in the project window, select the footage and choose file>interpret footage>main.
from that window you can set how you want ae to handle most aspects of the footage, one being how to handle fields. it will help to know a few things about your footage — is it typical 29.97 interlaced or was shot as 24p with a pulldown…? if it has a puldown, is it a typical pulldown cadence or advanced cadence…?
based on what you know about the footage, make sure the separate fields option is set correctly for your footage.
say you have typical 29.97, interlaced sd, ae should be set to separate fields (lower, for most sd footage). in addition, since you want to enlarge the footage, set the option for preserve edges.
Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW