Well, that is a fairly complicated (and much studied) shot.
A lot of planning went into it. In fact, Michelangelo Antonioni basically had the hotel set specifically built to accommodate that specific last shot.
The scene was shot right at dusk, so there would not be a huge lighting difference in the interior and the exteriors when the camera moved outside (not riding the iris at all).
In a nutshell....
The camera was traveling on a track that was built into the ceiling of the hotel room. The window grate was constructed so that it had a split in the middle. When the camera reached the point where it was about to push through the grate, the window pieces were swung open from side to side, allowing the camera to push through. At that point the camera left the track and was transferred to a very high overhead crane that suspended the camera below...it was not a camera crane, more like a construction crane. You can see a little bit of "floating horizon" in the latter part of the shot, which is usually the dead giveaway of a Steadicam shot... however, there was no Steadicam. There were however two gyroscopes attached to the camera... which explains why the shot has a slight Steadicam look to it in the last couple of minutes.
It's quite a great shot. It's a little slow for my taste though, the camera blocking is actually sooo slow that I'm sure the casual viewer doesn't realize what a production it must have been. Blocking all of the action and getting all the actors' timing down was probably almost as difficult as blocking the camera move. Supposedly it was also very windy on the day of the shoot, which made the floating camera move even more difficult.
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