Lighting 102: 1.2 - Position | Distance

Summary: By the end of this discussion, you should completely understand the following two statements:

1. Light has depth of field.
2. With enough light, you can turn a white wall black.


Leading Off: (1.1) Angle Exercise Discussion

Okay, the 1.1 section had what was admittedly a pretty rudimentary exercise. Which maybe explains why many of you didn't uh, actually do it.

This stuff is the equivalent of "wax on, wax off" in the beginning of Karate Kid, and you really want to explore these things in an environment where you are not also trying to make a real photo at the same time.

The point of this exercise is not so much to stretch yourself, but to just go and do it. Walk before you run. Start building an easy comfort zone and then stretch it.

For those who want to check out the results, you can see the tagged and posted results here. And it is good to see that most of you are navigating the Flickr posting and tagging issues just fine.

The thing that should strike you from this exercise is just how different a three-dimensional form can look when lit from different angles. I didn't mention it at the time, but some of you made the leap to considering angles above and below the object, too. The above composite is basically a matrix of horizontal and vertical light source changes. Click the pic for a big version.

(Thanks for the extra effort, Chris!)

Experience tells me that many more of you guys will be showing up for the "real assignments," but that is the dessert. You folks who are eating your veggies and doing the supposedly boring stuff are going to be much more intuitive when the more complex stuff gets thrown at you.

I'm just saying.

Light Position: Distance

My experience in learning to light (actually, in learning just about anything) is that I do not learn in a linear, sequential way. I tend to learn in fits and starts.

That is to say that I will plod my way through without making much progress, and then something clicks and I move to the next level.

Take snow skiing, for instance. When you start out, your only means of controlling your speed is by forming a wedge with your skis. It's called "snow plowing," and it ain't pretty. You form a wedge with your skis and use the friction of the inner edges to control your speed.

The first transition to real skiing is when you learn how to do a hockey stop, which is nothing more than lifting your skis up and planting them down perpendicular to the fall line with the uphill edges biting hard.

It looks so cool. And you think you are pretty hot stuff the first time you pull one off, with that awesome little spray of snow. (The sounds of screeching brakes actually played in my head.)

But what you do not realize at that moment is that the hockey stop is also the key to nearly everything that follows in intermediate skiing. It is all incremental from there. In other words, that little progression is the key to moving to the next level - and opens up many doors later on.

That is how I have come to feel about learning to understand flash distance as a lighting control variable.

So, pull out those slide rules, folks, 'cause this is where we introduce the concept of the Inverse Square Law

No, no, no. Not gonna do that to you.

Geez Louise, I have an engineering background. I worry about inverted yield curves in the bond market. I build cool stereo equipment from scratch for relaxation. I would eat math flakes for breakfast if I could.

But the Inverse Square Law still makes my eyes glaze over. Not that it is necessarily so hard to understand. (Although it is for many.) But because it just sucks all of the life and soul out of lighting.

Kinda like showing up at the hotel on your honeymoon night with one of those biological/plumbing textbooks from high school to make sure you can exactly figure out the precise plan for the evening's activities. Yeah, it may be accurate. But where's the creativity? Where's the experimentation? Where's the fun?

You gotta lose the math. Here is what you need to know about the inverse square law: The closer you are to the light source, the more powerful the light. Get real close and it gets really powerful. Get far away, and it gets weaker.

And here's the other thing: The closer you get to the light source, the quicker the lighting values change as you move in. When you get farther away, small differences in distance (from the light) become meaningless.

So, let's think about this in the context of a lighting scenario. Let's say that we have a subject about 6 feet from a light grey wall. Like, say, Jason, from last week:

In this case, the light was about five feet from him, and the wall was another ~6 feet behind Jason. As we moved the light around him for the first exercise, neither the light-to-Jason or light-to-wall distance changed much. So our wall is pretty consistently medium grey.

