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Understanding the Inverse Square Law

Lesson 7 from: Anatomy of a Photoshoot

Mark Wallace

Understanding the Inverse Square Law

Lesson 7 from: Anatomy of a Photoshoot

Mark Wallace

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Lesson Info

7. Understanding the Inverse Square Law

Lesson Info

Understanding the Inverse Square Law

Okay the next thing we're gonna do is sort of radical and uh it's gonna take us a second to prep it so we're gonna have all of our students come out here we're gonna move this out of the way and up on the screen here I'm gonna put this little um formula so I know I said I was bad at math and it's true this formula is called the inverse square law okay? You may have heard of the inverse square law before it is uh one of those laws in photography that I don't know it's talked about people like it but uh it can be confusing here we go so the actual definition is something like this was going to read this to you in physics an inverse square law is any physical law stating that a specified physical quantity or string is inversely proportional to the square of the distance from the source of that physical quantity I did doesn't make sense to me to make sense to you guys okay? I'm sure somebody out there's like oh yeah I know that I think of a basketball that zack told me yesterday think of a...

basketball as he grows so we're gonna do is we're gonna make this real and uh the inverse square law I will dramatically affect how you think about photography and how you shoot you first have to understand what it does now technically speaking uh speaking what I'm about to show you will only work from a point source of light so I asked to be round okay, so we don't have any round point source of light were using soft boxes and stuff so when you go home and you get into your studio and you meet her this stuff and you try this out and it doesn't work and you're like wow wallace lied to us now wallace live as well as lied no, I did not lie to you what's happening is this this physical thing only works if the source of the light is actually physically round okay, just go out teo wikipedia and you'll see all kinds of church and diagrams and stuff exploding it's really cool so what? We're going to show you the principle still work they absolutely still work but when you're meeting and you're not going to get specific results that I described you okay? So that's the disclaimer so what we're gonna do here is if we did have a point source of light right here and one foot away let's say this is uh we have distances that are doubling so have one foot two feet four feet, eight feet, sixteen feet and thirty two feet of course this isn't really distances because enough room, you know, you get your fine uh so what we want to know is how does light what is light? How does like behave across this distance as is travelling from the source of light what's going on with it okay and really what we care about is how does light fall off? How does this affect this one? We're shooting groups and portrait and how does it affect our backgrounds and our death? All kinds of stuff so normally what we would think is if at one foot we had full power we have full power full power that's gonna be a t shirt ok, if we have a full power at one foot don't flip yet over here two feet probably what your thinking is we have half power, right? No. What do we have one quarter power all right there's a reason for that because the amount of light is inversely proportionate to the square of the distance from the light which means this right here is two feet two squared equals four the inverse of four is one quarter so my mathematicians tell me so we get one quarter power or two stops less like so what just happened is we went lots ally uh tired just like that. Okay, the light just fell off over here at four, feet it goes from quarter power to what showing one sixteenth power we're just using this square loss so four squared equals sixteen the inverse of that is sixteen and we can keep flipping that over so go ahead and flip everybody all right so you can see that light is like lots of light and then it's falling off as we're going along it's just getting tired uh tired okay it's actually not getting tired is getting larger and look that up on wikipedia you'll see a chart not really understand this I'm not a fractions person we've made these into percentages so let's see the percentages of power this cool you have students like this they're awesome they're not students their teachers okay, so we've rounded these okay? Math people don't beat us up we've rounded them so it's lots of light hundred percent like boom power goes down twenty five percent a long way down to six right? And then over here it just sort of like cam cruising whatever I'm tired I don't give a rip so the light really calms down over here so what that means is if you're shooting something and you want really high contrast and you want the light to fall off and just really be dramatic, well, you need to have that like really close so that the light just runs out of energy over here and this is all going to be dark because our dynamic range we could sort of use that to make sure that we don't have any kind of issues there or let's say you're shooting aa group and we want to make sure everybody is illuminated right? Well, if we hang out all over here the difference in the power of light on this side of the spectrum it's almost the same we're going to get the same exposure here as you are back here, so if you have a lot of people you're probably gonna want to put him a little bit of distance from the light if you only have one and you want to make sure that's really a high contrast really? Uh g hate any light put it close okay gonna understand this clearly we're gonna have you guys put the signs down. We're actually take some pictures and show you this in real life so you can take your card out of here okay? Kelsey's gonna take this cards you can move that guy out of the way. Okay, let me need to move this out here. All right? So my friend john I need you to hold this please for me watching the leg were really