"What does ISO stand for?" my bright-eyed student asks. I am always asked this question in my digital camera classes. My answer? "What the letters stand for have very little importance. What is more important is that ISO is part of the magic triangle on all cameras that works towards ensuring a perfect exposure."
International Organization for Standardization - see, the words mean very little to the actual function that ISO performs on your digital camera. ISO controls your camera's sensor's sensitivity to present light. It is a function that is invaluable to all photographers, and must be learned if you want to truly tame your exposures.
Cameras (in an AUTO mode) and photographers have perfect exposure as their ultimate goal. To achieve a perfect exposure or exposure value (EV), three things must be set in concert with one another: the shutter speed, the aperture width (f/stop), and the ISO .
To put it very simply, your digital camera's ISO control helps your camera to "see" in the dark. This control enables your digital camera to record an image in low light situations. Even the simplest understanding will help you improve your digital photographs, but be sure not to confuse your camera's ISO settings with your camera's shutter speed; they work together, but they are not similar. One controls sensitivity to light, while the other controls the amount of light striking your camera's image sensor.
In our modern digital cameras, ISO settings have replaced ASA (film speed) once acquainted with our old film cameras. ISO numbers can range from a very low 80 and can go as high as 6400 on many compact point-and-shoot cameras. The more expensive DSLR cameras (single lens reflex cameras with interchangeable lenses) can sport ISO settings as sensitive as 12,800 and higher. The lower numbers 80, 100, 200 are reserved for brighter light, and sunny days. Mid-range numbers like 400 and 800 are necessary once the light diminishes, like the low light found in deep shade, at dusk and dawn, and inside a building. The higher settings of 1600 and greater are necessary when light is extremely low, like in concert halls, museum, etc.
Since a great deal of our picture-taking is done in low light (nighttime parties, museums, concert halls, sporting events), understanding when to adjust your camera's ISO setting will also help you to capture a stable image devoid of camera shake (blur). The more sensitive you can make your camera to existing light, the faster your camera will be able to fire its shutter. And a faster shutter speed will result in a clearer in-focus image.
However, there is a downside to higher ISO settings: digital noise. This graininess was always present in film of 400 ASA and higher; digital images have not escaped this phenomenon either. The more sensitive your camera's image sensor is to light, the greater chance of digital noise occurring. The good news is that newer digital cameras, especially high quality DSLRs, have been manufactured with firmware to help control this issue - not entirely - but digital noise at higher ISO settings has been reduced. Still, it's better to avoid very high ISO settings and try to illuminate your subject instead, by moving a person into the light, or using your flash.
Experienced photographers know their camera's ISO is good for something else besides low light shooting, and that's for fast-moving subjects. Due to the relationship between your camera's ISO setting and its shutter speed, it is easier to freeze action if you increase your ISO setting first, even in bright sunlight. Upping your ISO will increase your camera's shutter speed, and thus improve the clarity (focus) of your shot.
Another reason to increase your camera's ISO setting is to enable you to shoot without using your on-board flash. The light from your flash can be very unflattering when photographing people, because it can overexpose parts of your subject, resulting in a loss of important details, especially in the shadows. With a slightly higher ISO, you will be able to "suppress" your flash, and shoot your portrait or macro shots with natural light only, which is far more efficient in capturing true color tones.
Don't be surprised if you see your camera's resolution setting (image quality) reduced when you select a higher ISO setting. Because of the potential for digital noise, your digital camera will automatically reduce your image's resolution to a smaller pixel memory setting. This is normal.
Use your camera's ISO settings liberally. If in doubt about the most desirable setting, take the same shot using several different ISO values. When you review your pictures later, delete the ones you don't like. Unlike the expense incurred by yesterday's film photographers when they experimented with different settings, experimenting with your digital camera's ISO settings will cost you nothing but a few extra seconds of your time, and could ensure the capture of that perfect shot.
Tip: Some cameras (especially point-and-shoot cameras) have a safety feature where the camera will override a poor exposure setting and change the ISO to a better setting. But this ISO override feature must be turned on in your camera's set-up menu.