The Inverse Square Law – in pursuit of “applied” information…
Recently, I’ve been trawling YouTube, looking at the advice that is available to people who are looking to improve their photography. There’s some great information out there, but there’s also a raft of hollow tutorials, and the usefulness varies greatly.
I stumbled upon a video that was an interview with a well-known, online photo educator. The discussion was interesting and thoughtful. It contained many aspects of photographic technique and kit. In passing, the discussion touched upon the “Inverse Square Law” (ISL), and the importance of understanding it, as it applies to lighting for photography.
I thought that was interesting, as I hadn’t looked at it since college, so I thought I’d brush up, and do some research, by looking at some tutorials on YouTube with fresh eyes…
““What I found was, at first, slightly confusing, and then, as I watched more, actually quite misleading”.“
I would not like to point fingers at others who, in good faith, are trying to help, however, I would simply counsel them to research the subject before giving advice. I too, use YouTube as a resource to fill in the gaps in my knowledge and I have discovered that many of “the experts” aren’t being very helpful at all.
Do a Google Search like this: “Inverse Square Law, Photography” to see what I mean…Watch a few tutorials and judge for yourself the value of the information they provide.
Having viewed lots of these tutorials, I thought I’d write down some recommendations to the experts, so that they can amend their tutorials, followed by some tips to overcoming the effects of the Inverse Square Law, in an attempt to help others.
The Inverse Square Law, and how to avoid Fall Off V1.1 – Sunday, 11 August 2019
Author’s Note:We are only concerned with the Inverse Square Law, as it applies to light.
Firstly, here’s the law expressed as an equation…
Not very useful for most of us.
If you studied physics, or, applied maths, it’s reasonably simple. If you didn’t, it’s quite complicated. Either way, we are discussing how to avoid its effect. So, let’s put it to one side and promise to never refer to it like this again throughout the article.
How can we do this and still write a useful article? Well, let me give you a quick analogy; We use electricity every day. We plug the kettle in to the socket on the wall, flip the switch and it boils the water. We use the hot water to make the coffee. So, the task at hand is making coffee.
We all understand that electricity is useful but what are it’s first principles? I know that it has volts and amps and there are ohms in there somewhere. If I reached for it, I could probably explain the relationship between them, but these “first principles” about electricity are not important when applied to the coffee. We know how the kettle works; we know how to safely boil the water. We use electricity every day.
We don’t need to understand first principles when it come to electrical devices. In the same way that we approach electricity when making coffee, I will hereafter approach the Inverse Square law by the effect is has on light. That is light “Falls Off”, so a useful question of application would be, “Now we know it’s an issue. How can we avoid it?”
From this point I am going to use the term “fall off” for the effects of the Inverse Square Law, as applied to lighting. This is generally how it is referred to by pro-photographers.
Light “falls off” (gets darker*) as the light to subject distance is increased.
This “fall off” effect is greater when the subject is closer to the light source.
The further away from the light source, the less “fall off” there is.
Light spreading from a point source, according the inverse square law
*I used the term “gets darker” above. This is for brevity. The “effect” is – the appearance of light becoming darker. That’s what the sensor records. In fact, light does not “get darker” or indeed lose energy, over distance, at all. What is happening is that the light rays, or particles, are spreading apart. There is more space in between them, leading to the appearance that there is less intensity of light. The light covers a greater area, with less “density of rays, or, particles”, giving the visual effect of “getting darker”.
Remember: We are considering the “light to subject distance” not the “camera to subject distance” or “light to camera distance”. This sounds obvious, to those of us that have been doing this for a while, but it’s not obvious at all. In fact, many beginners have probably never thought about this. So, we’re talking about light to subject distance.
You can also take a look at this video. It’s is an amusing demonstration conceived by the notable scientist Richard Feynman;
“Richard Feynman’s Butter Gun” explains the effects of the Inverse Square Law. It explains the “fall off” using toast and butter.
- You get more “fall off” the closer the light is to the subject.
- You get less “fall off” the further away.
- This is the Inverse Square Law as it applies to us.
- We call it “Fall Off”
As a consequence, if the subject is nearer the light source, the light will have more contrast and as the light gets further away, it has less contrast.
And, those are the rules as they apply to us.
TA DA! That’s it. Isn’t it?… Only, that’s not it at all…
Back to the tutorials…
What these tutorials are doing is explaining the “first principle” of the Inverse Square Law, as it applies to a very crude and rudimentary studio lighting set up.
