These Two Photos Will Make You Instantly Understand How The Inverse Square Law Works

February 8, 2016 by Udi Tirosh 18 Comments

When I started to use artificial lighting, The Inverse Square Law was my nemesis. Not only it is not intuitive, but it is also not linear, and visualizing how a strobe distance from a subject will impact the photo is not trivial to say the least.

Photographer Derrick Bias shared a few priceless photos that show the exact impact that moving a strobe away fro ma subject has.

One trivial effect, of course it the fact that less light hits the model, but light fall off, background to model illumination ratio and overall contrast also play a part in this game. While I encourage everyone to take the time to learn The Inverse Square Law, and its impact on your photos these photos will provide an instant reference point if you are just starting out.

Lets look at the samples by Derrick.

The first thing that is really obvious is the fact that the background is lighter when the light is further away. This is because the light-to-model distance is not that different from the light-to-background. When the light is closer, than the light-to-background distance is about double than the light-to-model.
The other thing to note is the light falloff on the model. The closer the light, the harder the falloff
Lastly, the closer you are, the softer the light, this is really obvious in the next set of photos

There you have it, instant Inverse Square Law. You can see more of Derrick’s work (NSFW) over on his Instagram and Facebook page.

About Udi Tirosh

Udi Tirosh is the Founder and Editor in Chief of DIYPhotography, he is also a photographer, a relentless entrepreneur, a prolific inventor and a dad, not necessarily in that order

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Michal Rosa

“Not only it is not intuitive, but it is also not linear” – do you ever math?
- Jordan X Randall
  
  What are you saying?
  - Joseph Parry
    
    He’s being a dick about it, however the Inverse Square Law is a textbook linear scale. Not in the sense of 1,2,3,4,5 but in the sense of every time you double the distance you lose 75% of the light.
    
    That is a linear scale, you don’t lose more light at a different ratio etc, it always remains the same ratio of “Double the distance = 75% of the light lost”.
    
    I’m sorry Michal seems to lack the patience to type it as coherently as required to allow others to develop understanding, though such is the way of life.
    
    Cheers Jordan!
    - Michal Rosa
      
      I lack patience with stupid people. How can one finish anything resembling a high school and have a problem understanding a very basic physical law that’s applicable not only in optics? Just a rhetorical question (feel free to look up what “rhetorical” means).
      - Steve
        
        Sure I’ll look up “rhetorical”, right after I look up: “patronizing”, “self-righteous” and “arrogant” and see if all have a hyperlink to Michal Rosa’s profile. I’m so glad Michal took the time to jump on the forum given how busy he is re-engineering the Large Hadron Collider in addition to being the chief physicist behind the only working cold fusion reactor.
        
        “Michal who…”
        “Is he a photographer? I just know him as that narcissistic tool who is total dick to people on forums because he suffers from extreme delusions of grandeur”
        “Oh yeah, ‘that’ guy…”
        
        Don’t quit your day job Michal.
      - Michal Rosa
        
        Big words for an anonymous idiot.
      - Steve
        
        Great comeback from someone who hasn’t contributed a single thing to photography ever. Just keep wasting space on flickr servers while posting sarcastic and condescending remarks in forums so that one day you may be just as hated online as you are in real life. You’ve been doing such a great job at it so far.
      - Joseph Parry
        
        You lack more than patience.
      - James West
        
        Well, some people lack patience with rude people.
      - Michal Rosa
        
        I prefer to be “rude” rather than stupid. Sorry if you prefer to be on the other side.
      - James West
        
        I’m pretty smart, and I’m polite. I’m certainly not arrogant. O, what the hell…I remember something Kipling wrote: “A gentleman is someone who is never unintentionally rude.” In that spirit…you’re just a pedantic asshole. :)
      - Michal Rosa
        
        I am intentionally rude to stupid people but thank you anyway. BTW, as a smarter person than Kipling said “a witty saying proves nothing”.
      - James West
        
        Intentionally rude people generally have a low sense of self worth.
      - Rob Mulligan
        
        No James, “Intentionally rude to stupid people” generally means “arrogant dick head”. Correcting a possible error is one thing, being a dick head is another.
David Liang

You missed a hugely key element, the size of the light source affects the shadow fall off and softness of the light greatly.

“The closer the light, the harder the falloff”

Except if the light source is massive. Ever shot a portrait with a 60″ softbox 6 inches away from the subjects face? You won’t see any hard fall off, in fact it’s probably one of smoothest transitions you can get from a light source.
Mark Niebauer

Here’s Karl Taylors version . . .
https://youtu.be/373eg4BW-NM
Antoine Didienne

this is really interesting but I have a question to make sure I understand this correctly: the article says “the closer you are, the softer the light”. shouldn’t it be the opposite? shouldn’t it be the closer you are, the harder the light and light fall off subsequently? I don’t know… I might be stupid? ;)
- Paganator
  
  If the light source is closer, then it appears bigger to the subject, leading to blurrier (softer) shadows. The sun is absolutely huge, for example, but it’s so far away from that it looks small in the sky and that leads to hard shadows. If you put a large difuser between your subject and the sun, then the difuser becomes a larger light source, for softer shadows.