Lighting - Understanding Distance

01 May, 2008 02:00:00 Gareth

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For this tutorial all you are going to need is a single light source (preferably a head), a subject (preferably a real head) and a space of about 10ft x 8ft. What we are going to look at in this tutorial is the relationship of light with the distance over which it is used. Light allows us to see, to create mood, to change the appearance of subjects, define shape and give us amazing colours.

So let’s start with two basic statements;

1. The closer the subject is to the light source the more powerful the light is, the further away the weaker it is or for the sporting amongst you; the closer you are to where the cricket ball was thrown the more it’s going to hurt when it hits you!

2. The other is that the closer your subject to the light source, the changes you make have greater effect. The further away the less effect any changes will have. For this tutorial the lighting is off camera to the side of the subject as will be most portraits to create a three-dimensional subject.

Let’s have a look at the effects;

Image 1 (Light-Subject distance 6ft; Light-Background distance 5ft)
ISO100 : 1/125 : f8 : Manual : WB Auto : Centre Meter

Image 2 (Light-Subject distance 10ft; Light-Background distance 11ft)
ISO100 : 1/125 : f3.5 : Manual : WB Auto : Centre Meter

Image 3 (Light-Subject distance 1.5ft; Light-Background distance 8ft)
ISO100 : 1/125 : f11 : Manual : WB Auto : Centre Meter

Before you read on, look at each image and try to work out why the changes have happened. The ability to set your lighting will be greatly enhanced if you can learn to reverse engineer lighting and understand how certain lighting effects have been achieved.

Image 1

We have a fairy dull grey background (actually my light green curtains). This shot was taken with the subject pretty much equidistant from the light source and the background. Think of this as the light-subject and light-background distance.

Image 2

You will immediately notice that the subject is still exposed correctly but the background has taken a much lighter appearance. For this shot, the light has been moved further back, which of course makes it less powerful. To compensate and get the correct exposure on the subject, the aperture has been opened up (smaller f-number).

So why is the wall lighter? Well although the light source has been moved back, the light-subject and light-background distance is still fairly close and so we do not see any significant drop of light power to the background compared with the subject.

Image 3

The light source has been moved much closer to the subject and created a relatively small light-subject distance compared to the light-background distance. We’ve made our aperture smaller (larger f-number) to account for the more light power on the subject and expose the subject correctly. All of a sudden we find our background is now very dark, almost black. This is now because the background is around 5 times further away from the light source than the subject. Now re-read the first statement.

What we’ve just done is use the relationship of light over distance to control our background in just a few feet of space. Try this out, you’ll find that with a grey (or any light) background you can easily turn the background black. What we have also established is that we gain more control over the light when we move in close, read the second statement again! In effect we’ve created a shallow light depth of field. If you want, think about a macro shot, the closer you get in the greater effect any changes have.

For those of you who are more mathmatically minded (yawn), this is simply the Inverse Square Law in practice. BTW the inverse square law also applies to the f-scale; if you double your light-subject distance you’ll be halving your f-stop to get the same exposure, there’s a rule of thumb in there somewhere…

Good Luck