I understand that this is a genuine attempt to “keep it simple”, but it’s less than half the story. Almost all of them give a passive explanation of this property of light, without even attempting to apply, or even present the viewer with a practical solution. Presented in this way the information is not very useful. They may as well “describe water” to a drowning man!
In addition to this, several of the tutorials refer to a photographer’s ability to manipulate the exposure that falls on the background using a single light. They reveal, by demonstration, that they can change the tone of the background from white to black, using fall off as the basis of their working practice. Whilst this is all true, and quite interesting, using one light for both the subject and the background is simply poor advice. In 35 years of shooting in studios and assisting over 25 London based photographers, not one of them has ever mentioned the Inverse Square Law to me, or, used it to manipulate the tone or exposure of the background.
I was working as an assistant in and around Central London, during the 1980s & 90s. Of course this was a pre-digital, “film period” in professional photography. Transparency film was king, with our only practical choice being Kodak Ektachrome film or Fujichrome Provia. This meant that the issue created by the inverse square law was very critical indeed. Photographers did not have digital, post production tools with which to overcome this issue. Add to this the restricted, fixed, dynamic range that was available when using transparency film, and you needed to be spot on with your exposures, yet no one mentioned The Inverse Square Law… When faced with the issues that arise from it. What they discussed and requested was “another light to stop the background falling off” or “move that light to stop the shadows”.
Of course, there is a very good argument to understand and explain first principles. Having the background knowledge regarding exposure, depth of field, shutter speed, ISO, and the Inverse Square Law etc, are all essential if we are to apply these first principles concepts, in pursuit of improving our lighting, but the key to overcoming this phenomenon is, and always will be “application, not simply explanation”.
None of these tutorials ask:
“Now you know this fact about light, how can you avoid this?”
“How can we minimise this effect?”
They simply describe the issue, and offer no solution.
Let’s see if we can help a little more?
Here’s a chart that gives a graphical representation of the “fall off.” It shows the fall off in percentage terms, and this can be useful…
“fall off” in percentage terms
So the problem we face is that the light falls off. This happens the further the subject is from the light.
- The graph above shows that you’ll lose 75% of the light you have at 1m.
- By the time it reaches 2m distance and then at 3m it’s down to 11.11%.
- You can see that it falls off more, the closer the subject is to the light.
- The further away from the subject, the effect is less.
OK there’s our problem. That’s how light works. Let’s deal with it.
As professionals, it’s our job to manipulate the light so that we avoid ever seeing fall off, unless we want it, thus avoiding the effects of the Inverse Square Law. The whole point of shooting with lights is so that we gain control, and apply that control to achieve the desired effect.
Experts may like to consider this: A common explanation of light “falling off” gives a “light intensity example” of 100% light falling off to around 2%, this scenario is presented as “real world” and of course it is. The beginner is informed that they may encounter this, but it’s unlikely that they would encounter the effect from 100% to 2%. It may be useful to explain that this is the worst-case scenario, and that they will mostly be faced with a lesser challenge of say 80% light to 50% light… just as an example. What’s not presented is that most “final shots” will reveal a smaller portion of range of the ISL effect. There’s also no explanation that they are more likely to encounter the issue when the subject is larger or when there are multiple subjects.
Let’s get real…
What never happens: No one ever placed a point source light to the side of several objects and shot it from the front. Unless they were making a “tutorial” for demonstrating the Inverse Square Law” this would be folly. Yes, it’s interesting to see it as a demonstration, but a useful addition to any such tutorial would be to prompt the viewer to then place the light in front of the object, thus almost entirely removing the effects of the inverse square law… That would be useful “applied” information. It may prompt the viewer to realise that they have control of the light source.
Surely the object of teaching or training, is to explain the problem, and then, to reveal the solution(s) to the challenge(s) that may be presented to the viewer?
In addition to this, very few of these tutorials discuss the importance of the size of the source of the light, and the resultant effect that it will have. This is a major omission. I can’t find a tutorial that gives emphasis to the size of the light source, in relation to the size of the subject, and the influence that has on the result either?
It’s easy to understand that a point source light, placed close to an object, will have shadows and fall off. Increase the size of the light source and the influence that ISL has becomes less apparent. ISL is still present, but has less effect on the final result. There is less fall off. Move the large light source closer to the subject and the effect of the light source size overwhelms most of the effects of ISL, thus avoiding it… or if you prefer, controlling it.
We can think of a light positioned in this way as enveloping the subject.
