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Frankly, I couldn't get the answer. The closest I could find is from this webpage http://www.digital-photography-school.com/iso-settings
It states "In traditional (film) photography ISO (or ASA) was the indication of how sensitive a film was to light. It was measured in numbers (you’ve probably seen them on films – 100, 200, 400, 800 etc). The lower the number the lower the sensitivity of the film and the finer the grain in the shots you’re taking.
In Digital Photography ISO measures the sensitivity of the image sensor. The same principles apply as in film photography – the lower the number the less sensitive your camera is to light and the finer the grain. Higher ISO settings are generally used in darker situations to get faster shutter speeds (for example an indoor sports event when you want to freeze the action in lower light) – however the cost is noisier shots. "
If you were a film user before, you might have heard of people recommending to use ISO 100 for day outdoor shoot and use ISO 400 for indoor. Similarly, in the Digital Photography world, u can change these setting on ur digital camera. But why use ISO 100 (or 200) for outdoor and why use ISO 400 (or higher) for indoor?
Below is a ISO test I did using my camera.
Test Camera : Nikon D2H
Lens Used : Nikon AF-D 50mm f1.4
Aperature Set : f2.8
ISO : 200
Shutter Speed : 1/13 sec
ISO : 400
Shutter Speed : 1/30 sec
ISO : 800
Shutter Speed : 1/60 sec
ISO : 1600
Shutter Speed : 1/125 sec
ISO : 3200
Shutter Speed : 1/200 sec
ISO : 6400
Shutter Speed : 1/640 sec
If u notice, at low ISO (ISO 200 for this case), you can see that the picture are very "clean" while at high ISO (ISO 6400 for this case), you can see that the pictures are very "noisy". So, why would one want to use a high ISO then? What are the reasons for using a high ISO? Take a look again at their shutter speed. The higher the ISO, the faster the shutter speed. Which means, the less chance u get for camera shake. So, if u need to achieve a higher shutter speed, without changing the aperature, just by increasing your ISO, u can achieve a faster shutter speed.
So now, do a test on your camera to see what's the "best compromised ISO" setting for your camera (ie, acceptable noise, yet achieving decently fast shutter speed without introducing camera shake).
