JOHN MARGOLIES and Lowell Anson Kenyon represent something like the ultimately opposite approaches to architectural photography. Separate but simultaneous exhibitions give Washingtonians a chance to compare and contrast their complementary searches for fascinating fragments of the vanishing American scene.

Margolies' matchless array of color portraits from the extravagant! colossal! stupendous! era of American movie houses is on view at the National Building Museum. At the American Institute of Architects, Kenyon presents an idiosyncratic album of black-and-white vignettes from his distinguished 55-year career.

Margolies, 55, has spent more than 20 years driving America's blue highways and back roads in search of the individual and often quirky commercial architecture that is fast disappearing under the homogenizing influence of national chains and franchises. This show is devoted to movie theaters, from the glittering palaces that once lit up our cities to the cow pasture drive-ins that catered simultaneously to our passions for cars, sex and celluloid.

Kenyon, 60, says he took his first architectural photograph in 1939 -- when he was 5 -- of a cabin he'd built with Lincoln Logs. He went on to careers as a National Geographic photographer, chief of photography for the National Museum of American Art and the National Portrait Gallery, and teacher. All along he has been taking pictures that, for the most part, focus tightly on details and odd juxtapositions of homes and buildings that, for the most part, are at or near the end of their lives. His style is a self-effacing nonstyle that adapts itself to the subject; the unifying element of these 69 images is their technical perfection and their seemingly artless composition. Margolies' style, on the other hand, is in-your-face. He's a documentarian who's building a national inventory of forthright, fine-grained, color-saturated portraits of the individualistic "highway architecture" that, from the opening of the automobile era until the coming of interstate highways, made it possible at a glance to tell one American town or city from another.

Glance, for instance, at the Egyptian Theater in Boise, Idaho (built in 1927), and you'll know instantly that you're a long way from the Egyptian Theater in Ogden, Utah (1924), or the Egyptian Theater in De Kalb, Ill. (1929). "King Tut's tomb," Margolies explains. "Movie houses strove for the exotic and romantic, and in the Twenties everybody was fascinated by the discovery of the Pharaoh's tomb."

In many cases Margolies' photographs and aging moviegoers' fading memories are all that's left of these dream temples. Sometimes he has beaten the wrecker's ball by weeks or days or even hours; more often the demolition crews have beaten him, because he's just one man, driving from town to town, and it's a big country.

Exacerbating the agonizing slowness with which Margolies adds to his archive is his insistence on consistency. Whatever the subject, Margolies doesn't shoot unless and until he can capture the subject in full sun with a cloudless sky, with no people in the frame. This seems silly or compulsive until the cumulative effect of the photographs registers. Because the only thing that changes is the subject, we see clearly the building that Margolies is showing us, and nothing else. "I spend an awful lot of time in crummy motels waiting for the sun," he says. "Sometimes I just have to give up and go on down the road."

Kenyon, on the other hand, shifts his perspective constantly according to the demands of the detail or context he wants us to see. One moment it's the weathered paint on part of a screen door, the heavy, irregular checkering echoing the disciplined lattice of the screen; the next it's a long shot of ranks of snow-cushioned stone seats in the amphitheater at Arlington National Cemetery, evoking the unseen rows of headstones that surround it.

The amphitheater is the only identifiable place in Kenyon's portfolio; we don't know if the screen door's in Maryland or Montana, which deliberately delimits Kenyon's subject. Margolies, on the other hand, specifies the year and place each of his photos is taken, and has done his best to document each of the thousands of theaters, gas stations, restaurants and roadside attractions he has photographed, now numbering many thousands. A highly systematic pack rat, Margolies also collects period signs and random arcana.

"Sometimes it begins to feel like just too much to keep up with," he said at the opening, smiling. "But then I think to myself, Hey, you could be stuck in a real job,' and then I hit the road again." TICKET TO PARADISE -- Through Sept. 10 at the National Building Museum, 401 F St. NW. 202/272-2448. Open 10 to 4 Monday through Saturday and noon to 4 Sundays. Metro: Judiciary Square. Wheelchair accessible. MIRRORS AND MEMORIES: Architecture as Place and Witness -- Through July 28 at the American Institute of Architects, 1799 New York Ave. NW. 202/638-3221. Open 8:30 to 5:30 weekdays. Metro: Farragut West. Wheelchair accessible. CAPTION: The Tower Theater, saved from obscurity by John Margolies. CAPTION: Kenyon's "Church and Mail Box" (1988). CAPTION: Lowell Anson Kenyon's "Detail of Screen Door No. 2."