7 Tips to avoid issues with “Fall Off” that you may find useful when setting up light(s)
Preface: These tips are based upon 35 years of lighting, initially as a first assistant (the person responsible for the lights in a studio) and as a pro photographer. They take into consideration the first principles of ISL and encourage good practice and technique that will avoid it, without promoting it to first place, when considering your choices, creative options and settings.
Tip 1 – Position the light
Position the light(s) with shadows in mind. When setting up your light(s) you should be looking at the effect on the subject. If you don’t want shadows, move the light nearer to the front of the subject. If you want shadows, move the light to the side of the subject.
Helpful Hint: If you are using a constant light source like LED or tungsten, you can easily see the effect that the light has on the subject. If you are using a flash /strobe unit, without a modelling light. You’ll need to run a test, and “chimp it” on the back of the camera; Effectively, this is no different from what we used to do with Polaroid tests back in the day… just less mess. If you want to get it right… this is how it’s done.
- Front lighting avoids ISL effects on multiple subjects
- Side lighting is more susceptible to ISL effects, especially if there are multiple subjects
- “What about the background?”… We’ll come to that.
- You can reduce the effects of ISL by using more than one light source!
- Colin’s tip: Enjoy the challenge of getting the light(s) right. It’s always a challenge for both beginners and professionals alike. I have spent many hours setting up lights for pro photographers, so that when the “celebrity” subject sits down, the first exposure is really close to perfect. That’s a pro secret. Test before the subject arrives and make your subject feel like a celebrity. Test, test, test.
Tip 2 – Shape the light
Professional photographers rarely use single “point source” lighting. There’s simply no control. It increases the chances of fall off.
You should shape the light by using barn doors, flags or similar. You can concentrate the light using screes and shaper cards and tapes and anything that works for you. Use screens and cards to mask the light. Keep the light discreet. Test it, hone it, shape it.
Click here to view an interesting video on shapers
It could accelerate your light shaping learning.
- You can use anything to shape the light. If you have a budget there are some great shapers and reflectors out there, go have fun and experiment.
- If you don’t have the budget use cardboard or old packaging.
- Go to the Home Depot and buy some 8ft x 4ft polystyrene insulation boards. They come in white, and some even come covered in silver foil. You can easily make them black with a pot of matt black paint.
- Knock up an A frame out of some cheap timber, as stands. Duct tape the edges of the polystyrene boards and they’ll last a long time.
- For smaller shapers simply cut the large boards down!
- If you have no budget at all you can “skip dive” for this stuff, it’s everywhere, if you look. Start at your local refuse tip/recycling centre and you can help save the planet at the same time.
Tip 3 – Diffuse the light
In professional photography diffused, or shaped light is the default. I have rarely shot, or seen others shoot, with a single, bare point source light, pointing directly at the subject. It’s simply not controllable. It will lead to shots with “hot spots”, or uncontrolled reflections, harsh, amateur shadows and unwanted specular highlights. It just doesn’t happen in pro studios.
Softboxes, indirect lights, beauty dishes, fresnel spots, zoom-ellipsoid spots, strip flashes, umbrellas, or reflected lights afford the pro photographer more control.
Softboxes also allow the photographer to get the light closer into the subject…
“What? Wait! Hold on…”
Softboxes allow the photographer to get the light closer to the subject?
How can that be true?
First principle ISL states, the further away the light source is, the less contrast that appears on the subject?
“Surely my light should be farther away?”
In “real world” studio photography this is simply not true.
Yes, ISL applies, if you simply point a bare light directly at the subject… but who does that unless they want high contrast, deep shadow images?
Remember: A soft box, or, indirect light, closer to the subject, will give a softer light, not a harder light.
It will be a less contrasty light, a more even light. The closer the better… If you move a controlled, diffused, shaped light further away from the subject the light will start to produce shadows. The further back the more shadows. This is because the light isn’t from a “point” source at all. The source size has been increased to a larger area. This has the effect of softening, or, evening out the highlights and shadows, thus “cutting” the contrast. This type of light reduces the visual effects of ISL, because it has a much stronger effect on the sensor/film than the “fall off” effect of ISL itself. Once again, I stress you can’t remove the effects of ISL. We can’t change the laws of physics, but you can light so that ISL does not appear in the frame.
Our job is to avoid ISL by working with the appropriate lights, in a controlled exposure range, in which ISL will have no visual influence on the final image.
You should ignore tutorials that infer rules as though you will encounter the effects on a regular basis, when in fact, most of the time, you are lighting to either reduce, remove, or, avoid it entirely.
