Hi Chris...sorry to get to this late...just saw your question...Honestly the difference between iso 50, 64, 80 and 100 on the A7RII is, for the most part, negligible. The camera is new and I am still "getting to know" my options and if they work for me or not. The only reason I'm testing lower than the native 100 iso is to see if I can get those creamy smooth color transitions and shadows that can be pulled up as much as I need, and be totally noise free. I have tested these iso's against one another, looking at the files at 400 percent (one has to be compulsive about file smoothness to do this...:-/.....I come from a large format film (iso 100) and medium format Phase One backgrounds and I want to do everything possible to get my files on this Sony as close to my Phase One files (at Phase One base iso of 50) as possible. That's just me. I wouldn't recommend others even bother with this. I probably ought to quit posting my iso's when I post an image. On my comparisons, I have found that iso 50 for instance will give me smoother areas in say a blue sky or in very deep shadows (on extremely close inspection....pixel peeping) than more closely resemble my Phase One files with the same megapixel count. But to repeat, the differences between iso 50 and 100 on the Sony are negligible. Hope this helps, Eleanor