Some sections of the wall have collapsed, eroded by wind and rain over the years. Yang has taken photos to keep a visual record since 2007, carrying his camera while working. "It started as a hobby, but has grown into a near-obsession."

Taking photos of the Great Wall is physically demanding. Yang often has to hike more than 10 km in mountainous areas. The light is best early in the morning and late afternoon, when the wall also looks its best in the frame. He has to wake up at 3 am and stay in position until 4 to 5 pm on some days. The temperature drops to 20 degrees below zero in winter in Shanxi, and the wind feels like "a knife through the face".

Yang says his hands are often frozen, and he has trouble holding the camera steady.

But his troubles "are necessary" as the best preserved sections of the wall are hidden in the mountains. While he had friends who went with him to capture scenes on the wall earlier, only he stayed the course.

"Some found it too hard, and some didn't think what they got was worth the effort," Yang says, adding that the Great Wall is a heritage and its beauty should be appreciated.

In 2009, Yang started his photography project on sections of the wall in northern Shanxi, and visited the structure almost daily. The wall, he says, "has a life of its own". Even if it's the same spot, its appearance varies with time, weather, season and even the mood of a photographer.

"The picture composition, light and shadow are all important, but the more important thing is to discover and capture the beauty of the wall. You can stumble on beautiful scenery-rainbows, lightning, sunset or a sea of clouds."