The more I think about it, read, and look at the pictures, the more I think HCB was in fact sui generis, one of a kind.
It's helpful to think in terms of a painting. Consider, say, a painting of Children At Play. If it's an excellent painting, it depicts some sort of essence of Children At Play. In it, the children have not just scored a goal, the smaller child is not tackling a larger one. The children are simply engrossed in the game. Perhaps one child, in the corner, has been distracted by a coin he's spied on the pavement. The lines of the curb and the railing in the background are placed just so, as are the windows in the buildings.
The body language of the children is varied, as are their expressions. They are placed in the frame just so.
The moment depicted is not a special moment to the children, they would not recognize it as any different from the moment 10 seconds earlier or later. It is perhaps a completely typical moment in their play.
The painting is put into a canvas of such and such a size.
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So what?
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This right here is what HCB was up to. The Decisive Moment is not necessarily a human moment. It is not necessarily recognizable to to the people in the frame as special. It is not the moment the lovers kiss. It is not the moment the goal is scored. It is not the moment the milk spills. It is the moment when the painting of Children At Play, or The Lovers, or In The Kitchen appears in the viewfinder.
If you're out there looking for those human moments, those moments that the people in-frame would recognize as special, well, you can make some fine pictures. But your picture will be, most likely, of some children playing, some lovers kissing. They're not the pictures HCB was taking.
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How do you do it? Well, he tells us. You become engaged with the scene. You move, as the players in the scene move, and together you move the forms in the finder around. This is important, so I will repeat it: TOGETHER you move the forms in the finder around. Until a painting appears. Ideally. In reality, you grope towards it, shooting, wondering if that was it, or is this next moment it, or this one. You work the scene, you try, you shoot, you get closer, perhaps, to the ideal form.
In the end you have 2 or 3 or 20 or 36 exposures, and one of them, perhaps, will be the best one.
Movie mode could help here, not as a substitute for technique, but as an add-on. If you could shoot 300 frames, then one would be the best. If you could shoot 1000, then one of those would be the best.
But you still move, you watch, you engage, you hunt for that "painting".
A strong argument can be made that the only tool that really works for this is a small rangefinder. I'm not going to make it, because others have made it before, and you can probably go find that.
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And finally, it is perfectly clear why one would not crop. If you bought a rectangular canvas, would you paint a square painting on it? No. The painting is fitted to the canvas. If the painting is fundamentally square, you get yourself a square canvas, or you don't paint it.
There are infinitely many 3:2 paintings to be made, why fuss around with the square ones, or the 4:5 ones? You've got a 3:2 canvas. Put a painting on it.
