If you believe that the ending makes the movie, Blowup is for you. The first 30 minutes seem aimless and wandering, but they set up the main character and what is he is to discover about himself, about his occupation and about art in general. Antonioni builds tension (or frustration as you're watching it) not with plot, but with anti-plot. You want to scream at David Hemmings's character to: focus! screw those models! do something! But as the film unfolds you will see why Antonioni chose this actor, this profession and those girls. A wonderful manifesto about the dangers of voyeurism and what it does to a man's sexuality that is 40 years ahead of its time. The symbolism might get heavy handed at times (mimes, a broken guitar), but the sets are so full of creativity and the actors so beautiful (this will give my age away, but Vanessa Redgrave, who knew?) that you forgive Antonioni (he's Italian after all). Hemmings is Hugh Grant before Hugh Grant, but in this role at least, much more interesting. He's highly sexual, but unlike his painter roommate, his chosen art form represses him, all in the name of the shot. And when he finally gets the perfect shot in the perfect light, it's so perfect that someone steals it, and for good reason. Did those events actually take place or just through his camera lens? When the photos are the proof of what you see, then when that proof is taken away, did you see?
Review of Blow-Up
