Geek Articles

Simplifying the Complexities of Lighting in Photography

As someone who teaches workshops tailored to lighting, I’m always reminded how many people have a gross misunderstanding of how light travels and is used within photography. And while I usually start my workshops with an open lesson on the reality of how light functions. I’m frequently reminded, how, even the most experienced photographers, don’t have an accurate understanding of light and it’s fall off properties.

So I figured, if Roger is on this blog, taking apart lenses, debunking myths about filters, and showing uber nerdy MTF charts of the latest lenses, then I can show some of my expertise on lighting as well. And as always, I want to start with the Inverse Square Law.

Inverse Square Law

The Inverse Square Law is a pretty fundamental part of using lights within photography. If you plan on shooting anything other than natural light, I consider this rule to be a fundamental ‘must know’ law in photography. The Inverse Square Law, in mathematical terms, is broken down as follows —

Intensity is Proportional to 1 Over Distance Squared


In English, this translates to ‘Intensity is proportional to 1 divided by distance squared.’ And while I don’t expect all the readers to see a math equation and immediately understand what it means, it can be broken down into pretty simple terms. All you need to know in reference to the inverse square law is that it measures the falloff of light over distance. To put it simply, each time you double the distance of your light to your subject, you lose 3/4th or 75% of your output of light. This seems complicated, as most people assume that doubling the distance would lose half the power, but no one said that lighting was easy. Double the distance, lose 3/4th of the power.

Inverse Square Law Photography

© Zach Sutton

What this means, is that you don’t need nearly as much power as you think you do, and that distance is far more important than power. For example, a strobe like the AlienBees B800 is a standard light for many portrait photographers, and usually, has enough power to be able to shoot on location. At 320Ws, people consider this strobe to have plenty of power. But the inverse square law tells us the power has a greater falloff with distance, so if it’s 320Ws at 1 meter from your subject, it’s only putting out an equivalent of 12.8W at 5 meters away.

The fall off is more extreme at the shorter end of the spectrum (the power change from one meter to 4 meters (from 320Ws to 20Ws) is more significant than the power shift from 10 meters to 20 meters (3.2Ws to .8Ws), so the secret to lighting is that distance is everything. If you want your light to illuminate your subject, without having drastic effects on your background, bring your light as close as possible to your subject.

The Power Output Reality

The most common misconception when it comes to lighting is that the standard unit of measurement means something in regards to light output. Most lights are measured in Watt Seconds, which has foolishly became the standard for lighting gear within photography. Much like how speaker size doesn’t measure decibel rating in music equipment, Watt seconds doesn’t give an accurate reading of how much lumens each light is creating. In short, Watts is a measure of the electrical power the light itself draws, and so watt second is how much power it draws in a given second. Surely you’ve seen this if you’ve ever swapped out one of your traditional incandescent light bulbs for a LED bulb. While the wattage used is much less, the actual light output (generally measured in lumens)  is the same. A photography strobe’s actual power output is going to come down to a number of different reasons. And while Watt seconds is the universal term to describe light output, we’re developing a way to better deduct how much output each light is capable of generating (stay tuned).

But continuing on that trend, many people don’t even understand the power rating scale on their own lights. Using the Profoto B1 as an example – with a power scale going from 1-10, with 10 being the highest setting, where do you suspect half power to fall on the dial? Many assume 5, but the reality is that half power, or 250Ws, is actually 9. The same applies to AlienBees and just about every other major strobe manufacturer on the market, and relates back to the inverse square law and how light intensity is inversely proportional to distance. So 500Ws is only 1 stop more powerful than 250Ws, and 1000Ws is only one stop above 500Ws. Understanding this, along with the Inverse Square Law gives you a better understanding of the capabilities of your lights, and their output. Generally speaking, one of the biggest advantages of the large and cumbersome ProPacks is that they have output ratings of 2,400Ws and above, allowing you to better take advantage of a space, and spread out for your shoot.

Softness and Size Relativity

Another thing worth discussing is the extreme diversity of a single lighting modifier and how it corresponds to softness and hardness of light. Without question, my favorite modifier is my 2’x3′ Softbox, because it is considered medium by just about all standards, giving it an immense amount of flexibility. By adjusting its angular size, you’re able to get a broad range being hard light and soft light.

