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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They used to. If you look on older film cameras there are two flash settings: B and X. The B stands for bulb, and the X is for a modern “strobe” flash. When you use flash, the shutter speed of the camera becomes irrelevant, as the light is coming from the flash, rather than ambient light. A bulb has a much longer duration. On a camera with a focal plane shutter, you will have a “sync” shutter speed, usually 1/60 of a second, so the curtain fully opens. On a leaf shutter camera with a strobe flash, it can be 1/500 of second, because of the way the shutter works–the strobe flash is of very short duration, and freezes the action. A flash from a bulb is of much longer duration.
In the days of B&W studio photography, before strobe flashes became a thing, the subjects were lit with continuous incandescent light, and so both the shutter speed and aperture came into play. Being under those lights was extremely uncomfortable, so the advent of studio strobe lighting really changed things.
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