Edgerton synchronized his electronic stroboscope with a special high-speed motion-picture-camera so that with each flash, exactly one frame of film was exposed. The number of flashes per second determined the number of pictures taken.

Motion pictures are normally exposed and projected at 24 frames per second, but when pictures are made at a higher rate and projected at normal speed, the apparent movement is slowed down. Edgerton designed high-speed motion-picture cameras that could expose as many as six thousand to fifteen thousand frames per second. When these films were projected at normal speed (24 frames per second), very high-speed events appeared – and could be studied – in extremely slow motion.

When you take a picture, you usually photograph one image per frame of film. But in a multi-flash photograph, several exposures are made on a single negative. Multi-flash photographs are taken in a darkened room with the camera’s shutter wide open. The film is exposed only when the stroboscope flashes.

The rate of the flash determines the number of pictures on the negative: from a few, separate images (like Doc’s photographs of high-speed divers), to may overlapping ones (like his photographs of golf and tennis swings).

On the eve of World War II, the army asked Edgerton to build a strobe for nighttime aerial reconnaissance photography. The components of the system were bigger and more powerful than anything Doc had previously designed. The aerial camera looked much like a 35mm camera – only it was about two feet long. The flash tube was a monster, made of thirty inches of tough, quartz glass coiled into a spiral. The tube was positioned in a thirty-inch reflector pointing down from the plane’s belly or tail. Banks of capacitors, weighing up to five hundred pounds each, were slung on the plane’s bomb racks.

The flash’s most famous test came on June 5, 1944. Over the D-Day target areas, the airborne photographer opened the camera’s shutter and triggered the flash. The photographs of the quiet nighttime landscape showed that the Normandy invasion was not expected in the designated landing area.