In simple terms, angular size is based purely on the perspective of the subject. When you have a 2’x3′ softbox a foot from your subject, the light itself appears massive from their perspective, obstructing much of their view. Because the light looks large based on their perspective, it’s going to create softer light naturally. The smaller the light source, according to their perspective, the harder the light is.

And there is no better evidence of this than our own Sun. The diameter of the sun is 864,576 miles – quite obviously the largest light you will ever have access to. But because the sun nearly 93 million miles away from Earth, it appears small from our perspective (Just trust me, don’t go out and stare at the sun). That gives the sun incredibly hard lighting – harder than pretty much any other light source available to you. While obviously bare lights can still produce extremely hard and dramatic shadows and highlights, it cannot fully replicate the harshness of the sun on a cloudless day. This evidence can be seen by standing outside with a grid on a sunny day. Notice how the grid displays its pattern correctly on the ground below? Simply put, you cannot do that with a traditional strobe – without the use of some specialized tools.

A beauty dish grid projecting its pattern perfectly on the ground below, thanks to harsh sunlight

One of those tools comes in the form of something called a Hardbox. To my knowledge, Profoto is the only manufacturer who makes a Hardbox, but how it works is pretty simple. By seating the light within the modifier at an off angle, you’re able to drastically cut the size of the light source down, to better mimic the sun. This allows you to get the hardness of light found from the sun while maintaining control over your angle and power. Though this is just an example to show that your bare strobe isn’t the hardest light obtainable, as we don’t generally rent out speciality items like the Hardbox. 

So gaining full control over your lighting comes in with an understanding of both the inverse square law, as well as angular size relativity of your lights. Learning those two principles allows you better control of the softness and hardness of your lights and modifiers, as well as a better understanding of the power output of your lights and how they affect your scene and subject.


While I don’t expect you to remember every single one of the figures and facts I’ve presented here in this article, I do hope that you’ve learned something, and are better aware of how light functions when using it within photography. Like all things photography, you can nerd out and talk figures all day long, but to fully get an understanding of the principles taught, it’s best to get out there and practice them. We have a broad range of lighting equipment for both photography and videography, so give us a call and we can help you find which tool might be best for you.


Author: Zach Sutton

I’m Zach and I’m the editor and a frequent writer here at I’m also a commercial beauty photographer in Los Angeles, CA, and offer educational workshops on photography and lighting all over North America.

Posted in Geek Articles
  • Petrochemist

    As in speedlight Guide numbers?
    Unfortunately studio flash units have this vary very much based on the modifiers used. It’s fairly rare to use them without some sort of reflector/umbrella/softbox…

  • Douglas Dubler

    Zach I suggest you spend some time looking at Broncolor flash equipment. Without question they make the best light modifiers and their packs are adjustable in tenths of an fstop. That feature they have had for more than 20 years. I have used their equipment for over forty years. You can rent it from Samy’s.

  • Luigi Gallerani

    Question, should this equation work only if if the light source is omni-directional?
    as soon as you use reflectors or fresnel….. light decay is much less as you have a collimated beam! Extreme case: a narrow collimated beam like a spotlight or the extreme case the laser, does not follow the rule… In studio we usually have at least a parabolic reflector or fresnel lens, at least, a parabola and a fresnel lens is mounted on ALL flash units.

  • Peter

    Why would anyone talk about watt-seconds in the first place, whether or not that’s measuring the right thing? Did marketers think that “Joule” would sound too foreign?

  • Ian Goss

    The inverse square law only operates with point sources; as soon as your light source becomes larger, fall-off is more gradual with distance. That’s a good thing!

  • decentrist

    light is a circuit, not photon particles

  • Ian Goss

    The inverse-square law only works with point sources—the larger and more diffuse the light source, the more the law fails! This is good news for umbrella and soft-box users!

  • Sator Photo

    I applaud Zach’s attempt to return discussions on photographic lighting to its basis in physics. A with lenses, this helps us to cut through hype and question popular misconceptions.