Now take two more shots of Jason, from the same setup:

In the first, we moved the light way back. This, of course made it less powerful. But we adjusted the aperture (opened up) to compensate for that. So Jason is properly exposed.

But look at the wall: It is lighter. Why? Because the flash-to-Jason distance is about 25 feet, and the flash-to-wall distance is about 31 feet. Relatively, those two distances are not very different. So the light does not fall off much between Jason and the wall.

But for the second pic, we brought the flash in close. Like about one foot from Jason. But the wall is ~7 feet from the flash.

Relatively, that's huge difference between the flash-to-Jason distance and the flash-to-wall distance.

We close down the aperture to compensate for the brighter, closer light. So Jason is exposed correctly. But our light grey wall is now about 7 times further away from the light than is Jason. So it goes dark.

Jason is very close, where the light is powerful. The wall is at an intermediate distance, where the light is less powerful. As I move the light close to Jason - without even gobo'ing the flash to block light from the wall, I could easily make that wall go the rest of the way to black.

So, with my subject a few feet away from a light grey background, I can make the background black simply by moving my light in close to the subject.

NOTE: If little bells aren't going off in your head as to some of the doors that this light-distance variable opens up, keep knocking it around. This is a major thing.

Extra bennies? More power, (Argh, argh, argh.) Apparent size of the light source gets bigger, too. But that comes later.

So, moving in gives control and power. The light, in effect, has very shallow "depth of field," which is to say that the exposure that is correct for the close-in subject drops off very quickly behind him. Jason may be at f/11 or f/16. But just a few feet behind his head, you are already down to f/5.6 or f/4.

You gain the ability to light one plane without contaminating the other one. Sort of like selective focus, but with light. Expressed differently: Shallow lighting depth of field.

I could stick another flash on the background and light the two areas independently. That is control.

But sometimes you want a lot of depth of field to your light. Group shots, for instance. You want the front row and the back row to be in the same neighborhood, exposure-wise. Even though they may be a few feet away from each other. So you give up power in favor of even lighting.

That is the secret to this side-lit (but still evenly lit) basketball gym in this shot. The speedlight (main light, camera left) is about 75 feet away in the top row of seats. It is firing at 1/2 power, and I got f/2.8 at ASA 800. But it is lighting a huge area. And pretty evenly, to boot. (More info on the shot is here.)

So, here is the first of our actual lighting control sliders, for lack of a better term:

• Light Placed Closer = more powerful, and control of the depth of the correct exposure.
• Light Placed Further = less powerful, and a broader zone of even lighting exposure.

In other words, lighting has depth of field if you know how to exploit it.

And with enough light - and adjusting the exposure to compensate for the increased power - you can drop the exposure on a nearby white wall to black.

Trust Me, You Want to Actually Do This One

Who wants to guess this week's exercise?

It is similar to last weeks, except you will keep the lighting angle the same and vary the distance. Try to find a place with a clean background and some space to work with. I am thinking living room.

You'll be shooting a person or object in such a way as to use various lighting distances to control the relative tone of the background:

1. Find a nice lighting angle. Set the light a modest distance away - 5 or 6 feet. Shoot at max sync speed with your flash at say, at 1/8 power on manual. Adjust the aperture to get a good subject exposure with the light at the moderate distance.

2. Move the light back. Way back if you can. You may need to pump up the power to get a decent exposure. Maybe to 1/4 or 1/2 power. Adjust the aperture until the subject looks good, then note the background. It should be getting lighter.

3. Now move the light in tight. Real tight. As in one or two feet away. You'll probably have to dial your flash way down to compensate for the distance. Adjust your aperture for a good subject exposure. Note what happens to the background. It should get darker.

What you should find is that you have a surprising amount of control of the depth of field of the light. And this is before restricting or feathering the light in any way. And we will get to that later.

You should also start to be getting more intuitive about where you need to set you flash power to get a good working aperture from a given distance. Keep this up. You are growing a free flash meter in your brain.

