X100 auto iso behaviour explained

This post from photographer Robert Catto answered something that was puzzling me about Auto ISO on the X100:

“Came across some curious Fuji X100 behaviour on the weekend, just thought I’d throw this out there to see if anyone had seen it too.  A bit hard to explain, so bear with me… 

Saturday night, shooting a friend’s birthday party, turned off Auto ISO and changed to 6400ISO manually for that event.  (It was dark!)  Changed it back to Auto (highest setting 3200) at the end of the night.

Next day, out walking the neighbourhood in broad daylight, and kept finding the Auto ISO giving me hugely fast shutter speeds at f/16, and discovered the ISO was up at 3200.  In daylight, like the photo above, looking across Miramar to Stone St Studios - which was at 1/2000th @ f/16, and 3200ISO.  Huh?!

So…went back into the manual ISO menu (without turning Auto ISO off), turned that back down to 200, and…guess what.  The Auto ISO started choosing lower settings and giving me sensible choices of shutter speed - 1/60th at f/11, at 250ISO for example.

So my conclusion is this: auto ISO is somewhat influenced by the manual ISO setting you have the camera set on, even though you’re not using manual ISO.  If that’s up high, it assumes you’re wanting fast shutter speeds and ISO, rather than trying to give you the cleanest file possible by keeping the setting low.” 

He’s exactly right. Auto ISO uses the Manual ISO setting as it’s floor*. BUT there’s more to it. This is what I posted on Robert’s blog: 

“Found something else: this morning I started testing AUTO ISO again, with my manual ISO at 200. But I was still getting ISO 1600 in lighting conditions that didn’t warrant it at all.

So I had breakfast. And then I had a hunch. My Dynamic Range settings were set to 400 - which cannot be achieved below ISO 800. I set it back to AUTO and lo and behold: AUTO ISO came down to 400, perfectly normal for an overcast day.

So, not only does Manual ISO affect AUTO ISO, but Dynamic Range settings affect it as well. And apparently, once it’s affected by these other parameters it has a strong tendency to go to the max ISO value instead of the floor values set elsewhere. Which makes me wonder if it isn’t a bug rather than a feature.”

Lots of details to figure out in this pretty little thing.

*Apparently Nikon does this as well but I’ve never used Auto ISO on my Nikons. Colour me clueless about this one.