John Shearer was already a promising photographer at 16 when the photo director of Look magazine, who was preparing to cover John F. Kennedy’s funeral, invited him along to carry his bags. But when they arrived at St. Matthew’s Cathedral in Washington, the photo director instead handed him a press pass and told him, “Take pictures of people grieving.”

Instinctively, Mr. Shearer worked his way up to a reviewing stand. Using a telephoto lens that his father had bought him for the occasion, he clicked the shutter just as the president’s toddler son, John Jr., raised his right hand in one of the most poignant salutes in American history.

A moment later, Mr. Shearer was pulled from the stands by Secret Service agents. He fell to the ground, cracking his new lens.

“But I had my picture,” he told Westchester Magazine in 2017.

Other photographers had the picture too. But Mr. Shearer’s was particularly striking. It was somewhat overexposed, so that it illuminated Jacqueline Kennedy’s face behind her black veil. Over the years it has become one of the most reproduced images of that memorable moment.