Learn to Light: Direction of Light

Direction simply refers to where the light is coming from. It’s related to distance too, as how far away a light source is also has an effect.

Is the light coming from where you want, to create the effect you’re after? That’s the only thing you really need to know and consider. Point a light at an object, and you’ll get an area of highlight that faces the direction the light is coming from. You’ll then get an area of transition (sometimes known as the core) as the light moves from highlight to shadow. Then on the opposite side to the direction of the light, you will have a cast shadow.

At the risk of making you do something really basic, it’s worth recreating an exercise like the one in the video. Take a torch, a darkened room, and ideally a cylindrical or tubular object (you’ll get a better transition with one of these than say, a cube) and move the torch around it. Watch what happens to the highlight, transition and shadow as you change the direction, and the distance, of the light. The way these interact will help to create the illusion of a 3 dimensional object within the 2 dimensions of your photograph. All light will create a highlight, transition, and cast shadow on an object, although the difference between them can sometimes be minimal. Think how subtle the change is on an overcast day – the cast shadows are very vague and indistinct, and the transition is very gradual, but it’s still there.

The basics of direction are pretty simple, and easy to get your head round, as long as you have just one light source. If you start adding more, things can get complex, as for each light source you’ll have – you’ve guessed it – a highlight, transition, and cast shadow. These may not interact with each other in the way you expect or desire, so be careful when adding more than one light.

Direction – Inverse Square Law

So let’s bring distance into the equation. Here’s where things can get quite nuanced and subtle, but it’s also where some of the magic in lighting lies, as if you can get your head around the concept of inverse square law, you’ll have a tool that will serve you for life.

The distance a light source is from the subject is important because the further the light source is away, the more of the light will be scattered on its journey to the subject, and the less light will fall on it. Hopefully this isn’t too difficult to grasp – think of holding a torch at night and trying to light the side of a building half a mile away. The building won’t be very bright, as the beam of light from the torch will be scattered over such a large distance. Now point the torch into your hand, and it will be brightly lit, as the light only has to travel a short distance. You can see how the 4 characteristics of light start to interact – distance starts to become a quantity issue!

The nature of light over distance is governed by something called inverse square law. This may sound a touch intimidating if you’re not a fan of science, but don’t panic, it’s actually pretty easy to get your head round. What it means in practice is that if you double the distance from a light source, you’ll have a quarter the amount of light. So, if the correct exposure for something 4 feet away from a light source is f5.6, the correct exposure for something 8 feet away will be f2.8. 2 stops less = a quarter the amount of light.

Where people get confused is in thinking that “double the distance, halve the exposure”. This seems logical on the surface. The secret is to remember the rays of light expanding outwards in a cone from the source. Picture them covering one “square” at 4 feet, and then by the time they’ve travelled out to 8 feet, they’ll now be covering four “squares”.

In practice, these numbers are rarely quite so precise. The reason for this is that not many light sources are a single point of light, they’re often larger or more diffuse. So, the diagram in practice would be a bit fuzzier – the lines wouldn’t all emerge from one single point. The principle still holds though, even with very diffuse or soft lights – as you increase the distance from a light source, the amount of light drops off in an inverse square ratio. Just to repeat that in simple terms, if you double the distance, you quarter the amount of light.

So why does inverse square law matter, how can we use it and work with it as photographers? Well, the way in which light falls off in an inverse square ratio means that the relative differences in exposure between objects that are close to the light source is much greater than if those objects are further from the light source.

Look at the chart in the video. The difference in exposure between something 1m from the source, and 2m is 2 stops – double the distance, quarter the light remember – so it would be pretty noticeable. If you set your exposure for the thing that was 1m away, the thing 2m away would be 2 stops darker. Further down the distance scale, look at the difference in exposure between something at 7m and 8m from the source. The exposure is 1/28th and 1/32th of the correct amount respectively – this would be a difference in light and dark you’d hardly notice. The distance that has changed is the same – just a 1m difference – but the effect on exposure is significantly less.

In practical terms this can be employed a few ways. Let’s say you wanted to light a large group of people. For argument’s sake, let’s say the whole group of them are 10 metres wide. If you placed your light very close to the left hand person, and exposed correctly for them, the people even half way along the group would be much darker, and the person on the far right would be pitch dark. If, instead, you moved your light much further away, but still to one side, then the relative differences in distance between the left and right side would now be much lower, so their exposure would be almost identical.

Inverse square law also helps when you’re trying to create the illusion of space where you haven’t actually got any. Let’s say you’re taking a portrait of someone, and you want the wall behind them to be dark, but it’s painted white. If you bring your light source close to the person, expose for them correctly, and make sure they’re standing slightly away from the wall, then the amount of light hitting the wall will be significantly less than that on the subject. In this way you can make a white wall turn dark. This might not sound all that interesting, but later in the course, when we start creating actual lighting scenarios, you’ll see how being able to light in “layers” is very useful.

Now let’s see some of this in action.

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