The Canon EOS Digital Rebel XS/1000D Companion

Why There Are Two Ways to Control Light

So, you can limit the amount of light that hits the sensor by changing the amount of time that the shutter is open, and you can limit the amount of light that hits the sensor by changing the size of the aperture in the lens. But why do you need two mechanisms for controlling light? If your concern is just to ensure that the image is neither too bright nor too dark, wouldn't one mechanism be enough?

If brightness were your only concern, then yes, one mechanism would be enough, but because of the way the physics of light works, there can be a big difference in your final image depending on whether you try to control exposure using shutter speed or aperture. In fact, after compositional choices, the bulk of your creative power as a photographer comes from how you choose to manipulate these two controls.

How Shutter Speed Choice Affects Your Image

This next bit should be pretty intuitive. As you choose a faster shutter speed, you will have more ability to freeze motion in a scene. That is, when the shutter is open for a very short time, a moving subject will be frozen. When the shutter is open for a longer time, a moving subject will be blurry and smeared.

Figure 5-10. With a slower shutter speed, you can blur your subject to create a more dynamic image.

You may think that blurry and smeared is inherently bad and that you would always want your images ...