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A digital camera's sensitivity to light is measured in terms of ISO, named for the group that developed the standards for this attribute (International Organization for Standards). Most cameras offer a choice of ISO settings so that you can increase or decrease light sensitivity as needed. In dim lighting, for example, you may need to raise the ISO to expose the image.

Being able to increase light sensitivity is great in terms of exposure needs, but there's a tradeoff: As you increase sensitivity, you increase the chances of introducing a defect known as noise, which gives your photo a speckled look. Figure 1-6 offers an example, with the noise most evident in the dark background of the picture. Noise is also easier to spot when you enlarge the image, as illustrated by the magnified view shown on the right in the figure.

Today’s cameras are much less noisy at high ISO settings than in years past. In fact, if you're using a camera that's more than a couple of years old, better low-light pictures is a valid reason to purchase a new model. But because noise levels at different ISO settings vary from camera to camera, check camera reviews for this important information. Bear in mind that a model that offers an ISO range that tops out at a higher number than another camera isn’t automatically better; you may not be gaining anything if the noise produced by the higher ISO is significant.

FIGURE 1-6: Read reviews to find out how high an ISO setting a camera can use without producing noise, the speckly defect that mars this image.

See Chapter 5 to get a better understanding of ISO and how to deal with image noise.