Why are my time-lapses flickering?

Question

On my way on experimenting with time-lapse, I'm facing a flickering of brightness after I stitch the images together and make a video. I understand that if I use Av mode while I'm taking the pictures, per each shot, the camera will recalculate the exposure and that might result in different shutter speeds and in turn, this will result in the flicker when the images are made into a video.

But in this case, I'm using the manual mode and I'm setting the aperture, time and ISO manually. There's no readjustment left for the camera and supposedly all the shots are identical - parameter-wise (and they are, I checked). But still, when I stitch the images into one video file, it flickers. Why is that?

BTW, I'm not interested in how to fix this issue using a software. I'm just curious if this is expected and if there's a reasonable explanation behind it.

[UPDATE-1]

I'm using a Canon 6D. It was on a tripod for half an hour and I took 360 shots with the following settings:

• Aperture: F22
• Shutter speed: 1/5
• ISO: 200
• Lens: Canon EF 24-105mm 1:4 L IS USM
• With two ND filters: 8ND and a 4ND (I was shooting the clouds on bright daylight)

[UPDATE-2]

Here's my video on youtube. The quality is lowered but I believe you can see the blue sky flickering.

[UPDATE-3]

I just remembered that I have another test with the same exact problem. In this case, I was indoors and in a shade. I've had almost the same settings as before except my shutter speed was 1/3s. Even though I'm not 100% sure but I think I've had three ND filters this time (2ND + 4ND + 8ND).

[UPDATE-4]

Per suggestions in the answer, I shot another time-lapse. This time, the camera was in shade completely and with a hood on. The settings are like before (1/5s and with two ND filters) and the flicking is still there. Next, I'll test with higher shutter speed and without ND filters (but I think the result will be the same). Stay tuned (I have to wait for another partly cloudy day).

[UPDATE-5]

As the only test left (at this point), I conducted another failed test. This time I shot in RAW format and set the white balance in post-production using Darktable. But as you can see the flickering is still there.

On top of that, as part of the new knowledge that I've gained, I extracted the exposure value out of the exif data (called MeasuredEV) and plotted it in the following chart:

My eyes are not exact enough to say if the flickerings are actually matching the spikes or not. But I assume so. To be complete, my camera settings are; shutter speeds 1.6", ISO 50, and aperture F22. Also, I had three ND2, ND4, and ND8 filters mounted on the lens.

Answer 1

Have you tried selecting a fixed manual white balance? It's another of those pesky little things you need to pin down for consistent time lapses. Other things are Auto ISO, "Auto Lighting Optimizer", "Highlight tone priority" and similar camera-specific adaptive settings. Also autofocus.

Answer 2

It could be the the video creation itself. Have you checked to see that there is no flicker when you manually slideshow through the photos? Do you have the photos hosted anywhere. We could plot a brightness indicator of the photo overtime. Most naive would be total of all pixels.

Maybe the frame rate you want for the photo is not aligned with the output framerate of the resultant video. IE you want 18 photos per seconda but the video is 60 frames/seconda encoded?

Answer 3

Try using TLDF app to remove the flickers. The flickers in your videos should be easy to remove.

Answer 4

Can you try to shoot at full aperture (with a darker subject of course). The mechanical movement at f22 might be so small that the lens cannot do it consistently accurate.

With those Nikon lenses, for which the aperture is set mechanically by the camera, this is a huge problem. But if I set the aperture on the lens it works well.