The National Gallery of Art’s new retrospective of Harry Callahan looks at the photographer’s work on the centennial of his birth. But the exhibit might be most notable for inadvertently suggesting a truism about the career arcs of all brilliant artists: It’s really, really hard to keep creativity going indefinitely.

During his first half-century, the Detroit-born Callahan (1912-1999) was genuinely and consistently innovative. Working initially in mid-century Chicago, Callahan looked within his milieu, a Midwest then bustling with a mechanical heartbeat.

His finest works from this period are at once restlessly experimental and unpretentiously accessible. Callahan was a master of extreme-contrast photography, often letting uncompromising black and white hues speak boldly without the hindrances of complicating gray nuance. Ordinary trees in a park, for instance, are elevated by their pure black trunks and the unsullied white snow. In another image, four overhead telephone wires become an elegant arrangement of zips crisscrossing the sky.

At times, Callahan lets his subjects—a field of grass, or a façade punctuated by an intriguing arrangement of windows—bleed to the edge of the frame. At other times, he make still images full of motion. In one case, he shifts the camera back and forth to capture the spindly, haphazard trail of a flashlight. In another, he uses multiple exposures of leaves and branches to create the illusion that they’re swaying.

Some of Callahan’s most lyrical works involve the humblest of subjects: sunlight bouncing off the surface of water, the shadowy forms of bollards and birdbaths in a snowy park, and a series of plant shoots sprouting from fresh snow. These photographs possess a slight, gestural grace reminiscent of the works of Cy Twombly.

Some of these images overlap thematically with Callahan’s other great project of the period: documenting his wife Eleanor in various states of dress and undress. Some of the images of plant shoots echo the curves of Eleanor’s body, while decontextualized images of surf suggest the texture of her back.

In one celebrated image, Callahan photographs Eleanor’s head and arms in such a high-contrast way that her skin becomes as white as marble, her features traceable only by a few dark lines. (Props to Eleanor, who put up with her husband’s artistic whims over the course of a six-decade marriage, including sudden requests to pose naked in the middle of cooking or cleaning.)

Not all of Callahan’s early experiments are so successful. His layered exposures—often images of Eleanor sandwiched against landscape backdrops—seem forced, and his unusually close-up street photographs don’t feel as groundbreaking as the exhibit suggests they are.

The retrospective loses steam displaying last few decades of Callahan’s life. His photographs from this period—images of pedestrians, clapboard houses in his adoptive home of Providence, R.I., claustrophobic multiple-exposure experiments, and figures walking past façades in a variety of foreign locales—are increasingly repetitive and less bold than his earlier work. Where Callahan used neon signs in the 1940s to produce striking color abstractions in hues of peach, cherry, and aqua, his later color works are all too often wan and drained of verve.

Fortunately, Callahan’s mediocre pieces do serve one useful purpose: to emphasize how genuinely fresh his earlier images remain, all these decades later.