The Canon EOS Digital Rebel XSi/450D Companion

PHOTOGRAPHY 101

THINKING AGAIN ABOUT FULL AUTO MODE

105

FOOD FOR THOUGHT

When you were a kid, you probably learned about the color wheel and learned you

can mix the primary colors of ink together to create other colors. You probably also

learned that as you mix colors together, they get darker until they eventually turn

black.

Light works mostly the same way. e primary colors of light

—red, green, and

blue

—can be mixed together to form every other color of light. But, as you mix the

primaries of light, they get brighter until they eventually turn white.

A digital image is actually composed of three separate channels. One contains all of

the red information in the image, the second contains all of the blue, and the third

contains all of the green.

We’ll be referring to this three-channel color process at various places in this book.

Red Channel

Green Channel

Blue Channel

Unlike ink, when the primary colors of light (red, green, and blue) are mixed

together, they get lighter and lighter until they become white.

inking Again About Full Auto Mode

Now that we’ve identified all the mechanisms that go into making an exposure,

let’s look again at the Full Auto mode that you’ve been shooting with. As you

learned in Chapter 1, when you press the XSi shutter button halfway down, the