OK, it looks like I eventually found my own answer: it's because the cells have a "full well" capacity limit in practice...
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Replying to @cmuratori
seems like that applies to rolling shutters. Full frame DSLRs have iso settings for DSP reasons: http://photo.stackexchange.com/questions/28983/why-is-there-an-iso-setting-when-shooting-raw …
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Replying to @phort99
But that's not really true, because you could just record the whole value at the sensor. So it's not _really_ DSP reasons...
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Replying to @cmuratori @phort99
... it has to be some other reason, like the readout can't handle more bits or something?
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Replying to @cmuratori @phort99
Know what I mean? It's like saying "you have to set it because the low 4 bits are noisy and you might not want them".
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Replying to @cmuratori @phort99
But you'd never do that, you'd just record the low 4 bits too, and then just discard them in software if you didn't like them.
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So there has to be more to it than that.
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