Catherine celebrated her landmark birthday on Sunday and to mark the occasion, three new images of the royal, taken by Paolo Roversi, were released and will enter the National Portrait Gallery.
The photographer first met Catherine in November at Kensington Palace for tea and biscuits and admitted she was "apprehensive" about posing for him.
He told Italian newspaper Corriere della Sera: “At first the Duchess was apprehensive. She is machine-gunned by photographers every day but is not used to posing.
“Knowing my photos with models she was a little fearful of facing a real session, which required about four hours of work. But I reassured her that once we started it was going to be very easy. And so it was.”
The shoot took place later that month in the Temperate House at Kew Gardens, using natural light, and the duchess wore minimal make-up.
He said: “I didn't want her to look too Lady Duchess, too establishment, but as pure, as contemporary as possible. Or even timeless.
“In the end I wanted to take pictures in motion, so with that wonderful wide skirt I made her dance in front of my lens, a kind of accelerated waltz mixed with a pinch of rock ‘n roll."
The 250 shots were whittled down to 70 in December before Catherine then chose around 10 and Paolo selected his three favourites.
He said the "official" image - in which Catherine looks directly at the camera - was one they both preferred by the duchess was "less sure" than he was about the final choice.
The Italian publication suggested Catherine's husband, Prince William, and their three children, Prince George, eight, Princess Charlotte, six, and Prince Louis, three, chose the “official” sepia image as their favourite.