Copyright © Astronomy Roadshow All rights Reserved
Lens Aperture is the simple answer. The darker picture was set at around f11, the lighter picture had the aperture wide at around f4. The lunar landscape is lighter as a result but at least it recorded Buzz Aldrin correctly. Spotlights are not even required.
80 ISO film was chosen on the moon as the latitude of the emulsion speed was relatively slow, so the lighter lunar surface was not vastly overexposed, just a little.
As well as this, a slide can be copied onto negative film. This in turn is more flexible in the dark room and extra light areas can be dulled down a little if required without disturbing the rest of the image. Below left is Buzz; note the surface is a little overexposed but not too much. It was still a useful image rather than completely washed out. The fill-in light was from the moon itself. The reason why the ground in shadow was dark while Aldrin was fairly light is because the white suit reflected around 72% of light while the ground around 8%.
What is going on in these pictures from Apollo 11? In one shot the Lunar Module is in deep shadow and it can hardly be seen. The next shows it brightly lit. I know what it is... A spotlight! Aha got you all now. Yes it’s in a studio after all!
One final word; if a spotlight was used, where are the extra shadows? My own test shown above right - did not require a spotlight; just extra opening of the aperture and the background was not vastly overexposed; just a little but still usable.