(self-professed Hitchcock buff here, although I have to say Vertigo's not among my favourites
nonetheless, I can see why it's considered iconic by many, and this particular effect is just one of the reasons!)
Wikipedia says, "The effect is achieved by zooming a zoom lens to adjust the angle of view (often referred to as field of view, or FOV) while the camera dollies (moves) toward or away from the subject in such a way as to keep the subject the same size in the frame throughout."
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