The subject is "Doors". This could include old or new doors, gates, entryways, etc. By this, I mean something you pass through, not a rubber flap on a camera, etc. It must be a structure built by man, not a natural archway of stone, trees, bushes, etc.

Full particulars can be viewed here; http://forums.dpreview.com/forums/read.asp?forum=1008&message=35702639