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The British film director Christopher Nolan has become something of a specialist in the cerebral blockbuster.

Films like Inception and Interstellar both demand a bit of concentration, and yet both have turned out to be entertaining and popular. Others boast the last word in special effects.

But Mr Nolan is unashamedly conservative in one respect - he is a champion of celluloid over digital projection, and the need to conserve film stock.

He has been proselytising for these causes at the London Film Festival, where he gave a rare interview to Stephen Smith at the British Film Institute.