Here are your tags for the exercise:

• strobist
• lighting102 (no spaces)
• position
• distance

You would not believe how many shooters out there have a "standard" light-to-subject distance and just give up this wonderful means of control.

Don't be one of them.

Here's a feedback request for the comments:

To the beginners - does this light distance concept make sense to you? If not, did it after you tried the exercise?

To the more experienced - are you already thinking consciously of light distance in this way? If you approach it differently, how so?

Related reading:

Light Science and Magic text, 3rd edition, pp 36-39

(Briefly talks about the inverse square law in more traditional terms. Note: It appears this book is getting to be in tight supply. I am sure Focal will be running off more copies soon, if they are not already. It is a great book. Find it wherever you can.)

On Assignments with long, low fall-off light:

Big Gym, Little Lights
Lighting a Large Interior

OA's with close-in, tightly controlled light:

Compact Fluorescent
Flavored Vodkas

See all of the completed exercises for this section.

Discuss this section in the dedicated Flickr Strobist thread.

Next: Position | Review


Brand new to Strobist? Start here | Or jump right to Lighting 101
Connect w/Strobist readers via: Words | Photos

Comments are closed. Question? Hit me on Twitter: @Strobist


Anonymous dean burton said...

this is all about "fall-off". you have a good explanation here and a nice exercise. I will try it with my class.

June 25, 2007 3:20 AM  
Blogger Pete said...

Superb explanation. You have an excellent ability to deliver information in a way that will be absorbed by your readers.

June 25, 2007 3:31 AM  
Anonymous Kim Hogg said...

I'm not much of a math-head, nor do I have ANY experience with off-camera lighting. I guess that's why I'm here (and in the right place, it seems).

The descriptions made perfect sense (the photos an extra bonus!) and I can't wait to try it out. Looks like it's about time for my first submissions to the Flickr group.


June 25, 2007 4:07 AM  
Anonymous Anonymous said...

I can't try this out until my camera comes back from it's little "holiday" but I have a question.

Thinking in terms of a scientific experiment where you should only change one variable at a time. You say to move the light closer and reduce it's power at the same time and it will give you a sharp fall off in light. Obviously the light-subject-light-background has an effect but what about the power levels. Would it not have proved your point more if you'd kept the power level the same and adjusted the distance and aperture to suite?

Keep up the great work Dave.


June 25, 2007 8:37 AM  
Anonymous Anonymous said...

My experience with websites and blogs might indicate not so many people did the excersise for #1 because not so many people READ the whole thing.

Any way you can paraphrase these? I don't need to know how to ski.

June 25, 2007 8:54 AM  
Anonymous Jeff Geerling said...

I never understood the idea of the inverse law until reading your post... and you didn't even really explain it. That's awesome!

I've always seen these crazy mathematical formulas and diagrams, but never a picture showing what happens when you move a light in close on a guy's face. That was really helpful - thanks! I couldn't try last assignment because I had a lot going on, but I'll try this one out on a small project I'll be working on.

June 25, 2007 9:05 AM  
Blogger Rafael Andrade said...

i had some problems when it comes to publish my exercise on fickr. Even though i had tag it with the correct words it won't apear in the seach. Why is that? If you look for my first exercise it will be under my name Rafael Andrade. But you won't be able to see it if you look for the tag words - that i wrote but the seach seems no to find it. Can you help me?

June 25, 2007 9:24 AM  
Blogger Steve Thurow said...

The Jason distance exercise is a good visual of the inverse square law.

Caution Science content, I love Mythbusters,

June 25, 2007 9:29 AM  
Blogger David said...


You are correct. But some people are not even familiar with the term fall-iff, and I wanted to go even a little more plainspeak than that.


You're right, that would be simpler. But it would also lead to some unweildy apertures, which would in itself be confusing for some people. Back that 1/8-power flash up far enough and your working aperture might me, say, f/1.4.

And maybe f/64 for the close-up. So I am walking people through a process that will become intuitive: compensating at the flash output (power setting) when you make a significant distance move.


