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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


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