 Steve Beckle |
Meaning of ISO in digital photography I understand the concept of ISO in the film world -- a film's speed is related to how light-sensitive it is, which has to do with its chemical composition. What exactly happens in my digital camera when I adjust the ISO settings? Since there's no film, does adjusting ISO somehow change the light sensitivity of the optical sensor? Also, do higher ISO settings in the digital world result in the same graininess one sees in with higher ISO film?
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Kerry L. Walker |
Yes, ISO is the same concept for digital as it is for film. An increase in ISO in digital creates more "noise", which is the digital equivalent of grain.
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robert G. Fately |
As kerry rightly points out, Steve, ISO is the same for film and chip, but where higher speed films have more grain (a measure of resolving capacity, in essence) setting a CCD or CMOS to have higher sensitivity does not alter it's resolving capability - the number and size of pixels is unchanged - rather it "turns up the amplification". That is, an imaging chip will have some inherent level of sensitivity - say 100 ISO. Remember that the way a chip works is by converting photons of light that strike a given pixel well into an electric current. So, let's just say that a given picture element on the chip needs to be impinged by 1000 photons to trigger a "read". Now, if we instead amplify the chip's sensitivity so it only needs 125 photons to "ring the bell" so to speak, we have effectively set it's ISO to 800 (i.e. - it takes 1/8th the number of photons to generate a result, or 3 f-stops). Pretty groovy, yes? But, as always in life, there's a downside - that added sensitivity setting allows for the possibility of spurious readings - in other words, not just photons of light from the subject that went through the lens could cause a "read" result - so might photons of heat generated by the electronics on or around the chip itself. This is referred to as noise. To hear the audio version of this - take an amp and unplug the CD player (or remove the mike, or do anything to eliminate the presence of a source signal). Then crank the volume up to "11". That white noise you hear is the amplifier amplifying it's own self-generated signal. On an imaging chip, noise usually shows up in larger dark patches of the image, because that's where our eyes tend to notice it standing out.
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