Sorry, but the paraphrasing guy in my secretarial pool quit for lack of pay. I am running this series (the whole blog, actually) much as I would structure a more free-form discussion in the classroom. Hence the diversions and points made from tangential directions.

June 25, 2007 9:37 AM  
Blogger David said...


"Light Fall-OFF."


June 25, 2007 9:38 AM  
Blogger Mike said...

With regard to Related reading: Light Science and Magic text, 3rd edition, pp 36-39 You guys might like to know that there are second hand copies a plenty at just copy and paste: Light: Science and Magic: An Introduction to Photographic Lighting into the Title field of the search form and bingo, there they are. Happy reading. Mike

June 25, 2007 9:42 AM  
Blogger geodesigner said...

Hey Dave,

I don't comment much here, but today I'll make an exception. Your explanation was simply one of the best "educational discourses" I've ever read or listened to. Your power to transmit information is one-of-a-kind, you are incredibly skilled in your profession and as a master. When I finished reading today's entry, all I could possibly think was "never stop writing things like these".

Thanks for sharing all your knowledge. When we surf Strobist for "light", you know it's not just strobe light we're talking about.

June 25, 2007 9:48 AM  
Blogger Rafael Andrade said...

and there's another consequence in changing the distance between light source and object. When light gets closer to the object it becomes less softer. Shadows gain a harsh apearence and the diference between iluminated areas and shadows increases. That's because the light source as it gets near to the object increases its size, isn't it?

June 25, 2007 10:04 AM  
Anonymous Justin Liew said...

Dude, that was awesome. In 5 minutes I learned more than I ever thought I could from reading the Intraweb.

Oh, and for the record, I did do assignment 1, but you mentioned that there is no need to "stick these in the Strobit Flickr pool", so I gather a few other people did as I did and didn't do so. The exercise was easy to do and I learned a lot - uploading to Flickr is boring, and so I figured I wouldn't do so this time. But maybe I'll do so post-assignment for completeness :)

Keep the awesome work coming! :)

June 25, 2007 12:01 PM  
Blogger Francis said...

Very "back to basics" assessment. I'm not really a 'pro' photographer, but I'm gaining knowledge everyday visiting this site and practicing. Just recently I was playing an older Vivitar hot-shoe, and had my camera set at a 1 second exposure, and moved the same amount of lighting power and compensated with aperture. Interestingly enough, one of the shots from my subject was on a wall ( about 8 feet away). Aperture ring set to 'bout F/16, and unbeknown to me, his was standing in front of a low-key backdrop. The shoe, was roughly 2-3 feet away from (John) my subject, at about a -36º angle assuming his nose was 0º and gravity accelerates in a negative direction.

June 25, 2007 3:15 PM  
Blogger Jason said...

Speaking of the book Light Science and Magic being in short supply, I looked in 4 different major bookstores and couldn't find it. One of them was in Atlanta, still nothing.

Even when I ordered it from Amazon, it took 2 weeks for it to be shipped.

Glad I have it now though. It's great!

June 25, 2007 3:59 PM  
Anonymous Mithrandir said...

I think you should mention that the f/ scale tracks the inverse square law. If you double light to subject distance, you'll be halving your f/ number to get the same exposure, with the same flash power.

If you're used to exposure, you're used to thinking on that scale.

Even more, it gives you a rule of thumb for distance lighting ratios. If you want to make a white wall black, you underexpose around six stops (the f/ number changes by a factor of 8). So the ratio between your subject to light distance and background to light distance needs to be 8. So if your strobe is 2 feet from your subject, the strobe should be 16 feet from the wall.

June 25, 2007 4:02 PM  
Anonymous Karl Otto said...

(I'm a beginner)

Yes. It makes some sense now. I kinda messed about with the aperture until i found out i probably should have it f5 and then adjust the flash instead. (as you said)

I'm having a little problem understanding it all, but mostly i think it is because i am Norwegian and not fluent in English.

