ISO 320 is the A7R5's second base ISO setting. You will find at ISO 320 you will get better results than even ISO 200.
I normally take a set of shots at 1600, 3200 and 6400 ISO. I am taking 60 light frames and 10 dark frames. I don't do any adjustments to the files before stacking. I really need to get organised and do some bias frames now.
Here is pretty much my first successful Milky Way shot from a few months ago. I was combatting a bit of ambient light and quite a lot of cloud but I'm pretty happy with this. It was shot using my Sigma 16-28 f2.8 which is better at astro than I had anticipated, at ISO 1600.
Welcome, dear visitor! As registered member you'd see an image here…
Thanks for the information. Since my original post, I did some "experiments" at different iso settings. Best results were obtained at iso 320, and then increasing the exposure by four stops in Lightroom. The biggest difference compared to using a higher iso was that there was detail in dark foreground areas, while at high iso the dark areas were blocked. This is consistent with articles I've read about ISO invariance.