Tip 4 – Add additional light sources and reflectors as required
A simple way to reduce the effects of ISL is to add another light source, or a reflector. Bear in mind that ISL should not be your primary motivation when adding light sources. It’s best to think about reducing shadows, evening up the light. Making the subject look better. In thinking this way you’ll reduce the effects of the ISL as a consequence of making creative decisions.
Shadows are a real-world, natural phenomenon. They appear everywhere. Use reflectors to reduce their effect. If you place a main light to the left of the subject, causing shadows, place a reflector card or secondary light source to the right of the subject. This will reduce the effects of the shadows… and in turn reduces the visual effect of ISL. By thinking like this you have applied a solution to the effects of ISL (the shadows) and applied your options to the priority (the shadows), not the ISL that caused the shadows. This is a professional approach to overcoming ISL. More than one source of light can be effective at reducing shadows and therefore ISL. Rocket science it ain’t!
Of course, there are 1001 variations on lighting set ups. You need to learn 5-6 of them to have a battery of lighting set ups that could cover an entire career.
Tip 5 – Correctly position the subject
Using ISL as a primary reason to control the light that falls on the background is simply not advisable. I’m not really sure why anyone would advise a student/newbie to do this?
The professional approach is to correctly position the subject, so that the light(s) used to light the subject, does not influence the light that falls on the background… and then light the background with an additional light… separately!
There’s no point in protesting “What if there’s not enough room to do that!”. Shoot in a bigger space! You are compromising overall control with poor technique. This can lead to poor results with less quality.
Remember: Place the subject far enough away from the background so that the light does not influence the exposure of the background. In practice that does not mean that “no light” should fall on the background.
The secondary light used to light the “background” should give more light than the residual light used to light the subject. In other words, as long as it’s brighter, its effect is removed.
By doing this you now have control of the light on the subject and the background. The subject light and the background lights are now independent of each other.
- You now have control over both lights.
- You can now choose which light to prioritise.
- You can now independently adjust the settings for each.
- Now you can “balance” the subject light to the background light, or vice versa dependent upon your priorities.
You see. You now have more “creative” control.
And, by the way, after a few tests, this becomes much easier than it sounds.
Tip 6 – Choose the appropriate coloured background
As a rule of thumb, don’t use the main light as the “first tool” to control the tone on the background*.
Your starting position should be simply using a coloured background. If you want a white background, use a white background, with an independent, controlled light. If you want grey use a grey background, with an independent, controlled light, and, you’ve guessed it, if you want black, use a black background, with an independent, controlled light. If you follow this “common sense” tip you can see that it’s a better starting position and thereafter easier to control the look of background with, or, without a light. It gives you more opportunity to side step ISL, and more opportunity to balance the subject to background exposure without compromising or influencing your camera and lighting settings from a creative point of view.
*It’s easier to make a white background slightly whiter by independently lighting it, and the same thinking applies to grey and black.
Handy Hint: Matt backgrounds and matt paints make better incidental backgrounds, as the light is absorbed, and is less reflective. If you are looking for a black background, velvet has wonderful light absorbing properties, and hung properly, gives a very black, clean look.
Remember: If budget is an issue, and you cannot use a second light source, it may be worth learning how the background can be influenced using the techniques described within the tutorials, it’s clever and may be useful, but, in general, this isn’t how it’s done in a commercial pro studio. By using the main light to light the background you are not prioritising your options, you are limiting them. Using this technique restricts the creative choices available to you.
Tip 7 – Keep it simple, Think critically. Get it done!
I came across a really good article on cornicello.com that has value. It looks at the Inverse Square Law and other interesting aspects of lighting that you may find of some use. It demonstrates that moving controlled lights closer to the subject will give the opposite effect that most tutorials imply click here to view it.
As we have discussed, this is not a simple subject with easy graphs giving simple explanations. Neil Oseman has a great article that raises more questions than it answers, it does it in an informative way. He discusses, with subtlety and nuance, and I suspect, bundles of professional experience, issues that affect light and lights that bend the inverse square law by focusing them, avoiding spread and thus avoiding the law itself, or do they?.
Anyone know what a virtual Point light source is?
Can PAR lamps reduce the effects of the Inverse Square Law?
If you place a fresnel screen over the lamp and it focuses the light, does that reduce the effect of the Inverse Square Law?
Interesting stuff… you can find some answers by clicking here.
If you find this article useful, let me know by posting a reply. If you find a mistake, or an inaccuracy, let me know by posting a reply. I may update the article with any useful additions and credit you – Colin