Camera Mode: High ISO
If you're new to the world of digital cameras, you might discover when you're playing around with your camera's mode, that raising the ISO makes the image brighter. But, after going out and shooting some pictures, you realize that the photos with a low ISO look a lot better than the one's with a high ISO. That's because the ISO setting is not the same as the exposure setting.
The ISO setting determines how much light is needed to expose the image, not how much light is entering from the lens. Images with low ISOs look sharp and crisp while images with high ISOs look grainy. As a rule of thumb, you should leave your ISO as low as possible and adjust for exposure by opening or closing the aperture.
There are times when the ISO will have to be raised. Be careful when you do this. Try to only raise the ISO a little and compensate for the rest of the exposure with the aperture. Remember, an image in low ISO with a wide open aperture looks a lot better than a high ISO image with a closed aperture.
The size of your image sensor will affect how intense the ISO affects your image. An image taken on a professional camera with a large image sensor, using a high ISO setting, won't look as bad as the same image taken with a lower quality consumer camera.