When you think about it, there's only a handful of fundamental rules in photography. They have to do with the optics of lenses, the physics of light and the mechanics of photographic machines. All things being equal, when I teach I prefer to illustrate these simple principles with masterful photographs. That way, students pay them due respect and begin to realize the expressive and creative potential they hold.

Earlier this week, I revisited some of my favorite photographs by Jim Nachtwey during class. It was not a lecture about the ethics of photojournalism or a considered survey of Jim’s work. Nothing like that. My lesson plan called for an overview of aperture and depht of field.

Like any photojournalist worth his salt, Nachtwey has stopping power. But unlike most of his colleagues, Nachtwey makes me linger within the frame and re-imagine the crude world he shows me: what that same street would have felt like to a child walking to the corner store for a quart of milk or a loaf of fresh bread; how that particular room would have welcomed a couple undressing for the night; how a woman’s smile would light up her eyes before violence and cataclysm took over.

Nachtwey’s moral gravitas is found there: in this strange impulse to rewind history. As viewers, we are confronted with tragedy and in the same instant experience the deepest regret for it. But we don’t stop there. The photograph urges us to consider if this was the only possible outcome or if somehow this moment in time was preventible.