point of light.

[@byuphoto]

That is a very interesting insight. I don't recall ever referencing the 1' mark as the origin of light. I apologize if this was unclear in the video. Also, I thought we showed examples of how light intensity changes moving toward and away from the light. I would love if you could point out your specific concerns in the video with time stamps so we can clarify or make corrections. Thank you for your concern.

[@MrBigJonathanD]

@@byuphoto beginning at 1:26, the voiceover to the infographic begins to explain how the calculation is performed, using first a base of one foot, (which is an entirely arbitrary distance). Could just as easily be one meter. Or one Inch. Or five feet. Or 20 inches. At 1:40, the statement is made, "... so at one foot away, you get 100% of the light hitting your subject." Anyone hearing that knows intuitively that it's wrong, as 100% of the light is found at the point of the light's emission, not 1' away from the point of light. Light has fallen off between the point of light and that first foot. One can calculate the change in light following the same inverse-square-law calculation to determine that at 6", the light will be 400% the intensity of the light at one foot. But that's never explained. I would suggest that this video would be better served by choosing an origin point of two feet - or three feet - and working the math toward and away from the light - instead of starting at one foot, stating that it's "100% of the light" and then only calculating out in one direction.

[@byuphoto]

@@MrBigJonathanD Thank you for your concern in making sure this principle is understood correctly. You are right, the fall off of light and distance from the light source is all relative to the size of the light source. 1 foot was meant to be an example, not an exact distance that works in every situation. That is something we could have made sure to specify more clearly at 2:52 in the video. Thank you for watching and for your feedback and concern.