But he worked hard to win their trust.

"In 2005, for instance, I came across a funeral ceremony in a village of the ethnic Zhuang people in Napo county (in southern China's Guangxi Zhuang autonomous region)," Zhan said.

"Before that, I had shot several wedding ceremonies of different ethnic groups, but it was my first time experiencing a funeral ceremony. To grasp the unique opportunity, I tried my very best to ask for the family's permission."

He helped them with chores such as splitting firewood and carrying water, which made the family and the villagers treat him as a family member.

They allowed him to shoot the three-day funeral ceremony and the villagers introduced him to lots of Zhuang funerary customs.

To retain the most realistic images, Zhan insisted on recording them on film.

"Compared with digital photographs, negatives can't be modified at all," he said.

"Furthermore, the film can be kept for about 100 years with good preservation."

Buying film and printing photographs became his biggest expenses.

He sold his apartment in Mudanjiang in 2005 and spent 800,000 yuan during the seven-year project. "Almost all my friends couldn't understand why I decided to donate all the negatives," Zhan said.

"They advised me to leave them to my daughter, but I believed the films could be of huge value to the museum, which is a great platform to present China's ethnic groups.

"I hope we can finish the journey of visiting all the ethnic groups before my daughter goes to college, just like what I did before. It will be the most precious treasure for her growth."