AutoETTR doesn't let you change ISO when LiveView is enabled

[PaulJBis]

Hello:

Yesterday I was trying to make a sunset hyperlapse using AutoETTR and the advanced intervalometer when something really odd happened. First of all, my data:

  -I'm using a Canon EOS 550D.
  -I'm using a ML build from March 15 2014 (yes, I know, it's old, but I've been using it forever and it has always worked great, so I'd like to know what exactly went wrong before upgrading).
  -My AutoETTR parameters were: Exposure target at -1EV, Midtone SNR limit at 4, Shadows SNR limit at 2, Hightlight ignore at 0.1%, Allow clipping is Off.

Before starting with the time-lapse, I tried taking a couple of test pictures to allow AutoETTR to find the right exposure. I turned on LiveView (because I needed to see the framing for the hyperlapse), and the ISO set itself to 3200 (because it was dark). I took a pic, it came out overexposed (because it was dark, but the street lights were on) and AutoETTR said "the next picture will be taken at 1/2 ISO 100"... but the ISO stayed at 3200. I took another picture, and it came out again at ISO 3200... and of course, it was also overexposed.

I tried changing ISO by hand, but the camera didn't let me. Every time I changed the ISO, it set itself again to 3200, and AutoETTR didn't seem able to change it either. After doing several tests, it looked like:

  -If I turned off LiveView, both AutoETTR and me could change ISO without problems.
  -If I turned off exposure simulation, ISO could also be changed... but AutoETTR needs ExpSim on. (I should mention, BTW, that the Canon "ExpSim" warning was flashing all the time).
  -If I turned off AutoETTR, I also could change ISO.

Thinking now about it, I guess the reason I had never noticed this before is because I had always done day-to-night timelapses, so when I started them, there was always enough light for ExpSim to work properly. Yesterday I started the hyperlapse when it was already pretty dark.

My question is: what was forcing ISO to be at 3200? Was it ExpSim or AutoETTR, or were they interfering with each other? And what can I do in similar circumstances, when I do need LiveView on?

[dmilligan]

IMO, this is a good reason to split the "Always On" AutoETTR option. There should be an option that runs always after each picture is taken from QR data, but not continuously in LV. Right now, if you want AutoETTR to run after each photo, you also have to have it enabled where it runs continuously in LV (b/c these options are combined into one setting: "Always On"). This particular situation is one where you don't want both. Anyway, it's always sort of bothered me that these settings are combined, but I never really came up with a good justification for why they should be separate.

What's probably happening is that since AutoETTR is running continuously in LV, it is changing the exposure back to ISO 3200 as soon as it "meters" in LV again after the QR metering. The bad LV metering is probably coming from the fact that ExpSim has to boost LV exposure to make up for the fact that using a shutter speed slower than 1/fps is impossible. Since it's dark, your actual shutter speed is probably a lot slower than 1/fps of LV. The LV image will have the same apparent brightness (that's the point of ExpSim), but the noise will be quite different (it's using a different shutter speed, than the real one you've requested). The extra noise in LV data is probably causing AutoETTR to choose a higher ISO b/c of your SNR limits. So what's really happening is that AutoETTR is fighting itself. The LV AutoETTR metering and the QR AutoETTR metering are coming up with (very) different exposures.

Perhaps a1ex has already taken this difference in noise into account when using ExpSim in LV, and his heuristic is just off, or maybe it's just not been accounted for, IDK, I'd have to check the code.

So there are a couple of things you can try (until my request in the first paragraph is implemented):
- Use FPS override to allow slower shutter speeds in LV (IIRC, there is an option to link it to Canon shutter speed)
- Don't use SNR limits
- Disable AutoETTR and change exposure settings manually based on raw histogram ETTR hint in QR (I know this is probably not ideal)

[PaulJBis]

Thanks, I see what you mean. I guess I didn't remember that AutoETTR was on during LiveView too.

I have tried using FPS override, but if the shutter gets too low (like 0.5 sec. or so), it will pretty much make the screen unusable, since it will refresh it at whatever frame rate gets calculated from the shutter, i.e., 0.5fps or so.

[mothaibaphoto]

What's probably happening is that since AutoETTR is running continuously in LV, it is changing the exposure back to ISO 3200 as soon as it "meters" in LV again after the QR metering.

I'm not sure for 100% as this screen with QR metering disappears very quickly and it was some time ago, but as far as i remember, QR stated that next time ISO will be something lower, but persist shoot with the same ISO and overexpose image
So, probably, this explains any and all issues with timelapse ETTR exposure, at least I've met.
Thanks, @dmilligan, for insights and possible workarounds. I'll try it next time.

[PaulJBis]

Actually, now that I've thought about it, there is a very simple solution. Since I only need LiveView to check the framing, and the exposure will be controlled by AutoETTR using QR data, I just disabled ExpSim in ML... and it seems to work.

 

Amazing how one can remain oblivious to the simplest solution in front of one's eyes.

[mothaibaphoto]

Today I checked out this thoroughly. I can easily reproduce overexposed AutoETTR images but not with intervalometer. If i start intervalometer in the situation, described in first post, after a couple of takes exposure become correct.  Simply because there is no LiveView between shots I remembered, that first FRSP releases didn't disable LiveView between shots. I have such release for MKII. Even with this release I can't reproduce overexposed images with intervalometer. Moreover, i spend almost all day trying to confuse AutoETTR with different hard lighting conditions, but didn't succeded
This mean that assumptions, that this fixes problems i had with AutoETTR exposure before incorrect. And i can't reproduce it despite all efforts