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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The light is extremely bright. The battery of a camera can’t output that much power and have to charge up a capacitor over time and then release the energy all at once. The lightbulb could also burn out of it was powered continuously. Maybe today we can have more powerful lights with LEDs, but they would take up too much space and not fit in the camera body.
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It is of course possible to take a photo using continuous bright lights, and this is often done. For example in a professional photoshoot there will be lots of lights involved, or when photographing performers who would be distracted by a flash photographers may carry dedicated lights.
The price of this is that for a sufficiently bright light you need a large battery, and at least historically a bulb would also get hot and so need to be large to allow heat dissipation. A flash gets round this because it is only on for a tiny moment. The total amount of energy used per photo is therefore small, despite the light being extremely bright.
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The light is too powerful to be sustained continuously by the small-voltage battery that’s inside a camera. Instead it uses something called a capacitor. This capacitor is slowly charged by the battery with the energy needed for the flash. The capacitor can release all its energy very quickly and therefore – for a very short time – supply the large amount of energy needed for the flash.
So very(!) simplified: say the battery can supply 1 energy per second. And the flash needs 5 energy. Then the battery charges the capacitor for 5 seconds with 1 energy every second. The capacitor holds 5 energy. Then you take a photo. The capacitor releases the 5 energy in 0.1 seconds and makes a flash that lasts 0.1 second.
This is for conventional cameras. Phone cameras have smaller led lights and could in theory be on the whole time. I think it would blind too much though.
The main reason ‘flash’ exists is because of historical technical reasons. The original cameras (film) werent very sensitive to light. It required really long exposure times – like minutes. So to speed it up, they invented ‘flash’ – basically a paper covered in flammable material that they would light and be very bright to get a better photo.
Over time, flash paper was replaced with various new tech – flash bulbs were one-time use and very bright. Then xenon reusable bulbs.
These days, though, for most photography you don’t need a flash. Your camera phone is very sensitive to light and can even take reasonable pictures in dimly lit areas. The flash (now an LED) could stay on, but it uses a lot of electricity and can be uncomfortable for the people you are photographing.
So there are several considerations here:
* **Energy:** Studio “hot” (continuous) lights use a very large amount of power (500 w per light is fairly reasonable). Even still, a flash can generate more light within the narrow amount of time that a shutter is open.
* **Motion Freeze:** Flash allows better ability to “freeze” an image and reduce blur. The speed is faster even than a shutter speed of 1/250 of a second
* **Undesirable Light:** Flash is less likely to cause a model to squint or light your backdrops on fire
* **Portability/Efficiency:** Flash units can run on a small power source since they slowly charge up a capacitor. A disposable drug store camera runs a reasonably powerful flash on like a single AAA battery. This is much more efficient because all the light is released while the shutter is open so energy isn’t wasted
* **Power:** The actual power that a flash releases can be in the tens of thousands of watts!! This is because a fairly large amount of energy is discharged in a very small amount of time.
* **Overpowering Ambient Light:** Since flashes are extremely brief and synchronize to a shutter, you can use very fast shutter speeds. This will prevent ambient light from affecting the image. If you don’t want overhead lights, a window, the sun, etc. from affecting the image, you can use a very fast shutter and overpower the light with flash which is as bright as lighting (but for a very brief moment). If you *do* want ambient light to be part of the picture, you can adjust for this too and use both a flash *and* a longer shutter.
A few more misc things regarding flash in common use:
* **LED Flash:** “Flash” on many mobile phones is not really a true flash and just an LED light. Im not sure why it pretends to be a flash and only fires in bursts (on my Moto droid phone). Maybe its trying to prevent the subjects from squinting?
* **Combined:** Studio lights often combine a “hot” continuous light with a flash in the same light head. The continuous light is dimmer but lets you “preview” what the light looks like and helps you set it up/pose your model/subject.
TLDR: Photographic flash is very much like a lightning strike. A lot of energy is stored up slowly and then released very quickly.
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).
Several reasons:
1. It is easier, and uses much less charge, to power a single strobe of 6400 watts for 1/1000 of a second than to power that same wattage even for a whole second.
2. Flashes can be used to compensate for slow shutter speed. A mechanical shutter can only go so fast, but a strobe in a dark room will give you the effect of a very fast shutter.
3. Historical Development: The flashbulb originally was a magnesium filament in an oxygen bulb designed to explode and produce the flash. It was much EASIER and CHEAPER to design a bulb that would ‘flash’ than one that would maintain high intensity.
4. You CAN light the scene for longer periods, but it takes much more equipment and money to do so, while a flash is small and efficient and gives you the same result.
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