The Canon EOS Digital Rebel XS/1000D Companion

Changing the ISO

In the previous chapter you learned that the ISO setting controls how light sensitive the XS's sensor is. A higher ISO number means the camera's sensor is more sensitive. This means it can gather light more quickly, which allows you to use shorter shutter speeds and smaller apertures. For times when you need lots of motion-stopping power or a deep depth of field, the ability to increase your ISO can make the difference between getting and missing the shot. But in addition to creative latitude, increasing the ISO also means you can shoot in much lower light. In fact, the image sensor in the XS is so sensitive that you can shoot effectively in much lower light than you could ever manage with film.

As discussed in Chapter 5, ISOs are measured using a standard scale. The Rebel XS offers ISOs of 100, 200, 400, 800, and 1600. Since these are all doubles of the previous setting, you know that there is a one-stop difference between each one.

The XS also offers an Auto ISO option that will select an ISO for you. So if a faster ISO offers better low-light performance and the ability to shoot with really fast shutter speeds and narrow apertures whenever you need them, why shouldn't you just leave the camera on ISO 1600 all the time?

In Chapter 5, you learned that the camera increases the sensitivity of the sensor by boosting the amplification of the voltages that are read off the sensor after an image is shot. Unfortunately, any time you amplify any type of electrical signal, ...