ISO is a scale that indicates how sensitive the sensor or film is to light. The more sensitive it is, the less light required to make an image. ISO 200 is the ideal or default ISO for most digital cameras. Whenever possible, one should use this setting. ISO 400 is twice as sensitive as ISO 100 so half the light is required to make a correct exposure. ISO 800 would require half as much light as ISO 400 and so on. The cost to increasing the ISO is noise. Noise appears as small spots in the image. As the ISO rises, so does the amount of noise, each step making obscuring more detail and making the image less clear. Depending on the camera and the scene, the noise may be nearly imperceptible or it may completely ruin the image.