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Lighting - Understanding Distance
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Gareth on 01 May, 2008 01:00:00
 
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