Cinderella Costume Designer Reveals Truth Behind Lily James’ Tiny Waist - VIDEO EXCLUSIVE

Lily James’ waist in the 'Cinderella' posters has come under tough scrutiny in recent weeks, with some critics saying she’s had it Photoshopped.

But the film’s Oscar-winning costume designer Sandy Powell has lifted the lid on the secret behind the star’s tiny waistline – and says it’s all down to optical effects.

Speaking to us at the launch of the Cinderella Exhibition in London’s Leicester Square, the 54-year-old told us that it’s actually all down to the measurements of Cinderella’s iconic blue ballgown.

“It’s so extreme, the actual silhouette of it and the volume of the skirt is really huge,” she told Yahoo Lifestyle in the exclusive video above.

“I don’t actually know what the measurement is but it’s almost two metres in diameter, it’s really big!”

In addition to the voluminous skirt of the ballgown, the oversized shoulder detailing accentuates Lily’s tiny waist – making it look smaller than it actually is.

“And then we’ve got the exaggerated piece up here [on the shoulders] which draws your eyes to the tiny waist, which she has anyway,” she said.

“It’s a real sort of hourglass shape.”

So it turns out it’s NOT the work of the Photoshop wizards!

But the London-born costume designer, who’s won Oscars for her work on Shakespeare In Love and The Young Victoria, admitted it was the glass slipper that proved the most challenging.

“The glass slipper was the scariest challenge of all. How on Earth do we make a shoe out of glass?!” she said.

“I didn’t want to do glass that was completely glass that was completely see-through, as that would look ugly with the foot squashed into it.

“I started looking at chandelier crystal and found that crystal reflected the light, which was when I approached Swarovski.”

In the end, the glass slipper that Lily wears in the film was created by CGI – but the glass slipper that she holds in the crystal version, which is now housed in the London exhibition.

“We went through lots of different stages [with the glass slipper],” she said.

“Probably it was [the most challenging aspect of the film], in that until we got our first prototype, we didn’t know if it was going to work, and we didn’t have a back-up!”

Watch the full interview with Sandy in the video at the top of the page.

The free Cinderella Exhibition in Leicester Square is open to the public from 21 March to 10 April, but you must book tickets in advance at http://movies.disney.co.uk/cinderella/exhibition. The live-action Disney film is out 27 March.

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[Cinderella - first full UK trailer]