Pilots from the Photographic Reconnaissance unit, created in 1940, risked their lives by flying unarmed over Europe to take tens of millions of photographs, generating 36 million prints.

To make the 3D effect work, images had to be captured in carefully-plotted sequences which would overlap each other by 60% so everything would stand up when viewed through the stereoscope.

It made the job of the pilots - who, in addition, had to avoid enemy fire - an especially skilled and arduous one. Flying at 30,000ft, they were unarmed because of the weight of the five cameras carried on each Spitfire.