pushing the boundaries of space and the studio right now that's ok, now I've used a very large light source just to keep this easy so we can do is uh come on over here over here, everybody else come on up, come on, come on, come on! We're gonna be here do I hear way you come on over you're still the shot you're part of the team just like that okay you see we have everybody close too far from the camera and then what we're gonna do here is linda you might melt sorry um we're gonna we're gonna shoot this just a little bit all right there's another light somewhere on can I have a meeting okay somebody meter this I don't linda here where my camera's gonna be which is sort of over here and my my channel is not working for some reason so this is all sometimes this happens the uh the pocket was yours rarely fails but sometimes it does can you give me a transmitter here area all right, so what I did to fix that is uh, pro photos when they first turn on they listened for any kind of signal and sometimes they'll get the wrong signal and it will set the channel to a different channel since we just turn it on may be the first thing I heard was a radio station so didn't work so I turned off send a signal we're all good all right it happens probably wants it during issue so I'm gonna meet you right here on linda we get fourteen. Okay, now give me the usman's here we may have to change lenses guys but let's see if this works fact I can't even see lyndon this shot here is going to go with it with the lens that we have ok, so you can see here on this picture that we have way got chris sort of nice sorry linda, I don't have white england, so I couldn't fix you in there you see how the light is just totally falling off in these guys the way we're seeing a huge problem with this shot, so if you're shooting a group and you have your your subject one of them really close and another really far away, you always have this light fall off and, you know, we're sort of exaggerating this putting him this way, but this could have just as easily been this light going straight forward or maybe a couple of them and this could be a group six or seven people deep, especially company photos, if you had, like, fifty people or something like that, you would see that the people in back are all just there too dark, so the way to fix this is to change things so we're gonna do is we're going to take our group, we're all going to go back to that we'll go all way back to the white wall there you go get together a little bit like you just I'm feeling good I'm gonna take this back just a little bit way go excellent, all right, I'm gonna re meet her this in a little bit more power here remember your eyes with two hundred on this camera ok when you go over here centimetres at seven point one they're all way way far away very good thank you now take this a seven point one there we go good I don't know if I got that in focus and see how they're all it looks like the usual suspects isn't it theo usual suspects so there they are they're all lined up and they could even move around a little bit so a couple of you guys just take a couple before you go just like that right there take another shot with the guilty please step forward okay so you can see that we're getting consistent light now obviously this is a horrible set up with the background saul why it totally looks like they're prisoners but the principle is illustrated that if you want to get a large group of people to be evenly illuminated used the inverse square live to get them away from the light so that you're in that one percent range where the light is pretty darn consistent across okay we're gonna show you another thing so carly's gonna come out you guys are gonna go back to your chairs what happens if you want teo include or exclude a background uh or make something that's very very high contrast um I'll show you something really cool so carly come right over here and we're gonna put you like really close to this light there you go just like that gonna bring you back a little bit now carly's face is gonna melt when we do this he's gonna know me raiders of the lost ark okay, so we're going to do here is I'm going teo do something a little bit different I'm sure you how you can meet her for the shadows and how you can meet her for the highlights so first I'm gonna show you what I showed you earlier and so I'm gonna meet her straight to where my camera is going to be so when I do this my loomis fears out and I'm meeting straight out we got way too much power yeah take it all the way down there you go yes I got way too much juice it that that range so again I'm gonna meet her this one more time that's it f sixteen so lots of lots of light so what we're going to do here is I have metered too or my camera position is so I'd take a shot okay really? We don't care much about anything except for how the light is working what did you say that was six. Sixty okay, so carl yes good just like that carly's got that awesome look there, okay he's gonna pop all right now what we can see here is the highlights are a little bit blown out see that they're a little bit blown out now the reason for that may explain what's happening here when I have this loomis fear up remember it's taking into account everything one hundred eighty degrees well there's not nearly as much light coming from this side as there is from this side and so it doesn't know this meter does not know that this is not the light that I care about so it's averaging everything so it's it's essentially has an issue just like uh tcl metering does where it's stupid the meter is stupid it doesn't know so what I want to do is I want to take this instead of pointing at my camera and saying take everything into account wanted to ignore this ambient light that we're getting and I'm going to meet her right to the light and what I'm saying is when I meet her to the camera a meeting for the shadows I'm saying I want to make sure that my shadows are in the exposure and the highlights they can blow out if they want to what I'm saying now is I want my highlights to be metered perfectly and the shadows they can fall out they can go away okay so this f eighteen