I started photography sometime back, and always wondered when is high ISO handy in photography? Also can it be used to get any special effect in pictures?
My camera has Max ISO of 800 but I see some camera which boast of 64,000 etc. Does it really affect the performance?
Answer
ISO is very useful as it helps overcome read noise by amplifying a weak analogue signal prior to digitization (which adds a more or less constant amount of noise) thus giving a better signal to noise ratio.
That's all raising the ISO does, amplify the signal. It does not make the picture noisier because it only amplifies what's already there.
See this example. The ISO100 shot was significantly underexposed and suffers from really bad read noise. The same amount of light enters the camera in the second shot, but due to ISO the signal is amplified before readout, thus read noise is a smaller percentage of the signal and overall signal to noise ratio is better:
To using a high ISO actually helps to reduce noise when you have a limited amount of light (and you can't get any more by opening the aperture or shutter for longer).
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