June 25, 2007 4:53 PM  
Anonymous Karl Otto said...


I understand a bit more. Makes it clearer. Having some language issues, but that's okay. I have friends to explain for me ;)

Thanks for the course. And i did the last one to, just didn't upload.

June 25, 2007 4:57 PM  
Blogger Tim said...

Dave, awesome post and an excellent explanation of the inverse square law. I really like your angle of the light having a depth of field. I have a new blog geared toward portrait photographers and created a post about the laws of light today. I included a link to this post in my discussion of the inverse square law because I think it's a great primer on the subject without getting all "mathy". Check it out if you want: Sublime Light. Keep up the great work!

June 25, 2007 5:35 PM  
Blogger wavelength said...

re: last question in post. I had already started the idea, maybe gotten a click from the gears synching, but I think you just made the gears turn ;)

June 26, 2007 12:30 AM  
Anonymous Anonymous said...

Wow, that's great! Really was an 'aha!' moment to understand the control this provides. I'm a relative beginner, but it makes perfect sense.

To counter an earlier comment: I love the analogies like the skiing one. Please keep them!

June 26, 2007 12:58 PM  
Anonymous Alan said...

Great stuff, Mr Strobist! If I am not jumping ahead, may I suggst that, once you have the aperture and flash set for a desirable outcome, change your shutter speed to 1/4 and then to four times, eg start with 1/60" for set up, then 1/15 (4 times) then 1/250 (1/4) and you will see the background start changing.

With flash, set the aperture for the foreground and the shutter for the background.

July 03, 2007 1:35 AM  
Blogger Daniel Williams said...


Following Lighting 102 very closely. I much prefer this to 101, but you have to get the basics out of the way first. I love the way you spoon feed us our medicine in this lesson while disguising it as jello. Easy way for us to learn inverse square law and not immediately shutdown. Also have to say thankyou for not inundating us with lessons constantly. As a paper photographer I find little time to play with this stuff until late at night, and let's face it, some of us have other lives as well. Appreciate all the help you've been to my photography and everyone else's. Keep up the good work and I'm looking forward to moving on to the next lesson.

July 18, 2007 12:00 AM  
Blogger Robert Seber said...

Only a completely unfocused light source follows the inverse square law. Most light sources are at least partially focused.

July 30, 2007 6:53 AM  
Blogger falcopics said...

Excellent explanation for something I've figured out, well I guess really noticed, in practice. As a wedding photographer, if I shoot a couple in their first dance at say f5.6 with flash the background is pretty well lit. But if I open the lens to f2.8 the background darkens. Same TV in both scenarios. So what is happening is the flash has to work less to lite the couple at f2.8 so it is putting out less light so the fall off is faster. Makes sense, and demonstates that the flash is exposing for the near subject. It took me a while to figure this out because it is kinda counter-intuitive. Of course this wouldn't work where the ambiant lighting is equal to or greater than the camera exposure settings. While I am at it, another practical wedding application is the bouquet/garter toss. I have started backing up as far as possible from the tosser so the catchers get some flash exposure.

October 29, 2007 12:28 PM  
Blogger Matjaz said...

Robert, focused light sources follow the 1/rsq law just as much as the unfocussed ones.

Indeed, both groups break the law in the same regime: when source is not much smaller than distance.

Anonymous scientist, one pedagogically clean way to tweak just one parameter at a time (without going into cumbersome f numbers) might also be to expose for subject and move just the wall. Which could turn out impractical in some cases. :)


April 24, 2008 9:59 AM  
Blogger Therese said...

Hi David,

This is a great topic! Its just that 2 photos are missing...pretty please put them back?

Reading Strobist compulsively AGAIN

"Now take two more shots of Jason, from the same setup:
--------- missing photos ----
In the first, we moved the light way back. This, of course made it less powerful."

July 02, 2008 3:45 PM  
Blogger David Weaver said...

