Adjusting iso on the P7100

Started Feb 15, 2012 | Discussions thread
Billx08 Forum Pro • Posts: 11,373
Re: Adjusting iso on the P7100

phred14 wrote:

Please pardon the perhaps silly and slightly off-top question, but this is an ISO thread.

What does ISO mean in the context of digital cameras? In the old days of film it was related to the light sensitivity of the silver-based granules in the emulsion, presumably a function of size, chemical composition, and nature of the surrounding emulsion.

But what does ISO mean to a digital sensor? We have a patch of doped silicon, we've put some charge on it, and we expose that to light to bleed some of the charge off. More light, more charge bleeds off. We read the remaining charge, digitize it, process it, and call it a picture. Shutter speed controls how long the light hits the sensor, aperture chokes off (obviously has other effects, too) some of the light. Is ISO controlled with the amount of initial charge put into the sensor, or is it some configuration of the DAC, or what? What's going on at the nuts, bolts, and electrons level.

For this explanation, assume that Manual Exposure mode is used, not the P, A or S metering modes. Each photosite can only capture a certain amount of photons before it saturates and produces blown highlights. If this just barely happens at the base ISO (usually ISO 100 or ISO 200), if you reduce the exposure by closing the aperture by 1 stop or quicken the shutter speed by one stop, the image will be underexposed by one stop. If you then increase the ISO and shoot again, the data collected by the sensor will be the same, but the ISO is applied after the fact, digitally amplifying the sensor's signal data, and these values are saved in the JPEG and/or RAW data. That's why high ISO shots (using the metering modes) have so much noise. Their signal data has relatively low values because of underexposure, but some other types of noise (read noise, etc.) is multiplied/amplified, decreasing the signal to noise ratio.

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