ISO rating for Film Speed
ISO stands for 'International Organization for Standardization' and their film speed ratings are used to indicate the relative amount of light necessary to give a proper exposure. A normal film will be rated at ISO 100. A film rated at ISO 200 will give a proper exposure with only half the amount of light compared to the ISO 100 film, enabling you to shoot in lower light or with a smaller aperture or faster shutter speed. The ISO 200 film would be referred to as a 'faster' film. There are films available that range in speed from ISO 25 to ISO 1600.
So why not use the faster films all the time, what are the advantages of slower films?
The faster films have a more prominent grain structure the individual grains clump together to form spots that are visible to the naked eye, especially when you blow the photo up to A4 or larger from a 35mm negative. In certain circumstances this effect can be used creatively especially in black and white photography but mostly it is undesirable.
How does all this affect digital cameras?
In the digital photography world the phenomenon is called 'noise' not 'grain', the cause of the problem is slightly different. When light levels are low, the sensor has trouble reading the scene properly and pixels of random color are thrown into the picture. However to us photographers the end result is the same or very similar.
The 'better' digital cameras have, usually hidden away among the manual settings, a sort of simulation of the film speed effect. My camera for instance, a Canon EOS 300D, has an ISO range from 100 to 1600. This feature is not available when you are in fully auto mode but is available in all the other modes. I tend to keep it set to ISO 200 most of the time as the grain structure is not significantly worse than ISO 100 and it gives me that extra f-stop to play with. When I am shooting fast action and I want to freeze the action (not always the case - see shutter speeds and apertures) then I'll select ISO 400 or 800. If the light is very bad ie night time or indoors then a shot at ISO 1600 is often better than a blurred shot caused by using too slow a shutter speed or no shot at all.
Here are two images, you are looking at a small blowup from the center of each image, the top one was shot at ISO 100 and the bottom one at ISO 1600. The difference is fairly pronounced at this magnification (this is at 100% zoom) but, at more normal sizes the difference is harder to see. The grain becomes most obvious in parts of the picture which are fairly plain, such as the sky. These two shots, of course, were shot in daylight. The effect will be even more obvious in pictures taken in lower light levels.
Some digital cameras have built-in noise reduction technology. Next time you need to take pictures in low light try a few shots with and without it and compare the results. Also have a look at your photo editing software. Programs such as Photoshop have filters that can remove or at least reduce noise. Careful though that this does not affect the overall sharpness of the picture too much.
Getting away from the auto settings.
An explanation of the mechanics of exposure and the side effects of choosing different aperture/shutter speed combinations.
Overriding the automatic metering system.
What they are trying to tell you.
Setting up your camera to take a series of pictures at different exposures.
Another piece of the exposure puzzle.
An explanation, strictly for the jargon heads.