An upskirt warning poster in a subway station outside Tokyo. Photo by Jeff Epp.

The iPhone 3G in Japan has a special feature unique to that country: The camera always makes a conspicuous “shutter” sound when a picture is taken, even when the phone is set to “silent” mode.

The loud shutter sound is supposed to deter voyeurs from taking sneaky pictures up women’s’ skirts — or down their tops.

In Japan, upskirt and downblouse shots have become increasingly popular with the advent of high-resolution camera phones.

As a result, all cell phones sold in Japan make a conspicuous shutter sound, or say the word “cheese” when a snap is taken, according to Nobuyuki Hayashi, a tech reporter based in Tokyo.

On almost all new cell phones, the camera shutter sound can not be muted, Hayashi says.

“Some manufacturers have even put louder shutter sound,” he reports.

The shutter on the first iPhone sold in Japan could be muted in silent mode; an anomaly that many wondered whether Apple would correct in the iPhone 3G, Hayashi says.

Apple did: The shutter sound cannot be turned off, even in silent mode, Hayashi says.