Hi fennad. Most of colorist tutor out there use Log as material of color grading.
i don't know why no colorist color ther rec.709 footage LOL.
Actually, LOG footage is just a raw file with low level of brightness and color that contain so many information there. These information helps Resolve to create amazing 709 footage.
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If you have already 709 rec, there are just a little information in it, so you can not convert to log. You can edit it as look like log by decrease its level, and decrease its saturation.
If you need some log file, there are so many footage out there that you can download for free.