"There was too much retouching that needed to be done. There was insufficient light, the shadowing and highlights were blurred and the layers were indistinct. Even the eyeballs and the eyelids were not clearly defined."
Chen magnified the contrast and improved the image. When Chen met Mao's daughter Li Na in 1998, she said the fourth official photo was processed so well that "whatever direction you were looking at it from, you felt as if Mao's eyes were looking at you".
"She told me Mao liked it very much and chose the photo himself."
Chen also developed a method of replicating negatives and helped copy tens of thousands of negatives used by national media.
Today, Chen's hobby is collecting porcelain. He said the coloration of chinaware has something in common with photo processing.
Also, Chen says, he collects china because he wants to show his former friends and colleagues in Taiwan that he is doing well.
"They were my childhood friends. They said you should have stayed in Taiwan instead of returning to the mainland. You look poor and live in such a small apartment.
"I decided to show them I am not as poor as they think. I arranged lots of china in my study so no one could say the owner of so many antiques was not well off.
"I have just one shortcoming: Whoever says bad things about the Communist Party of China, I will argue with them."
Every time the Tian'anmen Square picture of Mao changes, Chen says he visits and takes a photo of the painting. But he has no apprentice and none of his three children has followed him into the profession.
"It demands practice from childhood, sensitivity toward art and shrewd eyes to discern shade and contrast on a picture. People do not use these crafts today, they have computers."
Chen also uses computers and digital cameras, but he is currently working on pictures of Mao using his traditional brush and pen and scalpel.
"Computers are good because they can reduce human labor, but they cannot reproduce the original scenes," Chen says.