Jeff Mermelstein, whose first retrospective is on view at Steven Kasher Gallery, has much in common with Helen Levitt. Both are quintessential New York street photographers for whom Gotham’s pavements are as indispensable as purple mountain majesties were for Ansel Adams. Yet the two differ markedly in what draws their attention as they walk the streets with their respective Leicas at the ready.

Ms. Levitt, whose work dates mostly from the middle of the 20th century and is almost entirely in black and white, is interested in how people deal with each other, whether it is a gang of children engaged in mock assaults or weary adults finding comfort with their companions. Her pictures are short stories: humane, sweet, and humorous. By contrast, Mr. Mermelstein’s pictures are one-liners: humane, yes, but bittersweet and punchy. They are in color, most of them taken during the last 25 years, and often focus on a lone individual; sometimes there is only the residue from someone who has passed by.

“Untitled (Bubbles in Air, NYC)” (1995) is one example of the latter. A string of soap bubbles is wafting down the sidewalk.Who blew them? No idea. The bubbles are in sharp focus and, in the 26-inch-by-40-inch Chromogenic print, quite beautiful. They float along casually in the bright sunlight, contrasting with the out-of-focus, gravity-bound buildings across the street. The picture captures the essence of what makes children delight in bubbles, and that makes adults feel like children when they see them.

In “Untitled (Burning Shoe, New York City)” (1995), a single work boot sits burning on the sidewalk while the legs of indifferent pedestrians pass by in the background. The toe and counter of the boot are a wraithlike white, and the laced midsection is charred black; a cloud of swirling gray smoke rises from inside the boot and hovers above it. The camera is low to the side walk, so we have a good look at the boot; like all experienced New York pedestrians in the presence of the ineffable, we shake our heads wonderingly before moving on.

“Untitled ($10 Bill in Mouth, NYC)” (1992) shows what Mr. Mermelstein can do with a single individual. It focuses on a middle-class woman of late middle age, her hair bleached and coiffed, her face etched with anxiety, crossing the street with a folded sawbuck clenched in her teeth. In the background, we see the white stripes of the crosswalk, the front of a yellow school bus, orange traffic cones, and parts of other pedestrians, but there is nothing to tell us why she is biting the $10 bill.We can guess, but there’s really no need to: It’s just one of the odd things a person sees in the big city.

In “Untitled (Bus Driver, NYC)” (1998), the driver is shot head-on from below as he sits imperturbably on his throne-like seat, the monarch of the street. A substantial, balding, white man, he exudes the imperial sangfroid needed to deal with passengers and traffic alike.The buildings of Midtown are reflected in the windshield so that his portrait is superimposed on the likeness of his domain. “Untitled (Man With Windex, New York City)” (1996),”Untitled (Book in Mouth, New York City)” (1993), and “Untitled (Christmas Tree Delivery, NYC)” (1994) provide three more examples of Mr. Mermelstein’s ability to dramatically isolate individuals from the surrounding urban rush.

The pictures that have two or more people in them tend to come from Mr. Mermelstein’s earlier work, some of it taken away from New York City. “Untitled (Family at Picnic Table, Yosemite National Park, California)” (1994) looks at the eating habits of people with pale white skin, blue eyes, and natural blond hair as if they were a rare ethnic strain just found by anthropologists. A model wearing a skimpy bathing suit reclines in “Untitled (Tanning Booth, New York Coliseum, New York City)” (1985) as she shows her perfectly tanned chest to a businessman straining to make his interest appear professional.

In another gallery at Kasher are nine pictures from “Ground Zero, September 11, 2001,” Mr. Mermelstein’s heroic record of the worst day of our recent history. All the instincts he honed as a human-interest street photographer with a bent for humor were put to service as a combat photographer.

For instance, the same instinct that let him see in a burning shoe an objective correlative for unnameable feelings about the city led him to shoot “Statue,” a veristic figure of a Wall Street type in his business suit sitting on a bench working with his laptop. The statue is rather schlocky, an unimaginative exercise in verisimilitude – except that on this day it is surrounded by debris from the fallen World Trade Center. Like everything else, it is covered with a fine white powder of ash. The man looks like one of the victims from Pompeii, frozen forever in his quotidian routine and tragic in his ordinariness.

“Tree and Skeleton of Tower Two,” “Ash Haze and Two Flags,” and “Smoke Cloud With Flame” are other pictures that capture the chaos of that day, as “Firemen With Tears” catches the human toll.

In the 1950s, television and air conditioning drew people in off their stoops so that the personal dramas that are the subject of Ms. Levitt’s work were more likely to be performed in private, out of sight of peripatetic photographers. The street remains a locus of our identity, but only as we pass along. Mr. Mermelstein has found a way to arrest individuals in transit so we can savor their idiosyncrasies, and to make objects on the sidewalk stand in for the people who abandoned them.

Until June 10 (521 W. 23rd Street, second floor, between Tenth and Eleventh Avenues, 212-966-3978).