Eli5: Why does a camera have to flash and time it perfectly when it takes a picture? Can’t the light just stay on?
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In order to take the picture, the camera collects light/data for a certain amount of time.
That’s called shutter speed and refers to how long a camera shutter, that covers a sensor, takes to open and close. It can be 1/8000th of a second, or it can take up to a full 30 seconds. The longer it’s open, the more light it takes in, and the brighter your picture will be.
A flash is just meant to make a subject brighter than the background. So the camera shutter is set so the background doesn’t get too bright, and a flash is set to make the subject brighter.
Flashes tend to be really bright for a tiny bit of time. This is both because of how the electronics for that light are built to avoid over heating and because it’s easier to have a continuous same level of brightness for a tiny bit of time than over a long period of time. The longer it goes, the more that brightness will fluctuate, which might seem like very little but to a sensitive camera sensor it might seem like a lot.
Photographers can use continuous lights instead of the flashes, and they do, but flash has a benefit where it helps to freeze motion. The flash is usually only ‘on’ for less time than a shutter is open or the sensor collects data. So if the shutter is open for 1/100th of a second, the flash might only be on for 1/10,000th of a second.
While that camera shutter is open collecting light, something really bright will get kind of set in stone but something not as bright will be more gradually set. So flash allows the camera to set how the brightness of a subject looks really fast, while letting everything else in the background that isn’t so brightly lit get set over a short period of time (the shutter time).
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