A Deeper Understanding of the Inverse Square Law

Improve your photography instantly by getting to grips with this crucial concept.

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.

In this class:

  • Using the Inverse Square Law in photography
  • Photography lighting tips
  • Studio lighting techniques
  • How to take better photographs

If you enjoy this class, check out Angles of Incidence and Reflection.

Questions? Please post them in the comments section below.

Comments

  1. Hi Karl,

    I have a question regarding an issue I experienced at an indoor shoot I recently did. I really want to understand it in more detail and as it has to do with the inverse square law, I hope you can help me out here :-).

    As I already mentioned, it was an indoor situation in a room with fairly dark green colored walls. My subject was positioned about 2 to 3 meters away from the windows, which were additionally partly covered by foil (to protect privacy within this psychotherapy practice room).

    I naively thought that we could still work with the natural light coming in from the windows, by adjusting exposure correctly (increasing ISO, getting aperture to a maximum and adjusting shutter speed).

    In post production I realized that I can’t work with these pictures. There was absolutely no definition in the subject (face, hair, etc.). Overall, the pictures looked way too fuzzy.

    Now, I really want to understand the cause of this issue and not just see it as an empirical value.

    Is my thought process right here? The light came bundled through the windows and diffused more and more the further it got into the room. About 2-3 meters away from the windows, it was already that diffused and of low intensity that nothing I tried to capture had any definition to it. All looking kind of fuzzy and mushy.

    Or is an additional factor also the camera and its sensor that need more light to create a decent output?

    Kind regards,
    Nina

    1. Hi Nina, this is an interesting question for a couple of reasons. The first thing we need to understand is that outdoors with sunlight hitting the earth the inverse square law doesn’t apply. This is because the sun is so far away that if you were to stand on one side of the earth or 10,000km away the luminosity value on you would be the same. However if you’re shooting indoors with a window as the only light source the reason you have more exposure on your subject if they are close to the window is simply because the window is now the light source and when they are closer to it they are effectively getting more of the light than if they were further away from it. When they are further away from it then much of the angle of light from the window is going up to the ceiling or down to the floor or left and right and they are in a smaller part of it, so essentially it’s the same as the inverse square law even though the light source itself outside is not being affected by inverse square law. I hope that makes sense! Anyway the bottom line is that based on what you have described you do not have enough light/exposure on your subject resulting in high ISO and noisy images. You described the images as ‘fuzzy’ and I guess you mean noisy, if they are fuzzy as in not in focus that’s something else.

  2. Dear Karl,
    In the first shot the shutter was on 1/15 sec and then you changed it to 4 sec which is 6 stops how did you know how many stops exactly to increase the light by or decrease the light by when changing the distance?

  3. desavoiecorp@gmail.com

    0m22s
    Where do you get these white blocks you use to hold your flat surfaces for your product photography ?

    I know I can build it but I am not very handy…

  4. Alhaidan

    Hi Karl,

    Just suggestion I think this video is more suitable to be in the photography lighting section.

    Best
    Alhaidan

  5. desavoiecorp@gmail.com

    I am confused here because at the end of this video when your last shot is farther away the light being much smaller relative to subject, the shadow is supposed to be harder but it looks much softer ???

    1. Hi, I think you are confused between the meaning ‘softer’ and ‘harder’ – this refers to how sharp the edge of the shadow is, so the shadow is harder if the edge of the shadow is a sharper line. It is also explained in the video several times why the shadow is less dense (density of shadows is how black or grey they are).

  6. 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.

  7. 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

  8. 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://visualeducation.com/class/introduction-and-understanding-light/

  9. 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.

  10. 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?

  11. 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.

  13. Laszlo Hajas

    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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