Why does the size of your light source matter?

This is part 3 of the learning to light series with Josh Larkin. 

Despite what you may have heard, size matters and that is especially true for your light source.

Portrait photographers often use large modifiers -- think 50" softboxes, 43" umbrellas, etc. -- to soften the light hitting the subject. And that's what we want most of the time (film noir shooters excluded), nice soft light that transitions slowly from highlights to shadows. Take the umbrella off of your light source, adjust its power output or your aperture or shutter speed for a correct exposure and what do you get? Hard light. Your beautiful transitions are gone, replaced by the hard line where your light ends and the shadow begins. That's what happens with small light sources.
 

The thing is, that huge softbox or umbrella will create hard light as well depending on how far it is from the subject. Why? Because it's all about the apparent size of the light. Not to you, as the photographer, but to the subject. If you have an umbrella handy, give it a whirl. Move the light stand back a good 15 feet from your subject and fire away. Hard light, guaranteed. Move it in to about three or four feet, soft light. At three feet away, that 43" umbrella is a huge light source that bathes your subject in light from multiple directions. From 15 feet, it's not so big anymore. In fact, it's kind of small, and the light is essentially hitting your subject from one direction.

Another way to think of this is to talk about the biggest light source of them all: the sun. Now, sure, the sun is a big light source, but, it's approximately 93 million miles away from the earth, so it therefore acts like a small light source. However, add an overcast day into the mix, and you essentially have a gigantic softbox over the light source, diffusing the light and allowing it to hit the subject from various angles, thereby making nice, even, soft light.

The basic idea is this: big light equals soft light, small light equals hard light. And as it turns out, that's a pretty easy concept to demonstrate. I decided to shoot a small Egyptian statue I had on hand. The figurine stands about five inches tall, so since it's really the apparent size of the light that dictates whether we'll get soft or hard light, I didn't need to go with an umbrella to get nice, soft light, I just needed to make the light appear big to the small statue. To do so, I mounted up a 10 x 14 inch Lumiquest Softbox LTp to my Nikon SB-600 speedlight and positioned it on a light stand at about 45 degrees camera right.

The lede shot was taken with the front of the softbox positioned 10 inches away from the figurine. At that distance and from the perspective of the small statue, this is a not just a big light source, it's a huge light source. I shot this at my D7000's max synch speed, 1/250th of a second, f/7.1, ISO 100. My SB-600 was zoomed to 24mm and dialed down to 1/8th power. Notice the transition from the lit side of the statue to the shadow side. Pretty smooth right? Now look at the shadow being cast by the statue. It's not that easy to see where it begins and where it ends because there's a gradual transition from the white board into the darkest area of shadow.

For shot two, above, I moved the light back to 28 inches from the subject. To compensate for the loss of light, I opened up my aperture to f/5.6. The exposure is a little darker on this shot, but what we're looking at here is the quality of light. While I wouldn't exactly call this hard light, it's definitely harder than the lighting in shot one. We can see that the transition across the statue's face is more abrupt, and in the shadow being cast by the figurine, the line between the lit and unlit area of the board is becoming much easier to discern. Same light, same light modifier, different apparent light size.
 

Okay, now we're at 66 inches from our subject, and to get a better exposure I've bumped my f-stop to 4 and flash output to 1/4th power. That gave me a good exposure for sure, but at just over five feet from the subject, my 10 x 14 inch softbox is starting to look like a pretty small light source to the statue. Have a look at the hat on the statue. In shot one, there's no real discernable transition between the lit and unlit areas of the hat. In shot two, the transition is still subtle. In shot three, it's much more obvious. How about the cast shadow? To my eye, it's pretty clear where the light ends and the shadow begins along much of the length of the shadow in shot three. Apparently, my softbox isn't so soft when it looks small to my Egyptian friend.

So as I said at the start, size matters. Practically speaking, there are a number of ways you can utilize this to improve your images depending on the look you're trying to achieve. For example, when we bounce our flash off of a ceiling or a wall to diffuse the light, we're creating a huge light source, right? Well, maybe we don't want it quite so huge. Solution, use the zoom head on your flash to tighten the beam spread, which essentially makes your ceiling or wall a somewhat smaller light source than when you hit it with the zoom opened up. Working on the fly but want to get good soft light in a portrait with only a small softbox such as I used above? Get that thing in close -- I'm talking like a foot from your subject -- boom, soft light from a small source that appears big to the subject.

Finally, did you happen to notice that blue/gray background getting lighter as I moved my light back from the subject? I did, and I think I might try and tackle the reason why that's happening in my next lighting post. Until then, thanks for reading and let us know in the comments what you think!

Read all articles in this series:

• Learning to Light
• Get Your Flash off Camera for Better Photos
• Apparently, size does matter...
• Control Over Your Lights With Distance
• Reflections
• Specular Highlight Control

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