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

MarshallG wrote:

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?

I think the reason is that CLOG 3 is an expanded dynamic range. What that means is that, from a midpoint, there are more stops of gray above and below that mid-point. If you shoot ISO 100, you can’t get enough stops of gray on the high end, so you need a reduced exposure.

Yes, essentially there is a base ISO recommended for these formats cause they are intended to maximize the DR possible for that particular sensor/log combination.

Usually a log profile will distribute a certain amount of stops both above and below middle grey per given ISO. Knowing this information will help determine how to best shoot a scene.

Just because there is a recommended base ISO doesn't mean you always want to shoot at that ISO. In some cameras like the Canon C300 Mark III, Clog2 allows for more DR below middle grey if you shoot lower ISOs, so you can keep that in mind depending on the overall contrast of your scene.

On a camera like the Canon R5, the sensor has much less dynamic range and only Clog 3, so keeping to base ISO makes more sense here since you don't gain nearly as much dynamic range above and below middle grey as you move through the ISO range, yet you'll deal with the noise floor a lot faster on this camera.

The sensor/log combination plays a big role here. I think Canon posts some charts on the stops above and below at each ISO using log on some of their cameras.