The Real Story Behind This Photo of Prince William and Kate Middleton

Photographer Mario Testino reveals the surprising way he got this iconic shot.

Jan. 21, 2014 -- When Prince William finally popped the question to Kate Middleton, royal watchers drooled over their intimate engagement portrait.

Now, photographer Mario Testino is opening up about how he got the iconic shot.

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"I waited a long time, an hour or two, to make that picture perfect, but I wasn't totally satisfied," he told Vogue magazine of the photo shoot. "Then, when I'd finished the shoot, they were about to leave and they suddenly hugged in front of a radiator. I took my camera and that was the picture that ran everywhere -- it was spontaneous emotion. ... You could see they were completely in love."

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Testino, who famously photographed Princess Diana over the years, first met the Duchess of Cambridge in 2008, and even then, he could see how happy she was with the Prince.

Likewise, William was "in love with Kate," he said. "I knew they would stay together."