How does dual native iso function?

[silvan schnelli]

I am unable to understand why, let’s say the Sony Venice at EI 3200 with a native setting of  3200 would have less noise than EI 3200 at native 800.

I have read the whitepaper https://www.photonstophotos.net/Aptina/DR-Pix_WhitePaper.pdf , but to my understanding, the paper talks more about the benefit of increased dynamic range by running a second capacitor in parallel.

How is it possible for cameras with a second native ISO to be more sensitive (higher conversion gain) and have better SNR with less incident photons. All whilst not making the lower ISO rating obsolete?

I feel like this would also suggest that I would need to have two separate light meter calibrations for the two ISO settings, if one setting converts x amount of photons into x exposure and the other converts y amount of photons into the same x exposure.
 

EI latitude representation of the Sony Venice 2
 

Edited April 3 by silvan schnelli

[Nicolas POISSON]

When bypassing the capacitor, both the noise floor and the clip threshold are lowered. This does not change the dynamic range: the ratio remains the same. It is like saying 1000 to 10 has the same ratio as 100 to one. What you gain in noise is what you loose in highlights.

The lower base ISO does not get obsolete as:

- the whole DR is unchanged

- base ISO of 800 is more convenient for well lit scenes, which are probably more common than low light scenes.

For instance, you are shooting outdoors during a sunny day. Using base ISO 800 and EI 800, you set-up your aperture so that 6 stops above middle gray and 10 stops below fit your needs. If you were to switch to base ISO 3200 EI 800, you would have the same exposure, but now only 4 stops above middle gray. Your highlights are clipped. To avoid that, you either need to close your aperture by 2 stops, or put 2 stops of NDs on your lens. At the end, you get the exact same noise and clipping relatively to your scene... but you need ND filters.

When switching from a lower to a higher base ISO keeping EI the same, you cannot apply the usual rule that higher ISO (indeed, higher EI) protects your highlights. This is the exact contrary.

It does not help that "ISO" is often used instead of "EI", which made no confusion until dual base ISO was invented.

Edited April 3 by Nicolas POISSON