Counting light in tiny buckets
How Does a Photo Get Caught?
A camera focuses light onto a grid of wells and opens a shutter just long enough to fill them.
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Step 1 of 5
A lens gathers light and focuses it.
Light leaves every point of your subject in all directions. A lens catches a cone of it and bends it back to a single sharp point.
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Step 2 of 5
The image arrives upside down.
Rays from the top of the scene cross over inside the camera and land at the bottom. Every camera records an inverted image and flips it afterwards.
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Step 3 of 5
The shutter decides how long light comes in.
Open it briefly and a running child is frozen but the picture is dim. Leave it open and the picture brightens while anything moving smears.
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Step 4 of 5
The sensor is a grid of light buckets.
Each well collects charge in proportion to the light landing on it, and a colour filter above decides whether it is counting red, green or blue.
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Step 5 of 5
Too little or too much light both spoil it.
With too few photons the counts are swamped by noise and the picture looks grainy. With too many the wells fill completely and detail is simply gone.
A phone sensor's well holds only a few thousand electrons, and once full it cannot record more — which is why bright skies go blank white.
The short version
A camera catches a photo by focusing light onto a grid of wells and opening a shutter for exactly long enough to fill them properly.
Try it yourself
Photograph a waving hand indoors, then in bright sun. The sunlit shot is sharp because the shutter needed far less time to gather enough light.
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