    One thing that needs to be debunked is the notion of “soft light” vs “hard light”. Photons do not come in two varieties, one soft as a marshmallow and the other hard like a billiard ball. Light is either diffuse or directional (collimated). If the light beams from a source are parallel (collimated and hence directional) then this means that you get very well defined shadows because less light is scattered (by diffusion) causing spillage of light off the highlights into the shadows in such a way as to contaminate the shadows, softening the shadow edges. You could talk about soft vs hard shadows, but a system more based on physics would be to use the terms “diffuse vs directional lighting”. Naturally this distinction is not binary, but you can get every gradation of shades of grey in between.

    Diffuse light is most commonly produced using multiple layers of diffusers and circular/non-parabolic reflective surface. Although many people teach the principle that the angle of incidence equals the angle of reflection, this really only works with an idealised reflective surface. In the real world no such an absolutely perfect reflective surface exists. However, the more mirror like the reflective surface on a light modifier is, the closer it becomes to an ideal specular surface. By way of contrast a white reflective surface tends to cause a great deal of light scatter as light bounces off it in such a ways to cause it to become less collimated and more diffuse.

    The are different ways to get directional lighting. A Fresnel lens was originally designed for lighthouses and focuses light so as to collimate the output to make the light beams more parallel. You can also use a grid to filter out beams of light that are wider than a certain angle of output. You can also make the reflector shape more parabolic to collimate the light output.

    The problem with the Profoto “Hardbox” is despite the name, it has nothing to meaningfully collimate the light output e.g. a Fresnel lens or grid etc. Sadly, it suggests that the name is largely an advertising gimmick because it is not based on physics.

  • SpecialMan

    The great thing about this site is that we’ve been trained; just as a marathon coach conditions runners by gradually building up the mileage, Roger has conditioned us by writing ever longer, more esoteric articles. So be as detailed and obscure as you want and we will follow you. Our eyeballs have the endurance of champions!

  • Hoping to. Essentially, we’re just hoping to build a standard and independent testing that measures the variance of lights, as well as actual light output and color temperature. But getting the standard correct has a whole heck of a lot more variables than lens do, since the life of the bulb, the color measurement of the walls, and so on all would come into play.

    We’re just wanting to make sure we get everything in order before we knock anything out.

  • SpecialMan

    Great article. As far as deducing light output, how about defining strobes in terms of the f-stop they produce at a specific distance and ASA? That’s one of the standards now…Or do you guys have some uber-scientific method that’s going to blow us all away with its elegance and precision?

  • Max Dallas

    No !

  • Jeremy Mills

    I would read all 3000 words of that. I think most of us would. 🙂

  • Athanasius Kirchner

    That would be great!

  • Graham Stretch

    Hi Zach, Athanasius.
    Great article, put me down for the next session on Better Beamers, I use one but cannot decide whether the flash zoomed to 50mm as recommended or a different flash zoom setting is better for different ranges so any insight you may bring would be great.

    Cheers, Graham.

  • Haywood Jablome

    Just straight up knowledge. No fillers, just the meat! I like your style.

  • Rupert

    intensity is proportional to the inverse of the distance squared.
    intensity is proportional to 1 over the distance squared.

    While the mathematical equation shown is correct, your verbal description is not.
    By including both “inverse” and “1 over” in your written description you have applied the inverse twice. I.E. indicating that the light gains power with distance.

    I don’t intend to take away from the rest of the practical lighting advice I assume is in this article.
    I have to assume because I was unable to get passed this point and will need to return and continue reading it when my brain stops hurting.

  • Hey Athanasius,

    I can certainly look into it. Admittingly, I know very little about fresnels and zooms within speedlights, but I have a fascination with the topic, so as time will allow, and if there is enough interest, I’m happy to put together another article that dives into this subject some more. There is certainly more to add, I just didn’t wanna bore you all with 3000 words nerding out about lighting.

  • Athanasius Kirchner

    Good article, Zach, this contains some very good pointers.
    Could you make another one dealing with simple flashguns and Better Beamers/fresnel lens addons? It’s a sort of arcane subject, but you have a firm grasp of artificial lighting, and I’d love to get some solid advice on them for birding.

  • Thanks Mike!

  • Mike Carnevale

    Good read

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