BT.1886 (not?) and ACES Rec 709 Output Transform

I'm starting this new thread because I've discovered that Resolve almost certainly outputs Rec 709 as straight gamma 2.4, not BT.709 when working in ACES color science. (I would hazard a guess it's the same in BMCM). This has had me confused for some time but today I ran some tests that are pretty conclusive. Hopefully someone will jump in and correct me if I'm wrong but some clarity from Blackmagic would help.

Settings:

Settings.JPG (103.55 KiB) Viewed 2485 times

Using Calman I calibrated two very accurate 3D Color Viewer LUTS for my Dell P2715Q UI monitor: Rec 709 BT.1886 and Rec 709 gamma 2.4. The gamma curves are different as expected

BT1886

Gamma Curve 3D Color Viewer - LUT calibrated to Rec 709 BT1886.JPG (35.05 KiB) Viewed 2485 times

gamma 2.4

Gamma Curve 3D Color Viewer - LUT calibrated to Rec 709 gamma 2.4.JPG (38.46 KiB) Viewed 2485 times

I then captured SMPTE bars in the Color Page viewer after separately applying the two LUTS to the 3D Color Viewer slot.

CONTINUED IN POST FURTHER DOWN TO INCLUDE INLINE IMAGES

I can't seem to add more than 3 jpegs within the post. Is there a limit? Size restrictions? I can't find any info.

I can't seem to add more than 3 jpegs within the post. Is there a limit? Size restrictions? I can't find any info.

Yes : only 3 images per post. it's a limit (open a next post, etc...)
For size, I think limite is <1MB (not tested the max)

contuned from above
BT.1886

3D Color Viewer - LUT calibrated to Rec 709 BT1886.JPG (105.12 KiB) Viewed 2479 times

gamma 2.4

3D Color Viewer - LUT calibrated to Rec 709 gamma 2.4.JPG (94.52 KiB) Viewed 2479 times

I then output a tiff and captured it in Windows 10 Pro's Photo Viewer

Tiff output ACES OT 709 .JPG (129.84 KiB) Viewed 2479 times

cONTINUED NEXT MESSAGE

If the images appear in this post as I think they're supposed to (they don't show in the preview) it's pretty clear from the near black chips in SMPTE bars that the Rec 709 output matches the 2.4 calibration much more closely. Given the variables of screen cap and jpeg they're probably a precise match. Here they are lined up and enhanced equally in Photoshop:

Black Chips.jpg (27.88 KiB) Viewed 2479 times

This would appear to indicate that if one is using the 709 transform to output from an ACES project it should be graded on a monitor calibrated to 2.4, not 1886. It may well be the same with BMCM.

The question is why do you need (to master in) BT.1886?

The question is why do you need (to master in) BT.1886?

I don't. I do need to know what calibration to apply to my monitor in order to grade properly. Without knowing what gamma curve is applied by Resolve on Rec 209 output it's a bit of a guess.