Originally posted by aelith:
all i know is that the higher the iso the grainer the picture.
book says iso is how sensitive film / ccd is to light.
Film grain due to ISO rating with film has everything to do with the photographic material used in the film and the physics of what happens when light hits a certain spot on the film. It's hard to make film more sensitive, meaning it responds to light faster (higher ISO), without sacrificing image quality. Someone with more film experience can explain the physics at work here.
Digital cameras are a different story. As light hits a single pixel spot on the sensor, it builds up a charge at that spot. When the exposure is done, the camera samples this voltage with an analog-digital converter. It depends on the camera setup, but pixels are sampled sequentially, not all at once. Increasing ISO on a digital camera usually means the voltage is amplified before it goes to the A-D converter. Thus, the grain you see in a digital photo is due to electrical noise being amplified. And since pixels are read sequentially over time, and noise changes over time, the noise on each pixel is different. It is sort of like superimposing television "snow" over your picture.
I've noticed at high ISO that I sometimes get lines across the frame, probably due to a sudden noise spike at the time the camera was sampling that area of the sensor. It's really only noticable on very dark pictures, so I never use the highest ISO setting when shooting at night.
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