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Author Topic: What does changing the ISO do, exactly?  (Read 1825 times)

BJL

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What does changing the ISO do, exactly?
« on: August 23, 2003, 06:47:34 pm »

I am hoping for more details from others, but this is the bit I know: the signal read out of the photosites goes through some analogue amplification before the A/D converter and this is I believe the main thing that gets changed with higher ISO settings. So it is like "electronic push processing" of the sensor output.

However, some digital scaling up might also get done; for example it seems that Kodak does this at least with JPEG output, probably in order to get more gradations of shadow detail within the restrictions of 8-bit format. Thus some highlights can get digitally blown out by this step that are preserved in raw files.


By the way, there is an official ISO definition of sensor speed, which is roughly based on the exposure needed so that an object of average illumination causes a photosite to be about 10% full (giving slightly over three stops of headroom above that).

This true sensor ISO rating cannot be changed by twiddling a dial, and anyway the JPEG output of my digicam puts the average subject much less than three stops below blowout, so clearly the name ISO is being taken in vain, and there are probably no real standards in use yet.
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Steve Kerman

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What does changing the ISO do, exactly?
« Reply #1 on: August 23, 2003, 04:57:06 pm »

I was wondering what changing the ISO setting on a digital camera does, exactly.  Does it actually somehow change the sensitivity of photoelectric sensor sites, or does it just add a scaling factor in front of the ADC when it's reading out the result?
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