RAW Video issue with Zoom Lenses?

[DJHaze596]

I have a Canon 6D and the 24-105mm Lens.  Works great but i'm noticing when i zoom in the Picture gets darker and i have to adjust the exposure.  it's an f4 lens which means it shouldn't do that.  My guess is RAW video works a little differently?

[Midphase]

I don't think so. Raw is recording exactly what is hitting the sensor at all times. Is it possible that while you zoom, you're zooming into a slightly darker area?

Also, the 24-105 isn't par focal so when you zoom your focus is drifting perhaps causing a perception of the image darkening?

[dubzeebass]

Pretty sure it is parfocal

[Scott Free]

Zoom lenses claim on constant aperture but it's not always the case. 

[a1ex]

To check this hypothesis, grab raw_diag from here, run the Optical black + DR analysis at both zoom extremes, of course, without changing aperture, and post the screenshots. Do that in both photo and movie mode. Make sure you have a light bulb in the frame to get the white point correctly.

[lostfeliz]

Just because the f-stop stays the same through the focal lengths might not mean that the amount of light transmitted (the t-stop)is the same.

[Midphase]

Pretty sure it is parfocal

Not according to several people who actually use them. Just do a quick Google search, or better yet, test it with your own if you have one. It also appears that the amount of focus shift in the zoom varies depending on the camera body that it's mounted on.

http://www.magiclantern.fm/forum/index.php?topic=8752.0

[MrTodd]

Hi

I regularly use an F4  24-105mm lens when shooting RAW/MLV. I must confirm I have never had any issue with changing light while zooming. I also use the VAF-5D2 in camera which definitely makes the lens non-parfocal.

In my experience your problem sounds like a lens based one not a camera or software based one.

Hope you get it sorted chum.

[scarluuk]

I also find this quite annoying about the 24-105mm lens but, however, it's not a firmware, build or camera fault but it's in the lens itself.
Just as Lostfeliz posted it's because of the light transmission isn't the some because of the design of the lens.
The t-stops are different, not the f-stop, or at least with most of the 24-105mm lenses.