ISO

The third and last key variable that determines exposure is ISO, which determines how sensitive the sensor is to light. A high number means high sensitivity, which can allow you to take pictures in dim conditions but typically causes pictures to be "grainy".

If you use "full auto" mode (the green square), you don't need to sweat ISO, as the camera will choose it for you. If you're in the "creative zone", you generally want to take pictures with the lowest ISO setting possible given the shooting conditions. As I've said before, I usually use "full auto" or Av mode. When I use Av mode, I set the ISO based on these guidelines.

- In bright sunlight, I use ISO 100 to ensure the sharpest image possible.
- If it's a little cloudy, I'll bump down to ISO 200. This minimizes the chance that pictures will be blurry
- Inside, especially if I'm taking pictures close up, I tend to use ISO 400. This is a good utility setting and, in my experience, the one most often selected by the camera in "full auto" mode.

My Canon Rebel XT also has ISO settings of 800 and 1600. I almost never use these as the pictures tend to be really grainy. If I want to take shots at night, I either use a flash or shoot long exposures. I'll write more on long exposures later.

What can go wrong if you choose the wrong ISO setting? Well, if it's too low and you're using one of the modes in which the camera chooses the shutter speed, your pictures may be blurry as the camera will choose a slow shutter speed to ensure the enought light hits the sensor to capture an image. If you use too high an ISO setting, pictures can become overexposed (too light) because the camera can't open and close the shutter fast enough. As mentioned previously, a high ISO setting can also cause grainy photos.