Life is full of compromises. One example in photography involves shutter speed and aperture: To achieve the right exposure, slower shutter speeds must be offset by smaller apertures. A similar balance exists between ISO and digital noise.
Making sense of ISO
ISO is a measure of your camera's sensitivity to light. Aside from some extremely simple point-and-shoot models (like the iPhone), most cameras include an adjustable setting for ISO. Your camera may have an ISO button on its body, or the setting may be hidden away in a menu on the LCD display.
A low ISO number (ISO generally starts at 100) indicates that the camera's sensor is relatively insensitive to light. Every doubling of the ISO (such as from 100 to 200, or from 200 to 400), doubles the camera's light sensitivity. It's comparable to a "one-stop" change in exposure. Suppose you were going to take a photo in dim light, and the camera was set to 1/30 second and f3.5. You might not be able to open the aperture any wider--f3.5 may be as far as it will go--and 1/30 second is too slow to take a sharp photo at that aperture setting. But if you double the ISO from 100 to 200, you can take the same photo at 1/60 second; and at ISO 400, you can take the same shot at 1/125 second.
Best of all, this technique works in most of your camera's exposure modes, including Program; so you don't have to work with your camera set to manual to take advantage of ISO. If you have the camera set to Program and you get a slow-shutter-speed warning--generally a little shaky hand in the display--you can manually crank up the ISO to take a sharper picture. Most cameras don't let you change any settings in full Auto mode, however, which is a great reason to shun Auto and use Program instead.
The downside of ISO
So why not set your camera to a high ISO and leave it there all the time, so you can always shoot with a very fast shutter speed? The problem is digital noise. When you make your camera's sensor more sensitive to light, you tend to get more errors--random pixels of unwanted color. Regardless of how low you set the ISO, you'll inevitably see some noise in a photo. But the higher the ISO, the worse the noise gets. Compare the same scene (below) shot at ISOs of 100 and 1600. When you see the entire image at Web resolution, they may look pretty similar:
When you see the entire image at Web resolution, they may look pretty similar.
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