May 12, 2010

Depth of Flash Exposure aka The Inverse Square Law or Flash to Subject Distance

The inverse square law or flash to subject distance video. It has so many names we decided to add another one - DEPTH OF FLASH EXPOSURE. This just makes more sense to us, so take it as you'd like. To us Depth of Field and Flash to Subject Distance have a lot in common. So much that it just computed easier if we renamed it.
Check out the video as we try to describe THE INVERSE SQUARE LAW in total lightenupandshoot style. See exactly how complicated this thing really is, but how easy to understand it should be with our new terminology...ok so maybe it's still complicated but we tried!
Now just to clarify, the measurements we give in the video are not exact. It is simply for you to be able to wrap your head around the concept that the closer the light, the less DOFE you have. The farther away the light, the more DOFE you have!
We are definitely not out measuring anything with a tape measure, but if you want'll see it's close.
We try to teach, like we like to any association with reality might be left far behind...but in the end, we know what we're talking about...and hopefully you do as well.
If you have any questions, just leave a comment and we'll get back to you!


  1. way to coin a new phrase!!

    DOFE is dope!!!

  2. brilliant guys thanks......................

  3. Yes, a very intuitive way to think about it. Thanks!

  4. This is the same thing that Zack Arias explained in his One Light Workshop DVD.

    And there is something already called the Depth of Light ;)

  5. F5,

    Flash to subject distance is something that a lot of photographers have explained!

    Zack's One Light DVD is excellent, and I recommend it to anyone, but he definitely did not call it Depth of Flash Exposure anywhere in his DVD.

    The Depth of Light has also been mentioned many times, especially by David Hobby over at

    This is just another way to help people get it a little easier!


  6. I wrote "fill flash for Dummies" back in 1996.
    The book deals with FILL flash, but uses the long accepted Inverse Square law as its basis.

    Rather than go though all the gimmcrackery and ersatz numerical sophistry and unlike the authors here who eshcewed giving the simplest and most efficacious method of using set values for their flash to subject distance ([i]confuisng at best, juvenile photography at worst[/i]), the Inverse Square law is simplicity itself:
    doubling the flash to subject distance = 1 stop.
    Yeah, they sort of say that but had they used known values, something simple like 1 f/stop = 10 feet ([i]cause we all can ad-subtract-divinde by 10[/i]) flash to subject distance becomes rote, as would f/stops.
    So, as in my book, flash to subject distance = 1, then any flash to subject distance equalling 10 feet equals 1/fstop.
    (Light is brutally simplistic (300,kps) and does not give one hoot what other ersatz math "gimmicks" ([i]or all that valueless talking as that they used-which only confuses the reader[/i]), 1 = 10 is wehat you should use to derive a sought value (f/stop), using 10 = 1 does the trick.

    ([b]"Depth of Light"?)
    More off-the-wall terminology from those who do not understand the Inverse Square Law.[/b]
    The less the Inverse Square Law (10 = 1) [flash to subject distance)) is understood, the more babbling confusion will be spouted about it) ([i]which is why 10 feet =1 f/stop works[/i])

  7. @Anonymous WHAT?I am more confused than ever now...thanks!

    I am not sure if you are complimenting, insulting or flat out plugging your book!


  8. My book is out of print-so not that.
    "Insulting"? How, since I wrote the book (1976) on "Fill flash"?
    Further, "Manual (read: STUDIO) Flash") is so vastly misunderstood, for those of us who were using (Studio) (FULL) manual flash as a rote technique for lighting long before "auto-anything" flash (back in those days) came along, those of you who /know not manual flash/ techniques, even a mostly cogent (the equally misguided OP video) didn't adequately explain the continuing Inverse Square conundraum
    *That is, no matter how well a thing (in this instance the formula (subject distance to flash)is explained, when hard math values are not a part of the working value, you must (MUST) have a math value to begin your permutations, which is why I inserted 10' (though the base distance could easily enough be 1 foot (/or 6 ibches/).

    The OP and extended conversation have helped in understanding the basics of the Inverse Square law, the continuing gibberish (without hard numbers) continues to be...; gibberish.

  9. The Inverse Square Law is a simple math problem:
    In 1973 I was in a class of 186 people who took a convolted test, one of the questions went as follows:
    "How lomg is a line"?
    Only six of the respondents answered correctly (I was not one of them).

    The correct answer was(is):
    "Twice the distance from one end to the other".
    (Which immediately also solves the Inverse Square question)
    OK, it's not a "question" but real problem solvers knew that the first time they read the "Law".