Gamma shift

Discussions - Using ACES
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Currently I am working on a project , since it has so much vfx shots Involved we decided to work on aces pipeline . Everything was going great till yesterday.
I am using Eizo CG319X as my reference monitor set’s to Rec709 2.4
Also ACES ODT set’s to 709
Everything looks great in my reference monitor when I export it to DP for approval every thing looks too dark in my iMac display also my DP and director are using iMac as well. So tried tagging gamma to Rec709-A now everything looks washed out . I tried YouTube and Vimeo test’s but the results are same like what I see in my iMac display . Also Tried changing my ODT to sRGB before exporting but it doesn’t work.

Am I missing something ? . It would be very helpful if someone help me out ( I know it’s a dumb question to ask in this forum but I really need someone to help me )

Thank you

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Something I don’t have a definitive answer/solution for but this topic came up earlier here and I shared the following. The problem isn’t related to ACES.

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Here’s some info on how Apple/Quicktime/AVFoundation handles video and files with 1-1-1 tags.

AVFoundation uses a value of approximately 1.96 as the gamma of video (the gamma value without the boost required for classic dim surround viewing environments). To derive that number, you first measure the response of a CRT. Most CRTs report a gamma of between 2.4 and 2.5 (depending upon the brightness and contrast settings on the monitor). You choose a value halfway between the two (2.45) and remove the contrast enhancement (the 1.25 gamma boost provided for viewing in dimly lit environments). The result is a value very close to 1.96 (2.45/1.25 = 1.96).

MacOS 10.6 and later converts the video gamma to the 2.2 display buffer gamma when the app performs the color match to the display.

Source: Evaluating Video Using QuickTime Test Pattern Files | Apple Developer Documentation

The following list summarizes the FoS luminance expectations (output in nits) on macOS platforms for video files tagged as BT.709 (NCLC = 1-1-1).

MacOS internal display

  • FoS Nits = Reference White Nits * linearBrightnessAttenuation * (Normalized Codeword)^(1.961)
  • The system uses a 1.961 gamma based on the assumption of a bright surround environment. The linear brightness attenuation varies between 0 and 1 and is dependent on the position of the display’s brightness slider. Reference white nits is the peak luminance capability of the Mac display.

External TV or monitor through HDMI interface

  • FoS Nits = Reference White Nits * (Normalized Codeword)^(display’s EOTF gamma)
  • Reference white nits is the peak luminance capability of the external TV or monitor. The display’s EOTF (electro-optical transfer function) gamma is the gamma used by the TV or monitor to decode the input signal to linear light.

Source: Evaluating an App’s Video Color Using Light-Measurement Instruments | Apple Developer Documentation

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