The Canon EOS Digital Rebel XSi/450D Companion

CHAPTER 5

WHY THERE ARE TWO WAYS TO CONTROL LIGHT

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Consider these shutter speeds:

1/60 1/120 1/250 1/500 1/1000 1/2000 1/4000

Each one is double the previous shutter speed. In other words, there is a one-stop

difference in the amount of light exposure generated by each of these speeds.

Now consider these apertures:

f4 f5.6 f8 f11 f16 f32

is one is not so obvious because most of us aren’t too familiar with calcula-

tions of circular area. But, trust me when I say that each one of these apertures

represents an opening that’s twice as big as the previous aperture. In other

words, there’s a difference of one stop of light exposure between each of these

apertures.

When it comes time to balance motion stopping power with depth of field with

overall illumination, you can take advantage of the fact that both shutter speed

and aperture can be adjusted by the same amount in opposite directions. In oth-

er words, the two values have a reciprocal relationship.

For example, let’s say your light meter recommends an exposure of 1/500th of

a second at f8. It doesn’t know how much motion stopping you might want, or

how much depth of field, so it’s simply trying to recommend a shutter speed and

aperture that will give you a good level of illumination.

But if you decide that you want more motion stopping power, and so increase

shutter speed from 1/500 to 1/1000 (one stop), you’ll run the risk ...