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Old Mar 16, 2005, 10:45 PM   #1
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i know this is a simple question with a complicated answer but anyway, here we go...

i was wondering how the iso worked on a digital camera and mainly why noise is created.

i understand how the noise is a byproduct of higher iso film (the larger the emulsion crystal size i believe) but i was wondering how this concept related to digital cameras (are the pixels used in conjunction with each other to get more light but less resolution - i dont know??? this is what i want to know)

thanks to anyone that can understand my question and answer it or point me in the right direction
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Old Mar 17, 2005, 12:26 AM   #2
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As far as I understand it is works in this way (much simplified). Single cell (pixel) collect light during exposure. Depending on how many photons it collected different voltage generated on this cell. Understandable that if too many photons collected voltage will reach certain point and will not grow anymore. Here we have ugly unfixable white spots if we overexposure our shot. Therefore there will be some kind of optimum light (number of photons) when out cell will produce the best results. Here we have so-called "native" sensitivity of cell. Bigger cell can collect desirable number of photons faster - this is why bigger DSLD sensors have higher minimum ISO level (usually 200). If there is not enough light then cells's voltage can be amplified and ISO number effectively becamehigher. Howeverwhen we amplify signal we also amplify undesirable noise (similar to audio) and noise (improper voltage) became more noticeable. It is also possible to look at this problem from another angle - if there is not enough light then our cell collect to few photons and therefore we can not make proper interpolation - our signal became too discrete, i.e. too noisy. This is why small sensors with big megapixel count (i.e. small cells) too noisy at high ISO and require lots of light (lower ISO) to be able to collect proper number of photons.

Hope it helps
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Old Mar 17, 2005, 5:59 AM   #3
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Your digital camera's sensor is really analogue: it does not directly output values ranging from zero to 255, rather each sensor has an output voltage. That voltage is amplified (analogue), then converted to digital. The more amplification, the higher the ISO, and more noise.
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Old Mar 17, 2005, 7:15 AM   #4
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thanks a bunch
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