The essence of photography is the capture of light, so it stands to reason that having an understanding of how light works can be useful, if not a necessity.
In this article, I’ll talk about how using some basic principals of light can give different effects in the studio. However, there’s nothing to stop these ideas from being applied anywhere. These ideas are simple, but are the basis of any photographic ‘look’.
Put on your propellor beanies, and read on!
In anything to do with light, it helps to understand the inverse square law. Put simply, it describes how things change over distance. For example, a light will not be as strong when it is placed further away.
More specifically, the distance relates as an inverse square, or to put it mathematically, one over the distance squared. In English, it means that if you put a light twice as far away, it will be four times as dim, as two squared is four.
By understanding this, we can now put a bit of numbers into what is quite an arbitrary thing, such as the position of a flash unit. We can tell that by moving the flash to half the distance, you’ll get four times the light on your subject, and so on.
One aspect that can be controlled by the lights is the shadow contrast, which describes the darkness of the shadows compared to everything else. This follows a simple relation:
The closer the light is to the subject, the greater the shadow contrast.
So how does this work? Let’s apply the inverse square law. If we bring the flash closer, then we know we are getting an exponentially greater amount of light falling on the ‘lit’ side of the subject. The shadow area, being a shadow, isn’t affected by how much light hits the subject. We say it’s lit by the room’s ambient light, the light reflected from the rest of the room, or from another light.
With the flash closer, and the light stronger, there is now a big difference between the lit and unlit sides. When we meter the camera to the lit side, the dark side is now very underexposed, causing dark shadows.
And the reverse? When we move the light away, we got exponentially less light. There’s less difference between the lit and unlit sides. Now when we meter the camera, we get an exposure that seems to lighten up the shadow areas.
This is incredibly useful in, say portrait photography. Sometimes a dramatic, dark shadow is needed, and from this we can see that the light needs to be close to the subject. If we need a more neutral light, we pull it further away, and re-meter.
Quick tip: Distance isn’t the only way you can change the amount of light on the lit side of the subject. Most flash units, if not all, allow the intensity or brightness to be dialled up and down. Need a less contrasting shadow, but in a smaller space? Dial down the flash!
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In this image, a large diffuser made from paper was used to give a soft light, as well as make a larger specular on the glass.
The other useful thing to control is the shadow softness. Technically, this is referred to as shadow fall-off, or shadow edge transfer, but for this article I’ll call it shadow softness. This one’s more involved:
The shadow softness is proportional to the size and distance of the light source.
Let’s say you’ve set up a flash unit to light a wall. A fly on the wall would see the relatively small flash bulb, off in the distance.
Now, let’s say we add a 50cm umbrella to the mix. To the fly, it now looks like the source of light is huge! The light comes from the whole surface of the umbrella, instead of just the bare bulb. We call the bulb the light origin, and the umbrella now becomes the light source.
So, distance is a doosie: the closer the light source, the softer our shadows get.
But how can we change the size of the source? Of course, there’s bigger umbrellas, bigger soft-boxes, even 10-ft-square panels of fabric would work. But there’s an easier way. If we move the light source closer to the wall, for example, the umbrella now looks bigger to the fly, and we get the same effect!
Generally, we can call both the distance and size the apparent size. Whether we make the source bigger or closer, the light will look larger to the subject. In other words:
The shadow softness is proportional to the apparent size of the light source.
The fact that we can control two things to change the size of the light source opens up a lot more opportunities. For example: if you need a softer shadow, but the flash can’t be placed any closer without appearing in the shot, a larger umbrella or diffuser will work. Conversely, If you need a hard light when up close, bare-tube1 or a snoot2 is the way to go.
Tip: A cheap way to make a large diffuser is to use sheets of fabric or, in a pinch, paper. Bigger is better (for softer shadows), and avoid seams and overlaps between pieces of paper.
Hopefully these ideas have made a few things click for you. Why is the light always so harsh on a sunny day? Although the sun is huge, it’s so far away, it looks like a pinpoint to us. So there’s virtually no shadow softness. This is great in landscapes, so that the shadows show off all the detail in the terrain.
But on an overcast day? The layer of cloud turns the sky into a gigantic soft-box, the source of light spreading from horizon to horizon, hence the shadows are incredibly soft (and well suited for portraiture).
Happy shooting!
Coming soon: A review of the Canon EOS 5D Mark II.
1 Bare tube: Using a flash unit with no diffusers or attachments.
2 A cone-shape fitting that lets the light through a narrow aperture.
Further reading: Creative Lighting Techniques for Studio Photographers
— Thomas Riggs