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Here is a weird one. Look at this black-and-white checkerboard. We have got equal numbers of black-and-white squares, and all of the squares are the same size. Obviously, white squares reflect light, and black square don't. So you might think that this grid, which is half-composed of reflective white squares, would reflect half of the light that's strikes it. But it doesn't. It only reflects 18% of the light that strikes it. Here is weirder one. This is true for most scenes in the world. It turns out that most scenes that you look at reflect 18% of the light that strikes them.
Now this fact has obvious applications for cocktail party conversation, crossword puzzles, and making very nerdy impressions on first dates, but there is a photographic application as well: because most scenes in the world reflect 18% of the light that strikes them, if your light meter assumes that what it's pointed at is reflecting 18%, then that's a pretty safe assumption. So what your light meter actually calculates is a shutter speed and aperture that will accurately reproduce the tones and colors of something that is reflecting 18% of the light that strikes it.
Again, for most scenes in the world, this is a safe assumption. Remember, different amounts of light will need different exposure settings, but your light meter assumption is always that you want exposure settings that are correct for 18% reflectance. Now as you'll see later, this assumption can occasionally let you down, but most of the time it will be fine. To further assist you, your meter tries to choose a shutter speed that will be safer hand-held shooting, though if the light is too low this may not be possible, and it tries to pick a midrange aperture. Most of the time you can just let your light meter do its, but as you see later, there will be time when you need to out- think it and takes a manual overrides to ensure that some tones and colors appear correct in your image.
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