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

This video may help. I actually have not watched it but Gerald does a good job usually.





I am basically just repeating without fully understanding myself what I'm saying. I'm an E.E. and so from an E.E. perspective, I really don't understand.

BUT... they tune the data capture to a specific ISO and this is called the "base ISO". Some cameras have two base ISOs. You want to use the base ISO because that will give you the cleanest output.

To me it is usually "hidden" what the base ISO is or you have to somewhat deduce it from context. That's probably 3 times more information than I actually know so I'll be quiet now.

I am only repeating what I vaguely remember from what I read, which may or may not be true, but…

… the issue isn’t so much the bright lights. The bright lights can be managed by the system. That is to say, a faster shutter speed or a higher aperture. The issue is seeing into the shadows after the « bright lights » have been taken care of. And that is where the sensitivity of the sensor to light comes into play.



The path to good Dynamic Range doesn’t come from being able to have a « high ceiling », but rather from having a « low floor ».

Brian Greenstone said:

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?

Click to expand...

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.

I normally shoot HDR PQ with my R5 and deliver HDR videos with Final Cut Pro on my Macbook M1 Pro. I have no problem with editing and grading either HDR PQ or CLOG 3 in same video but:

I have a question regarding low light shooting.

HDR PQ have base iso of 400, and Clog 3 have base iso 800.

If I'm shooting in low light.. city lights or similar and end up using iso 12800.

Is there a difference in HDR PQ or CLOG 3 noise wise shooting at iso 12800 caused by the different base iso?

shooting at iso 12800

* HDR PQ is 5 stops over prefered iso 400

* Clog 3 is 4 stops over prefered iso 800

Anyone with knowledge?

Brian Greenstone said:

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?

Click to expand...

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.

MarshallG said:

Brian Greenstone said:

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?

Click to expand...

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.

Click to expand...

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.