Auto ETTR - Sudden dips in exposure

[alderbaran2013]

I am interested in the Holy Grail sunset and sunrise timelapses and when I read that it was possible to take flicker-free ETTR timelapse using Magic Lantern, I was instantly hooked. However, my first few attempts using AETTR have been rather frustrating. During a sunrise shoot, the AETTR would ramp up the ISO and the shutter speed nicely but half way through the shooting, it would suddenly increase the shutter speed, causing a massive dip in exposure and immediately bring it back down in the next sequence. This occurred at regular intervals for the rest of the timelapse. Out of 288 shots this morning, 14 of them had sudden dips in exposure. I wonder if this is due a camera setup issue or a software bug. Allow me to share my camera settings and some screen captures of the problem.

Camera:
   Canon 60D + Canon EF-S 18-200mm IS
   Hoya Prond 3-Stops ND filter

Camera Settings:
   Quality: RAW
   Image review: 2 secs
   White Balance: Sunny
   ISO Auto Max: 1600
   Live View shoot: Enable
   Expo simulation: Enable
   Auto power off: Off
   Mode: Manual

ML Settings:











LiveView 133 (ISO 100 0.30s F/4.5) (Histogram displayed E-0.9)


Image Review 133 (Histogram displayed E-3.3)


A sudden drop in exposure occurred at the next frame, possibly due to the evaluation of E-3.3 in the previous frame.

LiveView 134 (ISO 100 1/30 F4.5) (Histogram displayed E2.8)


Image Review 134 (Histogram displayed E2.9)


The exposure jumped back up in the next frame.

LiveView 135 (ISO 100 1/5 F4.5) (Histogram displayed E0.5)


Image Review 135 (Histogram displayed E0.2)


Overall Exposure Graph (from LRTimelapse) showing multiple dips in exposure during shoot like an ECG


Hopefully, you can help me to unravel this mystery. I'm looking forward to a truly flicker-free timelapse minus the sudden dips.

[Audionut]

LiveView 133 (ISO 100 0.30s F/4.5) (Histogram displayed E-0.9)


Image Review 133 (Histogram displayed E-3.3)


A sudden drop in exposure occurred at the next frame, possibly due to the evaluation of E-3.3 in the previous frame.


Image Review 134 (Histogram displayed E2.9)


The exposure jumped back up in the next frame.


Image Review 135 (Histogram displayed E0.2)

This sequence is expected.  When there is overexposure (133), ML can only guess how much to reduce exposure to compensate.  It over compensated (134), but then since the was no overexposure, ML knows exactly how much to increase exposure for the next frame (135), which is why it's back to E0.2.

133 seems a little weird.  It might have something to do with being in movie mode LV, but I'm not 100% sure.  I suspect the lack of contrast in the highlights also caused some issue.  133 looks only slightly overexposed, but ML reduced the exposure greatly.

ML doesn't know what the exposure is going to do, so during an sunrise where there is likely to be overexposure from one frame to the next (same exposure settings).  It's this overexposure that causes issues.  As stated above, when there is overexposure, ML has to guess how much to adjust exposure, and it will err on the side of caution by adjusting the exposure to make sure there is no overexposure in the next frame.

So the sequence is, good exposure, overexposure (sun getting higher in horizon), underexposed from the compensation applied by ML guessing, good exposure because ML knows exactly how much to compensate from underexposure, rinse and repeat.

I've never done an sunrise timelapse so I'm not sure what the guys do to compensate for this.  One option would be the ramping module.

[alderbaran2013]

Thanks for the explanation, it makes sense to me now. I was concerned that I may have done something wrong with camera and ML settings. I loaded the CR2 + XMP files into Adobe Lightroom and the final result was much better than I thought.

[Audionut]

With a short interval between shots, any overexposure is likely to be minimal.  And when exposure is reduced, it's only the deep shadows that are going to be an issue (as far as noise is concerned).

If you want to cover all bases, you could enable dual ISO linking (to protect the shadows) with the relevant SNR settings, and set some overexposure protection by setting the exposure target to -1 EV or so.

[alderbaran2013]

Thanks, I'll give it a shot on my next trip. I left the interval at 15 secs because of the slowest shutter at 6 secs and image review of 2 secs.

[alderbaran2013]

In my previous sunrise timelapse, as the scene got brighter, I noticed that the sun became overblown and caused a lot of flickering that could not be mitigated with Deflicker. If I wanted to prevent the overblown highlights, should I go for the following setting?

Highlight Ignore          0.1%
Midtone SNR               0
Shadow SNR               0
Overexposure Target   -1 EV
Allow Clipping             No Clipping

Thanks

[Audionut]

You'll probably want to allow the sun to blow out, else everything else will be pushed into the noise floor.

Play with the Highlight Ignore percentage.  You want to adjust it so that the sun is blown, but nothing else.  The feature describes the amount of pixels allowed to be overexposed, so it's related to your focal length.

With the Overexposure target set to -1 EV, you should be covered for the times where the sun hasn't yet risen, but I'm not 100% sure.  With 15 secs between shots, you may be able to navigate the menu in between shots to adjust the highlight ignore setting.