The Canon EOS Digital Rebel XS/1000D Companion

Exposure Mechanisms

Your camera gives you two mechanisms for controlling the amount of light that hits the image sensor. Using these mechanisms, you can ensure that you don't end up with images that are too bright or too dark.

In Chapter 2, you learned about the shutter, a curtain that opens and closes in front of the shutter to expose it to the light that is focused through the lens. By using a faster shutter speed, the shutter will open for less time and thus expose the sensor to less light.

The second mechanism that the camera uses to control light is the iris, or aperture. This concept should be somewhat familiar because your eyes have irises in them, and if you've ever walked out of a dark movie theater in the middle of the afternoon, you know what it's like to wait for your pupils to close down so that your field of view ceases to be overexposed.

The iris in your camera is built from a series of interlocking leaves that, when rotated, create an expanding or shrinking opening, or aperture. The wider the aperture, the more light that gets let through to the sensor.

The aperture is positioned at the very end of the lens, just before the mount that attaches the lens to the camera body. As mentioned in Chapter 2, the shutter is positioned just in front of the image sensor.

DEFINITION: It's Curtains!

The shutter in your camera is composed of two curtains that pass in front of the lens. As the first one passes, the sensor is exposed. The second one then passes to close off the sensor. ...