In this discussion It is good too go back the the ProRes white paper:
– “While the ability to produce high-quality output is a key attribute of image and video codecs, it is quality preservation—or fidelity—that is the actual goal of a codec. … If an image sequence has visible artifacts to begin with, Apple ProRes will perfectly preserve these artifacts … The goal of every Apple ProRes family member is to perfectly preserve the quality of the original image source, be it good or bad.”
FIDELITY on the first generation and mostly through multi “reencoding” (renders) is key to understand this choice.
I have been doing many test on ProRes, DnxHD codecs and many others , just to conclude that that ProRes is slightly superior to DnxHD as far as Fidelity is concerned…but let us stay in the ProRes arena, looking at the “PSNR Comparison – Apple ProRes 422 Family” graphic it is quite obvious that the minimum bitrate to correctly preserve quality should not be under that of the ProRes 422 ( typically 147mbs )
I believe this is the RIGHT choice that was made as far as preserving QUALITY in FCPX (this also applies in avid :it is not a good idea to go under Dnx145 ). LT can be a correct choice for output but certainly not for editing-rendering, HQ is pretty good and about perfect , but 4444 is just fabulous for the big screen (DCP).
just my understanding
ZAP