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GH4 V-LOG contrast/gamma shift on external recorder
  • AlexMcVAlexMcV

    Heya,

    Not sure if anyone else has encountered this, I've done a quick search and come up with nothin...

    I've been using the atomos assassin with my GH4, and noticed that the footage from the assassin is slightly more contrasty/darker than footage recorded internally. I assume it's something the GH4 is doing, because it only seems to happen with V-LOG, not with standard. (I couldn't really be bothered checking with all the other colour modes...)

    Here's a clip of me A/B-ing it on a premiere timeline with the scopes up. V-LOG, then standard. https://www.dropbox.com/s/hiugamp6hfbved4/gh4-assassin-contrast-level.mp4?dl=0

    (obviously I don't own a colour chart, a set of pencils was the bet I could manage...)

    This was recorded 8-bit, as it was the only way I could record both at the same time, but it happens with 10-bit too. At 4K as well as HD. Both the assassin and GH4 are running the most up-to-date firmware, as of Jan 2017.

    Did I mess with some settings somewhere? Is my camera haunted? Is this something other people have noticed?

    It doesn't actually bother me that much, I guess this is just a "what's up with that?" kinda question. So yeah. What's up with that?

  • 1 Reply sorted by
  • balazerbalazer

    It's a bug in the GH4's firmware. When you select V-Log L, the GH4's internal recordings have the video_full_range_flag set in the H.264 stream's Video Usability Information (VUI). That flag should not be set, because V-Log L does not using a full range encoding. V-Log L uses standard BT.709 level range encoding. The recordings made in the Assassin have no such flag.

    Premiere reads the video_full_range_flag in the H.264 stream and compensates when decoding. That's why the levels are different.

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