Key word here: implied. You assumed, despite me explicitly saying 'almost' and also pointing out that there are limitations due to quantization error. Which is exactly why I said some posts ago that ISO 400 is the 'magic' ISO above which there's not much benefit to hardware ISO amplification for full-frame cameras, b/c this is where one electron is counted by one digital increment in your RAW file ('unity gain ISO').
That's why I explicitly said you can choose ISO 400 rather than 6400 or 12.8k some posts above, remember?
The way you phrased it implied that you could almost keep the entire (ISO 100) dynamic range at high ISO when the reality is that you can almost keep then entire ISO 100 DR up until ISO 1600 at which point you lose ~1 stop DR per stop increase in apparent ISO (not quite sure how this will actually scale). But fair enough, it was your comment that prompted me to test using ISO 400 instead of ISO 100 so yes, I do remember.
No, ISO 100 pushed is not always going to be the same as ISO 400, for tones below a certain threshold where quantization error is an issue (or where downstream read noise is somewhat more significant b/c you're only counting every 3-4 photoelectrons per digital increment).
I alluded to all of these from the very beginning, but I can't always write a novel every time I'm talking about a concept.
The magnitude of this difference surprised me a bit, I didn't expect such a large difference. Pleasant surprise.
Also, careful about resampled views in Lightroom. I actually do see a tiny bit of banding in your image - perhaps the A7 sensor is more outdated compared to the A7R? The magenta blotching can be reduced typically with the Shadows Tint slider under Camera Calibration. Also, magenta noise does seem to be more of an issue with the A7 than A7R, if I remember correctly.
You're correct about the resampling, it exaggerates the effect slightly but it's definitely there. This is actually similar to the pattern of purple blotches I see in the A7 files at high ISO that don't respond well to noise reduction that I complained about in the other thread.
The D810 surpasses even the A7R.
I've been very tempted to rent one of these to try out both the new AF system and the supposedly improved grip.
Also, that's a +5 EV push. Your initial claim that ISO 100 + 4EV falls apart to posterization and banding and noise compared to ISO 1600 is just not true - at least not with an A7R or D810. But, yes, quantization error and the effects of non-zero downstream read noise *will* have some effect at some extreme point. But even when it does, it's very easy to remove. It's usually as subtle as the noise that comes with higher ISOs. Not the magenta blotchiness and banding your example shows. Only time I see magenta blotchiness is in the JPEG preview, b/c of the limited quality/bit-depth of some of the preview JPEGs LR uses. There can be some significantly smaller faint magenta blotchiness for signals down in the RGB = 1,1,1 area, and those can easily be removed with color NR which thankfully rarely kills much actual image detail.
The initial claim was poorly phrased; +4 stops is where I start seeing blotchiness and extra color noise and is generally the limit of how far I push Exmor files. I generally have to push 6 stops to see the banding and posterization issues.
Also, realize LR's histogram is not the best judge b/c of the way it actually works. I'm not sure what exactly you're referring to when you're talking about 'posterization', but as long as your sampling is such that shot/statistical noise is sampled by at least one digital increment in your Raw file, the dithering effect of the noise will take care of posterization. If we do a little math and use some approximations, a signal of 16 photoelectrons varies by +/- 4 b/c of shot noise, and that signal is represented by, say, roughly 4 at ISO 100. The noise is represented by 1 digital increment, so you're fine. Yes, below this, you're not sampling the noise properly, so you may run into issues. That's where you'll benefit from using ISO 400. But if you're seriously trying to use image data from 16 photons and below... well, now, *there's* an extreme case.
This is exactly why I had issue with your initial claim that you could maintain high ISO dynamic range by just underexposing ISO 100 files. I suppose we have different definitions of high and extremely high ISO.
And like I said, above ISO 400, most of these issues are obviated, as you yourself have seen. From that point onward, you can do huge pushes and literally see no noise cost compared to shooting at the higher ISO.
Actually, you can perform quantitative tests to figure out exactly where this 'magical ISO' is.
Overall, I'm not sure what your point is though. You can generally use the technique I mentioned, save for ridiculously low signals of I'm guessing like 30 photoelectrons or less. I thought I was OK leaving that edge case out, at the risk of writing the novel I just wrote above.
And all that said, it does seem your A7 is not performing up to the level of the A7R or D800/810. Which is not too surprising - again, it's an older sensor.
Yes, switching to ISO 400 seems to give much nicer pushes up to ISO 6400.