Hong Kong's paparazzi stalk the city's tycoons, movie actors and pop singers. Philip Chan is after something bigger.

On a sunny day in the Kowloon City neighborhood, he watches his 14-foot-tall prey round a corner, far more nimbly than its size would suggest. Checking traffic, he steps off the curb and clicks—but the glare off the side is too bright and the shot is ruined.

The 27-year-old Mr. Chan gets back into position and waits. He knows the schedule: another double-decker bus should be along shortly.

"Buses are like different movie stars," he says, flipping through a seemingly identical montage of bus photos on his digital camera. "Everyone has their favorite."

Mr. Chan, an accountant, is one of Hong Kong's thousands of bus fans, ranging from young students to retirees, who are enthralled by the city's some 6,000 buses. They linger by bus depots and lurk near highway ramps, angling for the best shots. They form fan clubs, eagerly swap the latest gossip about changes in bus models and film their own rides, which they share online with anyone who wants to follow along (spoiler alert: they aren't always scintillating).