An exhibition devoted to a critic is an unusual enterprise. The particular curatorial challenge of this conceit is on display in the Metropolitan Museum’s “On Photography: A Tribute to Susan Sontag,” which places some 40 photographs from the museum’s permanent collection alongside selected quotations from the late author’s thoughts on the medium.

First, the good news: The show is well worth a visit. Almost every image is a subtle and alluring masterpiece, and the excerpts from Sontag’s writings are frequently provocative and occasionally brilliant. Equally important, several juxtapositions of criticism and photographs highlight the strengths of each. Sontag’s observations about such masters as Robert Mapplethorpe, E. J. Bellocq,Walker Evans, and August Sander pair well with exemplary images of these artists’ work, creating a rich interplay of art and criticism.

But the exhibition runs into several problems,mostly because of its reliance on many of Sontag’s more general (not artist-specific) ruminations about photography. Though insightful, some of her statements predetermine how viewers will react to the photographs placed nearby. Wonderfully enigmatic images such as Robert Frank’s “Fourth of July, Coney Island”(1955) and “[Railroad Crossing with Twenty-One Crosses, Near Indianapolis, Indiana]” (1930) by an unknown American artist become mere illustrations of Sontag’s statements that the flaneur is attracted to the “unofficial reality behind the facade of bourgeois life” or that “all photographs are memento mori.”

A related problem is that the show frequently does Sontag’s writing a disservice. Take this passage on war photography, which illustrates the great strength of Sontag’s criticism:

To be sure, a cityscape is not made of flesh. Still, sheared-off buildings are almost as eloquent as bodies in the street. … Look, the photographs say, this is what it’s like. This is what war does. And that, that is what it does, too. War tears, rends. War rips open, eviscerates. War scorches. War dismembers. War ruins.

Unwilling to stand aside and passively observe, she is a fervidly engaged advocate-critic. But the two war photographs that appear alongside this quotation – Edward Steichen’s “[Aerial View of Vaux, France]” (1917) and Alexander Gardner’s “Ruins of Gallego Flour Mills, Richmond” (1865) – fail to capture the spirit of Sontag’s impassioned statement. The Steichen, a plane’s-eye view of a village decimated by World War I, is too far removed from its subject to evoke Sontag’s sense of intimate witness; the Gardner seems ambiguous in its criticism of the effects of war. Aestheticizing the eerie stillness of burnt-out flourmills, this photograph turns a war-ravaged compound into a contemporary ruin.

Elsewhere, the visitor confronts the curious decision to pair Sontag’s observation that in Edward Weston’s nudes “the body is characteristically shown bent over upon itself, all the extremities cropped” with a photograph of a female nude in which three appendages are clearly visible, an arm is hidden behind a bent leg, and only the crown of the subject’s head is cropped by the upper edge of the photograph. Although this makes Sontag seem like a sloppy observer, the true fault lies with the curator.

Another curatorial error occurs in the presentation of a second Weston, “Bedpan” (1930). This image, in which a bedpan standing upright before a black background becomes an abstract birdlike figure, is presented as an example of Sontag’s observation that photographs can discover beauty in ugliness. However, Weston was in fact striving to illustrate a subtler dialectic between beauty and ugliness.

“Bedpan” alludes to Alfred Stieglitz’s photograph of Marcel Duchamp’s “Fountain” (1917), a urinal that the artist-cum-prankster notoriously displayed as a work of art, in what would become an iconic gesture in the development of conceptual art. Turning the playful urinal into the more sobering image of a bedpan, Weston effectively exposes the ugly underside of Duchamp’s flippant dismissal of the traditional qualities of art. In response to Duchamp’s light-spirited, irreverent, purely mental art, “Bedpan” reaffirms the inescapable messiness of material existence and human frailty.

Unfortunately, the complexity of this image, like that of many others (as well as several of Sontag’s quotations), is not drawn out by “On Photography: A Tribute to Susan Sontag.” This enjoyable but flawed show feels smaller than the sum of its elegant parts.

Until September 4 (1000 Fifth Avenue at 82nd Street, 212-535-7710).