A good way to think about this is using a flashlight. That way the continuous light to subject and background distance is easy to think about. So there is the cop in the face light, and there is the searching for lost keys in the backyard light.

The Inverse Square Law is VERY VALUABLE to know, and I would suggest knowing it. If you move a light source twice as far away you lose 2 stops of light. After a while this becomes intuitive. We already knnow this too. Just shoot down a line of people /objects at night with a flash ontop of the camera and you will see this effect. The farther away the darker it gets.

Since the desire is to control light knowing that something 6' away from the flash is going to be 2 stops darker than something 3' away is very useful.

It isn't the math, it is the physics of light that is important.

July 14, 2008 1:01 PM  
Blogger David Weaver said...

Just FWIW:
[Robert Seber said...

Only a completely unfocused light source follows the inverse square law. Most light sources are at least partially focused.]

Focusing simply places more energy per unit. Think of a focusing Maglite. There is no dimming available, just one constant light source. You are only concentration the light in focusing.

When using multiple strobes you need to understand that some light sources may be more intense. A focused beam moving father away still follows the law.

July 14, 2008 1:47 PM  
Anonymous Andy Peat said...

Thanks for this brilliant and insightful tutorial. Really looking forward to trying it out. I do however have a question relating to what someone else posted about changing one variable at a time, I understand that keeping the flash at a constant power would give unusable or unwanted apertures but why not keep camera apertures the same and modify the flash exposure only to compensate for the light fall off?
Andy, Notts UK

November 14, 2008 7:09 AM  
Anonymous Anonymous said...

I understand the lesson though my question is, would a darker or more even background be ideal for the shot? Or is it all just a matter of taste?

January 03, 2009 12:42 PM  
Blogger Sporked said...

This is brilliant. I've always understood the theory of the inverse square law, math and all. I never before now, however, made the leap to using it as "Depth of field" for light. I love this.

January 11, 2009 1:37 PM  
Anonymous Sheri Tanner said...

Thank you for the simple explanation of a very exciting 'light' mathematical law. I had asked a question on strobist group about a light problem I was having with a picture and was sent to this. I am a newbie photographer trying to master light. This may be one of the single best things that I have learned so far about light. (and I actually do know a little about light).

March 02, 2009 10:04 AM  
Blogger digitaldruid said...

No-one's announced a cut-off date for these exercises so I'm posting mine in the flickr group as a newcomer.

Loving the series so far, and looking forward to the rest of my kit turning up in the next few days! :)


September 13, 2009 12:29 PM  
Blogger madmanscam said...

....and then God said "Let there be light, and let David hobby look after it" :) :)I've been reading this blog in the last few days all the way from India ! I must say you are one fabulous guy who has the talent of of being able to teach things so well. You make things look so simple and understandable and you have a great sense of humor.These two things are the most important variables one should possess to be a good teacher(just like the two variables in photography (Now you will dispute that)) Spare me this time, I'm still reading through your blog and will continue to. Keep it coming. You're simply the best.Regards,Arvind

October 26, 2010 1:49 AM  
Blogger MikesMultiMedia said...

Basically what is being said here is put light where you want it, and remove it from where you don't.

Putting/removing done physically (close/far) and by power adjustment (High/low intensity).

Camera - adjusted for reception of the light around it.

June 20, 2011 1:10 PM  
Blogger Absolute and Alive Wedding Photography said...

Dave, great explanation to photographers how the light relates to the subject, this in mind, is also same as focal length and distance to subject. There is that connection, but in this instance we are moving light.

Good read - as always.

September 12, 2011 2:02 PM  
Blogger Albert Massiah said...

Hello Dave, if you have time to answer I would be grateful. When you move the like back doesn't the light get harder ? I could see the result it had on the background but the light looks harder when it is closer to the subject in these examples which is confusing me. Or maybe I need to look again. Thanks

January 01, 2013 2:43 PM  

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