A Deeper Understanding of the Inverse Square Law

The inverse square law is one of the most important, but, unfortunately, often misunderstood, lighting concepts in photography.

Quite simply, the inverse square law designates that a specified quantity of light is inversely proportional to the square of the distance from its source.

If this still sounds confusing, don’t worry! In this class Karl simplifies everything as he details how it relates to photography, why it is an important concept to understand, and how it can be applied to better control light.

Comments

  1. Hello Karl.

    Some questions of this lesson.

    1. Why step two in the inverse square law is 9 and not 8?

    2. When you take the picture, do you read the exposure, the RGB, what is this function called in Capture One or does it only work with Phocus?

    Thanks.

    1. Hi, can you tell me what times in the video you are referring too please so I can check before answering. Thanks Karl.

  2. Hi Karl,

    Apologies for my side-tracking off topic, but is there a function in Adobe Lightroom or Photoshop to make the RGB values of colours visible at the place of the cursor, like Phocus (or Capture One?) software does in the video?

    Hannu

    1. Hi Hannu, not as far as I’m aware I’m afraid. But if you’re on a mac you can use the mac tool ‘digital color meter’

      1. Hi, and thanks a lot! I never even knew such an app existed in my Mac, but there it is indeed and seems to work quite well.

        Cheers, Hannu

    2. Corey (KTE Team)

      Hi Hannu, you can use the Info tab (default F8) of Photoshop and the Develop tab of Lightroom Classic you can see the RGB values at your cursor

  3. Hi, can you tell me why the inverse square law is in the softbox class instead of the hard reflector class?
    Is it because the hard light reflectors are all nearly parabolic in shape and therefore can emit parallel light through the focal point and are less affected by the inverse square, or some other reason?

    Second, according to the inverse square law, the closer the light is to the subject, the lower the light density and the greater the contrast, that is, the harder it is.
    But why does the softbox get softer the closer it is to the object?

    1. Hi Jiansheng, this video is in our ‘getting started in product photography’ section and I demonstrate the differences of the inverse square law with a parabolic modifier compared to a point light source. Of course a point light source is the best way to describe the inverse square law. I don’t understand what you mean by lower light density? The fall off of exposure of light or the change in light exposure is always more rapid when the subject is close to the light source regardless of the type of modifier. Contrast is only less with a soft box because the size of the light appears bigger to the subject so the angle of incidence of the light can reach more parts of the subject. Please also watch this class: https://karltayloreducation.com/class/introduction-and-understanding-light/

  4. Hello Karl,
    Is the distance of the first point from a point light source important to create that pattern where light exponentially decreases from point to point? I would also like to ask why and how the inverse square law finds less application when using a homogenous light source such as a softbox or similar. Thank you for shedding some “light” on this.
    Regards.

    1. Hi Myles, the inverse square law still applies to a softbox but it simply can’t be measured in the same way as the light is emanating from its source in significantly different positions and not from the same point. I’m not quite sure I understand what you mean in the first part of your question?

      1. Hello Karl,
        What I was asking, in other words, is if it is crucial where the first point from the light source is set. At 1:03 you show an instance of the inverse square law but I do not quite understand first if and how you set a distance from the light source for point 1. Is there a rule to follow to do so? Also, you say at 1:14 to move the same distance to set point 2. Do you mean the same distance from the light source to point 1? I do not mean to nitpick but I really want to understand this.

        Thank you for your time.

        1. Hi Miles, I’m not entirely clear what you’re asking here but I think you’re trying to ascertain if the markers I was laying down if there was a rule to the drop off in intensity. If so yes there is and it’s similar to what I showed on the wall in this video but but it is not something you need to overly concern yourself with in photography terms because the rule only applies to a point lights source in somewhere like the vacum of black space. Anywhere else or with anything else such as a softbox or a white studio and other objects and then there will be bounce back from the light in the studio to your subject which will change the ‘rule’ of the inverse square rule anyway. And as every studio or space is different then it will be different every time. The best thing is just to understand the concept of what you saw in this video and be aware of it’s effects especially when it comes to having a light too close or too far to a subject and how that will affect contrast and backgrounds. If you want the actual mathematical run down then the inverse square law states that the quantity of light is inversely proportional to the square of the distance. Or in simpler terms if the source is twice as far away, it’s 1/4 as much exposure. If it’s 8x farther away, the exposure is 64 times less.

  5. Great video. Is there a way to set those tags in Lightroom like you do in Phocus that let you look at exposure in those areas as you adjust?

  6. Great example Karl, thanks for posting it. That said, it makes me wonder how effective my tiny home studio (16′ x 24′) is if I want even light throughout a setup.

    1. Hi Apaq, it will still have an effect especially compared to a light very close to the subject as opposed to bouncing it of the studio wall.

    1. Hi Louie, yes at some point soon it will be subtitled, it is a gradual and expensive process but we are working through all of our courses.

  7. Laszlo

    Dear Karl,
    Thank you very much for this new, great video on this very important subject. Very useful, easy to absorb and understand what the inverse square law is all about, especially with the practical demonstration of the apple.
    Best
    Laszlo

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