I'm not sure what Brakhage was trying to get at, if anything, with this very short exercise but it definitely was not one of his best efforts, in my opinion.
Basically the viewer is taken through a series of shots of an elevator door as it is being approached by the camera/filmmaker. With each shot the focal length and/or focus are slightly different, and perhaps the length of the take itself. That's it. For a man that redefined the experimental film world (and arguably the medium itself) DOOR seems noticeably underachieved, almost more of a test film than an actual work of art. Chances are, Stan had something far deeper behind the vision that I just didn't catch.
4/10. Locked out from meaning and beauty.