Crouched near the shore of a Russian lake where huge brown bears were stalking salmon, National Geographic photographer Randy Olson was a bit tense. "When something that big is within four feet of you, it's a little unnerving," he says.

Never mind his trip to Ghana, where he risked his life alongside illegal gold miners who had been driven by the military from a multinational mine. Or his moonlit trek in Sudan to an encampment of Kalishnikov-hoisting rebel soldiers getting psyched for battle.

Pittsburgh-based Olson and his wife, Melissa Farlow, have photographed both desolation and beauty for the magazine for 17 years.

Fifty large-scale color images representing series published in National Geographic between 1999 and last month are being shown in "The World at Our Door," their first major exhibition, at Silver Eye Center for Photography.

Farlow will give a free talk about their experiences at 7 p.m. next Wednesday at Manchester Craftsmen's Guild, followed by a book-signing. (Olson was also scheduled to talk but will be in India, Malaysia and Australia shooting an advertising commission.)

Even in less threatening environments, the pair push limits to get the photographs they want.

For Farlow's "Mustangs: Wild Horses," shot in South Dakota, they rose at 4 a.m. for three days, gathering gear and applying sun screen in the dark before driving across the desert to set up before sunrise. And then they waited, for hours, until helicopter-driving Bureau of Land Management contractors began a round-up.

Farlow had mounted three remote-controlled cameras in locations where the horses were expected to pass, and her efforts were rewarded with a breathtaking image that places the viewer directly in front of the driving hoofs and taut muscles of the galloping horses.

The same ingenuity paid off for Olson when one of the aforementioned brown bears dived before the underwater camera he had secured to photograph scarlet streams of salmon. He had time to shoot only two frames, but that was enough, capturing both the first image ever made of a bear hunting under water, and a spot among National Geographic's 10 best photographs of 2008.

Farlow and Olson generally receive assignments separately, but occasionally their schedules allow one to join the other and act as assistant. Those are among the few times they have together.

Two of the show's most compelling images hint at time apart. Olson's "Bird Hunters, Indus Valley, Mohenjodaro, Pakistan," including men standing knee-deep in water wearing decoy bird headgear, was taken in 1999, while Farlow's "Dios de los Muertos Harvest, Atlixco, Mexico," harvesting marigolds for graves, was part of a 1998-99 shoot for the National Geographic Society book "Long Road South: The Pan American Highway."

After hopping a ride to Sudan on an illegal flight in a chartered plane loaded with humanitarian aid, Olson noted in his journal: "I call Melissa on the sat phone -- she's been in tears over all this. I have about 200 flies on my body as I am talking to her -- periodically I breathe in and swallow one or two."

They were both teaching photojournalism at the University of Missouri, where each had earned master's degrees in journalism, when they were hired by The Pittsburgh Press as staff photographers in 1986. They remained with the newspaper until it stopped publishing in 1992. Spun into the freelance world, they soon became contract photographers for National Geographic.

The couple maintain homes in Sewickley and in Oregon and live in whichever house is closest to their current assignments.

"It's been interesting, but it's been tough. You never know where your favorite shoes are," Farlow quips.

While assignments are determined by the magazine, some are more personal than others. Farlow, for example, has loved horses since she was a child. A photograph of her riding her pony in Paoli, Ind., is among archival objects in a small display case, and she annually covered the Kentucky Derby when she worked for the Louisville Courier-Journal.

This affection shows in her written comments accompanying the "Mustangs" pictures, as when some of the horses moved closer and "started smelling me. I was thrilled to be able to experience this. To be able to feel their breath."

It's also evident when she reports that more than 150 of the 1,000 captured during the drive died of salmonella bacteria after relocation. "The rest were put up for adoption, abused, neglected or ended up in a slaughterhouse."

Journal excerpts in the exhibition, and on their extensive Web site (olsonfarlow.com), illustrate the thoroughness and intensity of their commitment. The effort put into each location is evident in the rich, storytelling-quality of each image.

"Preparing for each assignment is like getting a graduate degree in that topic," Farlow says.

The emphasis is always on the stories of the people and places they photograph. Their approach is what makes the work they produce special.

"Young and amateur photographers go to a place and want to find what's different, what's wacky about a culture. What interests me is how we're all the same," Olson sums up.

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