If one could actually manage “the tag” then all applications could actually deal with the “shifting”. There is no tag control that I know of in Media Composer once the media is in the DNxHD codec. The media is already created, but you shouldn’t see a shift.
Other codecs wrapped in QuickTime can have its levels changed via AMA by going to the source settings and you have a choice of expand, restrict or do nothing. Basically allowing you to change where the black and white points fall – either RGB graphic levels of 0-255 or SMPTE video levels of 16-235 (all in 8 bit speak). The histogram on the source settings page as two white bars to show you where 16 and 235 lie in the image. Adjust those settings accordingly. If you’re going for broadcast, DVD etc, then 16-235. If web only, then 0-255 is fine, but make sure you manage the export later properly so you don’t crush again by selecting RGB.
For my work, I keep everything SMPT 16-235 so I am ready for broadcast, then export with RGB (expand) for web deliverables. Easy to keep track of. Only if you go back and forth with After Effects and stuff will you need to keep track of those levels.
Michael
Michael Phillips