Assume the case of a Canon 6D Mark II. Its lowest ISO is regulary 100. If expanded, I can set it to
L, which is ISO 50.
Until now, I was always under the impression the lower the ISO the cleaner the image, so whenever possible I've been using ISO 50. Now I came across this chart from photonstophotos.net:
And that leaves me completely confused. According to this chart, I have a lower noise at ISO 100. ISO 50 seems to have a higher noise than ISO 300 too. Is that anywhere near correct? As long as I would not be clipping highlights, couldn't I just use ISO 160, 300 or 600 instead of lets say 50, 200 and 400 (depending on my needed exposure) and later drop down the exposure in post to get a cleaner image?
I've seen this question about ISO 50 but the two top answers are kinda contradicting:
Since your camera offers this 'expanded ISO" that provides for 80 ISO, you can assume that this ISO is sub-optimal, and could exhibit more noise or a loss of dynamic range than the 'native' ISO. [...] Some suggest Canon cameras are 'native' for ISO 100, and full stop ISO are best (100,200,400 etc.).
You can use it and it gives excellent dynamic-range and very low image noise but really barely any different from the ISO 100 setting.
The chart above would suggest the first answer is right, but that definitely doesn't hold true for the latter part of the quote, as ISO 160, 300 and 600 seem to be the best choices.
Can someone tell me if I interpret the chart correctly, and if ISO 160, later darkened in post, will really give me cleaner, less noisy images in the end than using ISO 50 (assuming that would be the perfect exposure) in the camera right away?