SAN FRANCISCO — At first glance, it looked like another throng of tourists passing through San Francisco’s Chinatown. Yet one thing made this group stand out: everyone was taking pictures, but no one was using a traditional camera. No heavy D.S.L.R.’s weighed down neck straps. No point-and-shoot cameras dangled from wrists.

Instead, everyone carried an iPhone and snapped photos with giddy enthusiasm. Most were San Francisco residents who had gathered in Chinatown on a warm Saturday morning to take part in an “Instawalk,” an ad hoc gathering of iPhone photography enthusiasts who share their pictures via Instagram, the popular iPhone photo service/social network that has more than seven million users.

The tours are led by Brian Roberts, better known to his Instagram fans as Doctor Popular. With help from the Instagram app, Doctor Popular’s remarkable photographs have quickly established him as an innovator who uses the iPhone as a powerful tool for creative expression, rather than as a pocket-size plaything.

The most important part of the tours he organizes occurs afterward, when participants gather at a Chinatown cocktail bar to drink and review the photos. “Have you tried the Synthcam app?” asked Doctor Popular, 34, a community manager at Postagram, an iPhone app developer. “It was made by a bunch of Stanford professors. It’s great for creating shallow depth of field.” That sets off a free-for-all discussion comparing the best apps for editing photos on the phone.