As I said, Matti, observation and anticipation. The children are sliding down the slide, running around to the top and sliding again, so it's not too hard to predict where they will be. Kertesz was presumably on a bridge, so out of line of sight of the people below. so no one noticed him (always good to photograph people who are heavily absorbed in something, they won't see you). He thus had enough time to wait for the 2 unpredictable elements in the picture (the boy running towards the slide and the old lady at top right) to place themselves ideally. Allowing for the difference in attitudes between the time this picture was taken (1950?) and now, it's still not too hard to get pictures like this at an amusement park or similar venue. This is a very good picture, but it was taken by a human being, not God!
Originally Posted by matti