How to read a dynamic range chart?

Question

Background

In short, I know what dynamic range is, how to expose properly, etc: don't clip highlights or crush shadows, higher ISO generally = more noise (though not always), etc.

I recently bought a Blackmagic Pocket Cinema Camera 6k g2. The dynamic range chart for it looks like the following image, but I don't understand how to read it, and their support won't explain it either.

Reading this answer gave me some more hints, but I still have some questions.

Questions

1. Let's consider the vertical 100 ISO bar. You have a 0 in the center of the chart, 3.9 stops above, and 9.5 stops below. According to the answer I just linked, I assume the line separating the two is middle grey, and it shifts based on the ISO you select, correct??
2. If the above is correct, then the further you go toward 1,000 ISO, then you can maintain more detail in the highlights, but less in the shadows??
3. If the above 2 are true, then what does the 0 represent on the side of the chart?? (obviously it says "stops", but it doesn't make any sense due to the above).

Thanks!

Answer 1

The maximum dynamic range of a sensor is achieved when some photosites reach full well capacity and some photosites record the minimum possible. I.e the maximum difference has been recorded... and that's all the DR rating tells you; it does not tell you how many values are recorded in between the min and max.

The minimum and the maximum values are fixed for a sensor of a given sensitivity (native ISO). Therefore, if you increase the ISO above the native ISO you necessarily loose information from highlights as they are pushed into clipping and the recorded DR decreases. That isn't usually a problem if you are increasing the ISO due to a lack of light (highlights/DR). The minimum remains the same.

With a dual ISO sensor, when it switches into its' low conversion gain state at the higher native ISO it becomes more sensitive to light and is able to discern lower levels of exposure... in this case the minimum is decreased; and the maximum also decreases (full well capacity is lower). This is a benefit as long as you are increasing ISO due to a lack of light (not enough to reach FWC).

The Black Magic 6k G2 is specified as having dual native ISO at 400 and 3200... yet that DR graph looks nothing like what raw data/DR from such a sensor should look like. The only thing I can assume is that there is a lot of manipulation applied; and to me it looks to be digital manipulation, not just analog amplification. E.g the ISO "step" is at 1250. And if that's the case, it can mean whatever they want it to mean. I.e. it's after processing, probably with their "video dynamic range" LUT. I would assume the dividing line for "above/below" must be middle grey.

This is what the raw DR looks like for a dual ISO sensor. This is the Z9 with dual native ISO's of 64 and 500. There is no "above/below;" e.g. 10 stops of DR means the highlights just below clipping are 10 stops brighter than the shadows just above clipping... what you do with that is up to you.

https://photonstophotos.net/Charts/PDR.htm#Nikon%20Z%209