The Inky Blackness of Space
You may have run into the occasional black background when shooting close-up images with an on-camera flash. You may also have noticed people in large venues with cameras trying to light up the room with a camera flash. You may also have heard people claiming that the Apollo moon landings were faked. Allow me to tie these three concepts together.
Light is a ubiquitous presence in our lives. Yet most people don't understand how it works at all. They take it for granted since it's pretty much everywhere. If they go into a dark room, they turn on the light so they can get back to ignoring how light works.
Turn on a light and its rays radiate out in all directions. However bright that bulb is, the light it casts is brighter near the source than farther away. As the light travels, it spreads out to cover an ever-widening area. But just how rapidly that area grows can be surprising when you stop and do a bit of arithmetic.
Shine a light on a wall that's one foot away. It will cover some given area. Now back away to two feet and notice the change in area. Since you've doubled the distance to the wall, the width of the spot cast on the wall by the flashlight will be doubled as well. So will the height; But since area is proportional to height times width, the total area lit by the flashlight will be quadrupled (two times two equals four). Since the intensity of light emitted didn't change but the surface area grew by four, the intensity per surface area fell to one quarter of the original. This is the inverse square law and holds true for any change of distance. The light that reaches any given spot falls off inversely based on the change of distance. If we backed up to four feet, the light on any given part of the wall would only be one sixteenth (four times four is sixteen).
So let's translate this into our close-up photography scenario. And instead of shining a light on a wall, let's assume it's your camera flash shining on your subject with sufficient intensity to expose it properly. If that's all you can see in the frame, everything will work out pretty much as you expect. If you back up to increase the distance, your flash will compensate by putting out more light. There are limits of course, and at some point your flash won't be able to put out enough light to do the job, but within a reasonable range of working distances, you can ignore the inverse square law.
But if your subject doesn't completely fill the frame and you can see any of the background behind it, distance becomes important. If your background is twice as far away from the flash as your subject is, the light on the background will be only one quarter of what is lighting up your subject due to the inverse square law. And since exposure stops equate to doubling and halving of brightness, that decreased brightness on the background will mean it will appear two stops darker. The amount of light on the subject will be cut in half twice to reach the one-quarter background brightness as created by the relative distances involved. And it only gets worse if the background if the background is even further away. Two stops darker will already be noticeable, but if that background gets too much darker (further away in relation to the subject distance) it will rapidly approach pure black. There simply won't be enough light falling on the background for it to show up in the image unless we risk burning out the subject exposure by keeping the shutter open longer. Some people like the black background effect, but even they can't deny that it doesn't look natural.
And that brings us to the problem of using a camera flash in a large venue. You're sitting in the arena looking at the stage and someone distracts you temporarily with a camera flash. In this case, the subject on stage and the background are both quite distant from the camera and flash, but still their image is likely doomed to underexposure. It would be highly unlikely their flash could be powerful enough to light up their intended subject that far away. Despite the lack of subject and background distance variance, both are so far away that the inverse square law will spread the light from the camera flash enough to make any chance of lighting either nearly impossible. Normal camera flashes just can't reach out to cover that much distance. The light continues to spread and although it will reach its target, by the time it gets there it will have dimmed considerably. The house lights will totally overwhelm any contribution, however minimal, it may make to exposure.
And that brings us to the Apollo moon landing hoax, or so some people say. This weekend is the forty-fifth anniversary of the Apollo 11 moon landing where Neil Armstrong and Buzz Aldrin became the first two men to walk on the moon, Or they became the first two men to fake doing so on a Hollywood sound stage under the direction of Stanley Kubrick. The conspiracy theory goes that NASA simply couldn't have managed to pull something of this complexity off for real. But at the time, the United States was engaged in a high stakes game of one-upmanship with the Soviet Union and that NASA was forced by the government to fake the landing to force the Soviets to compete by pumping more and more of their national economy into keeping up with us. The USSR did launch Sputnik and Yuri Gagarin ahead of us, so this time the US was determined to at least appear to beat them. So as Kubrick was finishing up 2001: A Space Odyssey, NASA approached him to direct the fake moon landing footage. Or something like that.
Conspiracy buffs point to a number of issues they believe bolsters their case. One of these is the observation that in all the photos they brought back as well as the video footage, you can't see any stars. Furthermore, the astronauts didn't even talk about seeing stars. The idea is that since we can see so many stars when we look up at the night sky from earth, there should be even more visible from the moon where there's no atmosphere in the way and no light from urban sprawl to get in the way. But since computers back then couldn't possibly calculate where the stars all should be in any given shot, the decision was made to leave them out entirely.
NASA explained the lack of stars by pointing to the exposure difference between the lit moon surface and the blackness of the sky. That is, the stars were there, but they didn't show up in the images due to exposure differences. But then why didn't the astronauts at least talk about the stars. Surely the night sky on the moon should have been extraordinary.
The problem with all this is that it wasn't night. All the lunar landings happened during lunar daytime. It would have been far too hard to see on the dark side of the moon. The sky on the moon is always black, day or night, since there's no atmosphere. But the sun was still shining on the lunar surface as well as the lunar lander, the astronauts themselves and all their gear. The stars didn't show up in any images because the landscape was far brighter than the light coming from any given star apart from the sun itself. Even though the sun is 92 million miles away, the closest other star to the moon is more than 20 trillion miles away. The light from them is subject to the same inverse square law as is light from any other source. The sun itself and the light reflecting off the lunar surface were far too bright for any stars to be visible.
By the way, sunlight itself follows the inverse square law in everyday life here on Earth as well. Given this, you might be wondering why we never notice backgrounds being darker than foreground subjects when lit by sunlight. The answer of course is simple: while the light does fall off based on the square of the distance, everything on Earth is roughly 92 million miles from the sun. An extra few feet or miles between subject and background makes no meaningful difference in the light falling on each. If we were to take a trip to the planet Mercury, the sun would be bright indeed (and hot), but here on planet Earth, everything is pretty much the same distance from the sun.