How does the Auto-ISO algorithm work?

connloyalist

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As the title says, I was wondering: When I have auto-ISO turned on, how does my E-M5 Mark II decide how far to push the ISO? Does it try for a specific shutter speed? It seems to adjust the ISO to make shutter speed come in around 1/100s ~ 1/125s? I couldn't tell any difference between IBIS turned on or off. I use manual lenses so I thought perhaps it takes the focal length set in the IBIS menu into account. No, not that I can tell.

Does anyone have any insight here?

Regards, C.
 

Giiba

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It is not very smart...

You can effect it in 'P' mode by adjusting the slow limit speed in flash settings, and the camera will try to keep that as a minimum before raising the iso. In other modes it does as it pleases with no regard for things like ibis.
 

ac12

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Depends on which camera and which shutter mode.

In P mode:

EM1-mk1 and EM10-mk2
  • mechanical shutter; shutter speed drops to 1 / (FL x 2), then the ISO starts going up.
  • electronic/silent shutter; shutter speed drops to a stupid slow 1/13 sec, then the ISO starts going up.
    • On these cameras, do NOT use the e-shutter on Auto ISO in low light in P mode.
    • As far as I am concerned, this is a firmware BUG.
EM1-mk2
  • mechanical shutter; shutter speed drops to 1 / (FL x 2), then the ISO starts going up.
  • electronic/silent shutter; same as the mechanical shutter
  • 3rd option is a user defined min shutter speed, where the ISO would start going up.
I have no idea how Auto ISO works with a manual lens.
Unless you tell the camera what lens you have on it, it won't know the FL to compute the min shutter speed.
 

connloyalist

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In theory, with a manual lens they could assume the flocal length set for the image stabilizer is the focal length to be used for this calculation?

Regards, C.
 

KBeezie

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I would imagine with a manual lens (least the way that makes sense to me), assuming that you haven't set a focal length into the IBIS setting :

- Aperture continues top open as needed until it hits maximum
- Shutter begins to drop as needed until it hits the slowest set (in this case I'd imagine something like 1/30th, but I know you can sometimes force it by changing the flash slow-sync speed higher)
- When both Aperture is at it's maximum limit, and Shutter at it's slowest set, the ISO goes up.

But I doubt that's how it actually works, and I usually shoot manually anyways in which case auto-ISO would just simply raise til my exposure is centered.
 

ac12

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@ac12 thanks, that's very informative. I had no idea my two camera bodies (em10.ii and em1.ii) were following a different algorythm to set the ISO. Any idea on how this works on Pen-F?
For the mechanical shutter, the EM10-mk2 and EM1-mk2 work the same. It is with the electronic shutter that they differ.
Basically the EM1-mk2 fixed a bug that is in the EM10-mk2 and EM1-mk1.

Nope, don't have a Pen-F to test. I would guess the same as the EM1-mk1.
 

Rob Trek

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Hi. I made a video some time ago on auto-iso for the E-M10ii and PenF. I think it's still relavent but I plan on updating it with some of the finer points mentioned in this thread.

 

ac12

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Rob, it appears that a firmware update on the EM10-mk2 updated the Auto ISO logic, to utilize lens FL to set the shutter speed floor. 1/(FL x 2).

I suggest that you do a mechanical and electronic/silent shutter instruction also.
I use the e-shutter at a concert, for no noise. And that is where I ran into the LOW shutter speed floor on the EM1-mk1.
I have yet to hear back from Olympus on this bug.
I did not test for the bug in A or S modes.
 

Rob Trek

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Rob, it appears that a firmware update on the EM10-mk2 updated the Auto ISO logic, to utilize lens FL to set the shutter speed floor. 1/(FL x 2).

I suggest that you do a mechanical and electronic/silent shutter instruction also.
I use the e-shutter at a concert, for no noise. And that is where I ran into the LOW shutter speed floor on the EM1-mk1.
I have yet to hear back from Olympus on this bug.
I did not test for the bug in A or S modes.
Yes, I noticed the shutter is slower in silent mode on my em10ii. It also affects A mode, but not S mode. Personally, I prefer the slower shutter speed but it needs to be consistent regardless of shutter mode.
 

barry13

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As the title says, I was wondering: When I have auto-ISO turned on, how does my E-M5 Mark II decide how far to push the ISO? Does it try for a specific shutter speed? It seems to adjust the ISO to make shutter speed come in around 1/100s ~ 1/125s? I couldn't tell any difference between IBIS turned on or off. I use manual lenses so I thought perhaps it takes the focal length set in the IBIS menu into account. No, not that I can tell.
Hi, I was using a 45mm adapted lens today (IBIS set to 48mm on my E-M1 mkI, firmware 4.x), and the body wouldn't drop the shutter below 1/125s in Aperture mode. Playing with it some more, it seems that the IBIS setting has no affect on min shutter speed, and it will hover between 1/100 and 1/125 in low light (until it hits the high ISO limit; mine is set to ISO 8000).
 

S-Osolin

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From my observations, auto iso is somewhat dependant on lenses. When on shorter range of up to 50mm it will constantly dip to 1/60s, while higher up at around 100mm, it'll rarely go under 1/160. I'd guess that it defaults to some some middle when it doesn't know what focal lens it's got.
 

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