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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You absolutely can! Many professional studios do this but a bit of context on the “why” it exists and on why it’s not a feature on my devices or cameras now.
So traditionally you know how you fill a bathtub with the tap? It can take a while and there isn’t a lot of flow. One way to mitigate this is to fill a second vessel up (let’s say a barrel) slowly but when needed just yeet all that water into the tub at once.
Same thing happens with the electronics in a camera, the battery can provide low power so you need to fill up a “barrel” in this case a capacitor. When you hit the button that power is dissipated very fast into the flash at once. Now with more modern devices and lower power LEDs it much more able to keep a light on (you see it with the flashlight on your phone) but its still VERY power hungry and will take lots of power which could better be used elsewhere. If you have ever accidentily turned the flashlight on you phone by accident and found it hot and with 50% battery after 10m that’s what’s happening.
The priority for devices (and cameras) are not specifically the flash as it provides less benefit to the user so a “flash” is good enough and something people are used to.
In old cameras they used a very bright [Xenon strobe lamp](https://en.wikipedia.org/wiki/Flashtube) which is *very* bright (far brighter than a traditional filament lamp could manage) but can’t be sustained, certainly not from a battery supply – so they just trigger it as the shutter opens which is actually very easy.
In modern phone cameras, the LED that’s the flash can stay on for a while longer but at maximum brightness it will get hot very quickly – often the “flash” uses a far brighter level (more power) than the LED could actually survive for more than a second or so without overheating & burning out.
In “torch” mode the LED is powered at a lower level so can stay lit without overheating.
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A bright light will cause people to squint. A quick flash is over before you have the chance to react.
If you want a super clear and bright image, you want to capture as short of a slice of time as possible. On a lot of older photos you’ll notice the image is clear, but any bright spots or lights are “smeared out” into squiggly streaks. This is because a lot of cameras had a bit longer exposure times, possibly because it wasn’t very adjustable or because it was very imprecise (no eay for the camera to tell how much it would need), and the flash is ehat gives you the short bright clear image. So it’s capturing light, then flashing, then capturing some more, and then closing the capture.
I have used continuous bright lights for photography on occasion… aside from everything others have mentioned, trying to keep your eyes open in sustained bright lighting gets fatiguing both for the subject and the photographer.
Very bright lights need looks of power and make lots of heat.
A flash bulb is actually much much brighter than a standard light, even an LED. They use a lot of power and because of that, leaving them on for long periods of time creates a lot of heat. Studio lights will be just as bright but they are not limited by size or portability so they can have more and larger heat sinks.
Flash bulbs used to be arc lamps that would be fired by a capacitor. You’d head a whine when the capacitor was charging because you need a lot of power to make an arc. Before that they would use carbon filaments which were single use; you would put a bulb in the flash holder, take the picture, then pop out the bulb and throw it away.
There are several discussions about continuous vs a flash, but cameras only have a fully open shutter when below around 1/250 sec. If you shoot faster than that, the 2nd curtain starts closing before the first one has fully cleared the lens. This causes a bright band and dark band. It actually is a scrolling slot across the sensor or film.
So there’s actually a 3rd thing flashes can do: they can pulse their power across the entire time it’s open so that the slot always has some light, which is additive.
Yes, there’s a pretty large one somewhere outside right now. Powered by fusion, even. Has a bad habit of wandering out of frame every single day, but seems to do so on a regular schedule.
Our puny Earth lights can’t sustain similar levels of power all across our orbit. We can, however, either get a light to shine very bright for a split-second, or pretty bright continuously. Continuous lighting can be very expensive, and pretty uncomfortable if aimed directly at someone, so we don’t always use it.
The lamps we often buy for our homes are even punier, and often don’t provide enough light for a camera to snap a photo without blurring or creating lots of image noise.
The battery in your camera or smartphone can maybe take thousands of images in one charge, run the light for a few minutes, or some combination in between. Some smartphones cannot run the light at the same time as the camera. Maybe there isn’t enough power available, or maybe they’re enforcing use of flash or external lighting.
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.
With a flash, there’s a small – moderate chance the subjects will be blinking.
With blindingly bright lights, there’s an almost 100% chance the subjects will be squinting.
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