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What does ISO have to do with HDR PQ or C-Log?

Started 1 month ago | Questions thread
John Sheehy Forum Pro • Posts: 26,688
Re: What does ISO have to do with HDR PQ or C-Log?
1

Brian Greenstone wrote:

I'm hoping someone can explain this to me, but I'm totally confused about the relationship between ISO and dynamic range. When shooting in CLog3 you're supposed to use ISO 800, and when shooting HDR PQ you're supposed to use ISO 400. I have no idea why, or why shooting at ISO 200 will reduce my dynamic range. Those things seem to be entirely unrelated to me as I've always assumed ISO to just be a signal amplifier to boost what's coming from the sensor. But I don't see how that has anything to do with the number of stops of dynamic range that can be captured in those various HDR/Log formats. If anything I'd think a higher ISO would give me *less* dynamic range since it might cause the signal from the sensor to clip. I don't get it.

Can someone please clarify this for me? Also, I read somewhere that the ISO needs to be in multiples of 4 for best results. Ergo, ISO 400 or ISO 1600. ISO 800 or ISO 3600. What's up with that too?

These formats require sufficient raw headroom to do what they are expected to do.  Such headroom simply does not exist and is not possible at lower ISOs.

Based on your numbers, I would assume that HDR PQ exposes what would normally be raw ISO 100 at ISO 400, and CLog3 uses the raw ISO 100 to be exposed for ISO 800.  This give 2 and 3 stops more headroom, respectively.

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Beware of correct answers to wrong questions.
John
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