There’s no shortage of stunning material in the National Gallery of Art’s retrospective of Harry Callahan. The photographer, who died in 1999 and would have turned 100 next year, was relentlessly inventive, producing everything from curvaceous nudes to geometric landscapes. His technical experiments with black and white and color photography—capturing the paths of moving lights and using multiple exposures within the same print—remain impressively innovative today. But, my favorite Callahan works are his simplest: He was genius enough to make high-contrast photographs of humble strands of grass emerging from the blankness of a layer of fresh snow. The result is the photographic correlative to Cy Twombly’s minimalist scrawls.