Subtle exposure shift bug in Canon 7D

I have noticed that when using the Canon 7D to create timelapses (speeding up video, not using still images) there are subtle shifts in exposure even though the Aperture is locked, the ISO doesn't change, and the shutter is fixed. It seems that the camera is still doing some adjusting according to the luminance of the subject (I shoot quite a few skies/sun/clouds).

Any ideas?

could auto white balance be effecting this?

could be the lens. I had a friend who had a 7D that ended up having a problem with exposure. At first he reckoned it was the lens, which was a zoom lens with a constant f/stop, meaning, if he were to have it wide open at the biggest aperture, it sure stay wide open as he zoomed in and out. However, when he did zoom in and out (yes, in manual mode with aperture and shutter locked) he noticed slight shifts in exposure, as if it would shift towards another aperture, but the quickly switch back. So he returned the lens and got a replacement, only to find the problem still existed. He then exchanged his 7D and alas! The problem went away. Oh, and also, to rule out the possibility that it was just the camera, he had tried the first bad lens on a T2i to see if the lens would work fine on there, but it didn't. So he came to the conclusion that BOTH the lens and the camera had some weird glitch in it that allowed the aperture to shift during zooming. I know your problem doesn't involve zooming, but it could be similar electronic failure or mis-communication to the camera, where it will shift to another aperture and then quickly shift back. Of course, it could be the camera chip, but I thought this bit of someone's experience might prove useful. Good luck figuring it out, and good job maintaining quality control and noticing the change in exposure!

I think it might be in the firmware, but specifically during a certain setting and certain lighting situations. I recently had the same thing happen on my T2i. From the observations I made - I think it has to do with the variation of light brightness across the shot - from extreme bright to extreme dark and the shutter speed. Here is an example where exactly that happened:

It makes no sense for this to happen. It is an annoying flicker every once in a while. The lens was set manually for both focus and f-stop as well as the camera was in manual mode across the board. (F 4.5 - 1/800 sec) - A big difference between this and many other time lapses I have shot without an issue is the shutter speed. I think that is the culprit. The slower the better. Hence, the next time I will test with a slower shutter speed and a higher F-stop - just to see and perhaps substantiate it.

Ouch, that is a painful flicker. Hope either of you find a solution, I don't want this to happen to my future time lapses either...

The exposure of a photo is not 100% precise. There is always a small variation in the exposure length. The longer the exposure the smaller the percentage of variation. (and the more noise... oh well, can't have it all.)

And there is definitly a variation in how open the aperture is. Remember most DSLR open the iris wide open to give you the best possible view in the viewfinder. So when taking the shot it has to close down the aperture to what it is set and then open it back again.

Two ways to conquer that part: a) shoot wide open. that way the aperture doesn't open and close with every shot. (but of course that brings short exposure time and shallow DOF) or better b) use an old fully manual lense that is not controlled by the camera. Nothing moves inbetween shots, no flicker from that source.

There are so many reasons why you should shoot stills instead of video for time-lapse. 4k, long exposures, waaaay less compression, to name a few.

Also, with most video I have found totally manual lenses are much more stable. * As varius mentioned. (non canon lenses with an adapter mount - Olympus, Zeiss or Lecia are all good).

Also Canon lenses finely tuned for stills - the focus ring is made for auto focus and much to short for video (to pull focus). Although, I have heard Canon are going to bring out a video range of lenses soon.