Lighting Distance Formulas

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  • #169498

      Hi Roger, James and the Forum,

      For example, when lighting a daytime interior scene from the outside with large light sources coming through diffusion, outside a window. What is the calculation for the distance of the light + diffusion (outside the window) to the subjects (inside the room)? Is there a mathmatical calculation for how far away the lights should be or is it totally dependent on space? I’m guessing you want all your subjects to have similar fall off inside the room so would inverse square law apply?

      Many thanks,



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      Andrew C

        I’m curious what others have to say on this too.


          Here is the calculator I wrote on the subject

          Gabriel Devereux | SOFT-LIGHT CALCULATOR

          I’d recommend having a bit of a read on a few other topics in this forum.

          I find you can assume most laws of light with a firm understanding of the inverse square, basic geometry and a few orthogonal laws (lamberts cosine). As you introduce different levels of reflectivity and specularity of reflectors (very non-Lambertian), it gets increasingly more challenging – silver stipple and gold stipple being an ultimate pain, but typical soft reflectors – Ultrabounce, Muslin falls into the realm of Lambertian-‘enough’ and Mirrors fall into the world of the exact opposite (just basic geometry).

          Re-Inverse Square Law, it’s a statement of proportionality, not absolute magnitude. By that, I mean the whole double the distance half the output is an assumption that the energy distribution is proportionate to the above ratio. In optical far-field and such, the above statement applies. Still, people’s common misconception about Fresnels, Ellipsoidal Spots and Lasers and that they don’t conform to the inverse square is untrue.

          However, one thing I will say. As an industry, a craft, we have somewhat limited our technical understanding of what we do, using the subjectivity and preference of the term. Some say some aspects of light calculation are impossible, and so forth.

          That’s just all categorically untrue. It’s just that the education around our industry doesn’t represent this. EMF calculations exist for RF propagation, and the same laws apply.

          As an example of my rant – we say one camera has a ‘superior’ colour science to another while not understanding the fundamentals that all are an approximation of an HVS response. A manufacturer takes artistic liberty in its CMF OETF. The Alexa doesn’t have ‘superior’ colour science to another; it just looks nicer, it may have spectral primaries that are more indicative of its look concerning RED Monstro spectral primaries, but one can achieve the same result – blah, blah.



            Continuing on this! Re how far away your light should be from your reflector. Have a play with the calculator. The amount of your reflector you illuminate, the amount used, and so forth.

            That’s just more look at the beam angle, draw an isosceles triangle and see what happens.



              im curious about you guys but i`ve always had a really hard time with math and equations, so i find myself really having to abstract all of these ideas into a mental 3d model i can understand, it just turns out light is one of the most difficult things to abstract in your mind without being reductive and completely deforming how light really works, im curious about how you go about these things on set.


                The general consensus is –

                Buy a light, a diffuser, a reflector and play around with it for a few years (or days) and, you’ll get the hang of it.

                The alternative is to hire a gaffer with said above experience (preferable to young cinematographers).

                The alternative is to give high-school geometry another crack – your understanding of a camera, power and so forth is far greater in complexity than most of the equations applicable.

                It’s just taking the time, maybe downloading python, and exploring that aspect of it. In my head that’s the only alternative to real world practice. Equations are useful as you build them you learn and see patterns in light propagation. Therefore on-set you can make judgement calls just based of that knowing – you don’t need to whip out the calculator unless the time to rig and test greatly outweighs the 10-30 minutes it’d take to calculate it.



                Roger Deakins

                  I think there is some confusion between fall-off and the size of a light source. They are not connected. The fall-off of light from a source follows the inverse square law. How soft that light is on the subject is governed by the size of the source relative to that subject. That is simple geometry.


                    I can try to give my two cents here. Not as experienced though as other members here, so let me know if there is something I got wrong. Don’t get hung up too much on the math though.

                    1.) First important for you to know is that every exposure setting translates into an absolute number of illuminance you need. Quick way to remember is that with 24fps and a shutter of 180° (1/48s), shooting at T2.8 and 100 ASA, you need 100 footcandles of illuminance. You can measure that when u put ur lightmeter into footcandle mode.
                    The rest is pure math. If you wanna rate ur Alexa at the native 800 ASA, which is 3 stops more, you need 1/8 the amount of light for the same exposure, so 12.5fc, let’s say ≈13 fc. You wanna shoot at T4.0? Okay, so you need double the amount of light, which is 25fc at 800 ASA.

                    2.) But how do I know which light to use now? Do I need a 2.5K HMI or a 4K? As said above also by the others, don’t get hung up too much into the math. Good gaffers know from their experience what to use.
                    If you still want to prepare yourself, you can use the tools the manufacturers give you. Arri for example has to Photometrix App, where you can choose your desired fixture and dial in the distance and it gives you the amount of illuminance in fc or lux. Just play around with it. Every manufacturer will give you data of certain illuminance values in certain distances.

                    3.) How to put that into practice? Well, for a project, most of the time your budget is the limiting factor. So good way is to calculate if the biggest light you can afford still gets the job done, will say gives you enough light at a certain distance.
                    Your production can afford an M18 HMI maximum? Great, because when you dial in the numbers, you will find out it gives you 13fc at distance of 80ft or 25m. Which is fairly enough.

                    4.) So far, we are shooting in direct light. But what happens when we bounce or diffuse our light?
                    Bounce or diffusion always take away light. If you want to keep your exposure, you have to put that into account. It is hard to measure exactly how much it takes away. I didn’t have the money or equipment to test it myself, but I read a few tests, e.g. the one from Matt Porwoll (check out his blog!). I did some math, and if I am right, you get roughly this loss of light when the bounce or diffusion is at around 50% of the distance between light and object:
                    Diffusion: ≈ -1 stop of light loss
                    (216: -1 1/3 stop; 251: -2/3 stop; Full Grid: -1 1/3 stop; Half Grid: -1 stop)
                    Bounce: ≈ -2 stops of light loss (UltraBounce)
                    Bounce & Diffusion: ≈ -3 stops of light loss
                    (with Bounce at 1/3 of the distance and Diffusion at another third,
                    e.g. Light – 6′ – Bounce – 6′ – Diffusion – 6′ – Object)

                    5.) Talking illuminance, let’s take our light loss into account with our numbers from 1.): So if you want to create a daylight lighting situation just as Mr. Deakins does often, with HMI bounced into UltraBounce and 251 on the windows, you know now you loose 3 stops of light. If you still shoot at T2.8 at 800 ASA, you now don’t need 13fc but 100fc, or 1070lux. 13fc btw equals ca. 140lux, if you use metric system as I do.

                    So roughly, you can say this is your light loss and this is how you can calculate. Of course this changes when you change the distance between light and bounce or diffusion. But this is why you shouldn’t be too much focused on the math; it’s is never accurate enough. But it gives you a chance to roughly encircle the type of light you need and the amount of illuminance you need. Especially for low budget productions this comes in handy.

                    Let me all know if I am right here!

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