I have been working on a few projects that use RED files for the source footage at 3840x2160 resolution, and when you create a new timeline based off of the source it will create a timeline that recognizes the source raw size and then use the I-Frame only preview codec at 1920x1080. So the source footage is being sized down in the timeline to accomodate the smaller size, but in the motion tab of the source footage Scale is represented at 100%. This is universal from what I can see like if using 1920x1080 source footage and the preview files are set at 720x480 the source is still shown as 100% scale. So when you scale up beyond 100% with source footage larger than the output size does Premiere still leverage the raw size and not result in a loss of quality? Or is it only an issue if you render it and use the preview files for the export? Some clarification on how the relationship between these elements work would be great.
As an aside to this topic, I used Pluraleyes 3 to sync footage through their app and export out an XML to import into Premiere, and instead of a typical sequence being made when using canon HDSLR footage at 1920x1080 i-Frame Only preview files, Pluraleyes made the sequences at 720x480 DV24P Preset for the preview files, and I didn't realize it at first and noticed the first export using one of those sequences did not look as good as it should have and I used the previews to create the export, so all of the synced sequences had to be moved to the usual sequence settings.