In Jessica Craig-Martin’s “Russian Tea Room” (1999), a waiter’s jacket and the stack of napkins and plate of strawberries he carries are transformed into a black sea and a few white islands. This remarkable photograph, in which the photographer used heavy contrast to turn recognizable forms into an abstract investigation of presence and absence, is but one way to interpret “Black and White,” the title of a group show currently on display at Greenberg Van Doren Gallery.

As in many summer group exhibitions, the theme is less memorable than the individual pieces, which explore topics as disparate as racial politics, the medium of photography, and night and day. In fact, the exhibition feels merely like an excuse to show off the range of the gallery’s impressive roster of artists.

Katy Grannan, who had a well-received solo show at Greenberg Van Doren earlier this year, is back with four signature photographic portraits of outcasts and eccentrics. In the quirky “Michael & Evan, Red Hook, NY” (2003), two boys stand side by side before a white curtain decorated with black silhouettes of birds in flight. The boys wear earnest expressions and obviously consider the portrait to be important, but their intricately patterned outfits clash terribly and neither has properly buttoned his shirt. That the boys’ preparations for the photograph have made them less presentable is playfully alluded to by the pair of glasses that the shorter boy has removed from his face and holds by his side. In order to look better, he has inadvertently affected his ability to see.

Paul Graham’s two photographs from his “American Night” (2000) series are wide-angle shots of expansive parking lots, each populated by a lone African-American male. Both pictures are overexposed, so that the entire image is seen through a thick haze. Though the pictures were taken in broad daylight, this cloudy light has a nocturnal quality that recalls the white nights of northern summers.

The series from which Mr. Graham’s photographs are taken is formulaic and unsubtle, juxtaposing images of solitary African-Americans with color shots of pristine suburban houses. Hung here without the opposing house images, these two photographs are liberated, yet still deliver a political message. This is one of those unexpectedly positive transformations caused by inclusion in a group show.

The exhibition is less kind to other works. The three Richard Diebenkorn works on paper are abstract studies of line and form, but their inclusion in “Black and White” distractingly draws attention to their color. The effect is to render Diebenkorn’s challenging works easily dismissed with a passing glance.

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“Dreamweavers” is an amusing exhibition of photographs that examine dream or fantasy worlds. Chelsea’s Yancey Richardson Gallery has gathered 18 images that use some combination of computer manipulation, elaborate set creation, and tabletop models to create scenes that tease with their alluring implausibility.

Gregory Crewdson’s “Untitled (Birds around Home)” (1997) shows four birds of different sizes and colors gathered around the carcass of a fifth bird, which has been marked by peck holes and on whose back appears the word “Home.” Mr. Crewdson has been celebrated in recent years for ambitious photographs that use gigantic movie-sized production teams to create scenes of suburban disturbance; too often, though, the real drama in these photographs exists in the production rather than the product. The more modest “Untitled (Birds around Home)” is funny, surprising, and subtly disconcerting – exactly what his later work aims to be.

Robert and Shana Parke-Harrison’s “Reclamation” (2003) is a photogravure with a deliberately antiquated look. Two men in suits and bowler hats pull a football field-sized sod carpet over a dirt field. The photographers have employed paint, multiple negatives, and constructed sets to create this surreal, literary image.

James Casebere’s “Siena (horizontal)” (2003) captures the simple interior of a sandstone structure. Light spills from a well-lit arched doorway into the center of the large foreground room, creating a reflection whose rippled edges indicate that the floor is covered by water. The photograph seems to have a seductive directness, but the more one looks the

less it makes sense. How can the front room be bathed in water, yet none has seeped into the back room? Close examination shows that the back wall looks like a painted surface. The explanation of these mysteries is that the photograph is of a constructed model, not a real building. Mr. Casebere gives just enough indication of the image’s fictions to undermine our sense of visual certainty.

Like “Black and White,” “Dreamweavers” exposes some risks inherent in the summer group show. While the former lacks focus, the tradeoff for the latter is the uneven quality of its images. In the realm of the imagination, all are not equal, and it is the unfortunate fate of certain “Dreamweaver” fantasies to be rendered mere commonplaces when hung beside the more energetic, challenging creations of their peers.