``It's a cross-section of her personality ... she was a little girl before and now she's a woman. Obviously there's life experience in her face, but she's still beautiful.''
Moss said hat she had not changed much since she first worked with Day.
``I've got crooked teeth, bow legs, a wonky nose,'' she says.
A former model and self-taught photographer, Day is renowned for bringing a hard-edged documentary look to fashion image-making.
She forms close relationships with many of her sitters, resulting in candid and intimate portraits.
The commission will go on display on February 12 and its unveiling coincides with the National Portrait Gallery's Face of Fashion exhibition on portraiture in fashion photography, which opens on February 15.
The exhibition features the work of acclaimed photographers such as Day, Mert Alas and Marcus Piggott, Steven Klein, Paolo Roversi and Mario Sorrenti.
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