okay actually I'm sorry put that ana where we have eighteen eighteen eighteen let me do that one more time because I think I didn't meet her right? I'm almost positive I meet in this wrong we're in czech ballet yeah so I uh definitely meeting so it's twenty so that's a spidey sense sometimes when you just shot something at sixteen and you know so twenty all right now we're gonna take the same shot and look at that so the shadows definitely much different so this is the shot that we did before even getting to go by yeah there you go that's the shot before look at those highlights you can see how her skin is blown out we don't see any of the texture this one you can see that we have a muchmore of the skin and those highlights or not blown out anymore so we're saying shadows go away also noticed the white background it's almost totally gone if we had this uh normal ambient light was a little bit more them able to just totally fallen out of place and we could really isolate things so let's do this one more time so we have this background back here in fact I mean it's sort of fudge this so let's take a step just wants to play right there you go so let me meet her that to the light again fourteen fourteen okay and see how dramatic that follow force just right there okay now we can see see the background it's almost totally non existent right not there so take about four five steps back keep going there you alright um and come a little bit closer there you and me that if you would please him like six three all right, now we've placed carly a little bit farther from our light yeah, we're gonna get some watch out the background changes so you see it we could include or exclude our background based on how far we are from the light I haven't changed my white balance there's all kinds of issues with this picture but if I get this to let me go back and forth there we go to the back ground out more background the farther away she gets from the light the more we get because that light is going to be evenly distributed so it's pretty darn cool right? We've got this uh, inverse square law theories that was called we can use that to control how things look with groups with models with backgrounds with meeting for highlights and shadows and all that kind of stuff you don't even have to know fancy math just know that close dramatic fall off far not dramatic fall off if you put something too close in a group we're gonna have issues with that put a little bit of a distance away. Okay all right, so let's look and see what's up next move this guy out of the way here quick question yes from cathy walters and move this whole thing up is that how can you tell how far away you need the subjects to be from the light so that you get them in that one percent range or is that a smile an air good question so um yeah so how do you know when you're at that place where the light is falling off so much there is a principle uh some people call trial and error um and we prefer infact I forgot the photographer that told us that she shoots for national geographic we just shot interview michelle westmoreland thank you so you were talking about michelle was moreland a couple weeks ago and she said instead of trial and error she says uh it is um experimentation and knowledge and I love to think of it maurin that term it's like you know what you'll have happen is you're gonna learn how to um through trial and error and I'm just going to try something and it doesn't work we're gonna try something sort of learned your light source like this light source if I've shot with this for a year two years I'm going to know that eight feet at a certain power level that's where my that's going to start falling off so one of the things that we're going to talk about tomorrow is something called failing with purpose and the principle of failing with purpose is exactly for that is how do you figure that stuff out how do you know where that that sweet spot is? And so what we do we do this about? Oh, I think about once a week, maybe once every two weeks we'll bring in a model or a subject and I'll shoot for about three or four hours with no expectation of success at all. The exercise is I'm trying to figure out I just got this new light I don't know at full power on this pack where the light is going to start to fall off that's all we're doing today is I gotta figure that out and so will spend two or three hours figuring that out. Um and the exercise is not to get good photos is to know exactly where the inverse square law kicks in for this specific thing um and after you've done that for years, then you'll know based on your experiments and your your knowledge base so the other way to do that somebody has my meter thank you all the way to do that is pretty simple, so what you do is you take your light meter and you gook what is this meeting right here and you can turn this around so it's opposite I'll meet her that that's eight take a step back that's four five take a step back as three six now in that sweet spot as three two as three, two, you'll start seeing. They become consistent and that's. The easiest way to tell. But then what you do, and I'll show you. I have this notebook where I write down stuff like a madman of things like that with this octu box on this pro photo packet, this power, these distances worked well for this. Um, and then you'll have your head filled with knowledge that drive other people crazy.

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Ratings and Reviews

Brian Geoghegan

Mark Wallace, Brilliant at what he does, so clear to understand, he is amazing, well done Mark great workshop, I learned so much. Thank you, kind Regards, Brian from Ireland


Mark really knows his stuff. He was very well prepared and Mark did a great job teaching this course. Mark went through all the steps from beginning to end in great detail. He also answered questions from the audience an online viewers which helped fill in any blanks. Great course.

a Creativelive Student

I loved this workshop! Many things I struggled to understand about exposure and many other things became so clear! Just wow!

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