Very interesting. I'd never heard of ISO invariance, so thanks for that davy.
On first watch it seems to fly in the face of the original photographers and Int Standards Org of ISO in cameras. That standard and how it works is exactly how I use my Canons (and in the past Nikons and even my Panasonics daily happy snap cameras).
Namely, to 'see' if I've landed on target I set ISO 12800 and 4 second single frame. Grainy as heck but I'll most certainly see color and know if I'm on target in just 4 seconds! Centering becomes just a few mount movements and just a few single 4 sec frames. So it doesn't take much time to frame it nicely! Then, that ISO "standard" means that when centered, I know that if I halve the ISO to 6400 then I'll both halve the noise AND get exactly the same brightness of image by setting double the exposure time. IE ISO6400 and 8 second exposure. And so on ... ISO3200 means 15 second exposure will give the same image brightness AND half the noise of ISO6400. And ISO1600 means 30 seconds will give me the same image brightness and half the noise of ISO 3200. And, tada, the fact that each step keeps the same image brightness, means that whatever astrotoaster slider adjustments I performed on that very first ISO 12800 single 4 second shot, I can simply leave ... That same AT settings of all the sliders will work perfectly for 12800/4 6400/8 3200/15 1600/30 800/60. Makes VA easy with a DSLR. Really easy. But, if I find I really struggle with lack of detail then if 30 secs was dim (say) by doubling exposure time to 60 seconds I'll get an image twice as bright and twice as much detail. But I will therefore have to adjust the AT sliders as the image is twice as bright.
So I use that halving of ISO and noise with each step, and doubling of exposure time, to my advantage when doing VA. So initially I was thinking boy that video which shows how the A7 works stuffs up my workflow so do I really crave for a SONY A7S now? Maybe not? But, re-watching I really cannot believe what I am seeing there! I think it means set ISO 100 (so the lowest noise possible by the camera), then in AT simply bump up the brightness and see what it looks like. If it is not bright enough, then set ISO 200 and repeat. As, tada, the video shows that the detail seems to ALWAYS be in the image EVEN when at ISO 100! That's incredible!
I think I am going to have to go visit DigitalCameraWarehouse down the road and beg one of the guys there to loan me one, or find out where I can rent one! I have to try that.