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.

 

Comments

[Fuji X100 ~ Blogs, rumours and]

[...] trouble with the X100 ISO? This blog post by Patrick la Roque might help. He has a website you should check out, frequently trying to do something different with the X100. [...]

[Erricose]

Thank you so much
I had the same issue

Regards from Belgium

[Patrick La Roque]

You're quite welcome :)

[Nick Alamanos]

I can't figure it out also, i took it to a wedding this weekend and even though I had max iso set to 3200 it would chose really low iso setting like 250 for indoors, and supperlow shutter speeds like 1/4 of a second. So i turned auto iso off and set it manually.

[Patrick]

@Nick:
That's odd. As I mention in the post, Auto ISO can easily be tripped into selecting higher ISO values due to the selected dynamic range settings and base ISO. But I've never encountered the opposite: the camera NOT going high enough. It will however quickly drag the shutter as soon as it runs out of ISO for a given aperture.

From what I understand the X100 runs the following list of checks before evaluating the needed ISO value (this is for aperture priority mode):
- Base ISO
- Dynamic Range (which can force a higher base ISO than your chosen setting)
- Chosen Aperture
- Exposure compensation
- Top Auto ISO value
- Selected minimum shutter speed in Auto ISO.

Once it establishes the base settings it checks for exposure compensation, then tries to give the lowest ISO possible for the current aperture. But as soon as it runs out of ceiling it starts dragging the shutter below your minimum value. In very low-light situations this can quickly go down very rapidly if the aperture isn't wide open and/or exp comp has been increased.

I don't understand how it could start lowering your shutter speed before increasing your ISO… This is very odd.

You could've increased your base ISO and forced AUTO ISO to start from a higher value, but it still doesn't explain the behaviour. Are you certain you weren't using spot metering instead of multi or average? This can produce quirky metering results depending on where you're pointing… A lightbulb in a night scene for instance would make the camera meter for much brighter values.

Do you remember your settings at the time by any chance?