Hey Phil…
depending on how you want to deal with the difference…..
prior to importing the media into PPro…
go to Prefs>General
and enable/Disable “Default Scale to Frame Size”
when this is enabled, Media will inherit the size of the sequence they are placed in.
In your case this would cause some non proportional vertical scaling in a 1080p sequence.
if this is enabled, clips won’t scale to the sequence and maintain their original resolution within the sequence.
Here you will be full width but have the option of positioning or proportionally scaling the clip to fit within frame and create the black bars (pillar box) you describe.
If your non matching source media is the 1st clip introduced to a 1080p sequence, you will be prompted if you want change the sequence to match the source….which you don’t…
After that….as far as rendering….there shouldn’t really be anything special to look out for as far as I can think.
hth,
Alex Udell
Editing, Motion Graphics, and Visual FX
Let’s Connect on Linkedin
Examples: Retail Automotive Motion Graphics Spots
Example: Customer Facing Explainer Video
Example: Infotainment & Package editorial