Like, for instance, I’ve seen photos of the night sky with crazy details of the stars and they say that this picture was taken with “12 hours exposure”. What does that mean exactly and what does it do?
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“Exposure” in photography is a combination of three things: 1) the size of the hole through which the light is being gathered (“aperture”), 2) the sensitivity of the film or electronic sensor (“sensitivity” or “ISO”), and 3) the time that the sensor is exposed to the light (“exposure time” or “shutter speed”).
The combination that’s right for a given set of circumstances will depend on various things, such as the amount of light that’s coming off the thing you want to photograph, the amount it’s moving with respect to the camera, and the required distances that need to be in focus.
For most everyday photography in daylight, a normal shutter speed (exposure time) would be around 1/30 to 1/500 of a second. Photographing stars would require a much longer exposure time because they’re much dimmer than a normal daytime scene. An exposure time for stars using a normal camera would often be several minutes. Any reasonably advanced camera would support this kind of long exposure, but it would normally require a special setting on the camera.
Such a long exposure will require the camera to be locked down on a tripod or similar. (For a sharp picture, the camera has to be still during the exposure and nobody can hold a camera still enough for such a long time.) Because the stars are moving in relation to the camera (the stars are still but the camera is fixed to the Earth and the Earth is rotating under the stars) the stars will often appear to move during the exposure and the resulting image will show the stars as lines (in a “star trail” picture). An exposure time of a few minutes is enough for the stars to move enough to start forming these lines.
To have a long exposure of several minutes (edit to add: or hours, as in your example) but still keep the stars as fixed points rather than star trails, the camera can be mounted on a special device that very slowly moves the camera to compensate for the Earth’s rotation and keeps the stars stationary in the view the camera is pointing at.
No expert here, but I think it comes from having to expose film to light for chemical changes on the film to happen. The longer the film was exposed to light, the more details you could make out on it.
In the older film type cameras, exposure is the time that the film is exposed to the scene that it is supposed to capture. The same concept is used for digital photography. In older cameras the shutter on the camera opens then quickly shuts (results in the distinctive sound of cameras). If a high speed event is captured, this exposure time is very low to avoid blurring.
Sometimes, to capture stars or low light events a long exposure is needed although the subject and camera must be still – pre digital age. This can be used to make special effects like the stars making an arc through the night sky. Or a longer exposure to deliberately have a “motion blurring” effect
Photographs collect light over a period of time. If the photo were taken ‘instantly’, no photons would reach the inside of the camera and you’d have no picture. If the photo were taken super fast, very few photons would get in and you’d have a super static-ey image. Photos are usually taken as quickly as they can be without becoming static-ey in order to avoid blurring from camera or subject movement. This length of time is the exposure. Really dim subjects can require super long exposures in order to get enough photons to make a good picture.
Stars are very, very dim. When you take a picture, you are exposing the frame for a period of time, usually just a fraction of a second, but you can control that amount of time. When they say a 12 hour exposure, they mean that literally. The frame has been exposed for that time, letting an incredible amount of light in. Of course, any movement in the cameras field of view will come across as a blur, so the movement of the earth comes into play at that point. For stars, you don’t need hours, just enough time to capture enough light. Lens choice and camera sensitivity also help.
When there is low light, the camera needs to sit there collecting light for a longer period of time in order to produce a usable photo. Most consumer/smartphone cameras handle this automatically. In a dark environment, your phone camera will actually take slightly longer to take a photo. However you might not notice this because camera manufacturers know that people aren’t going to be willing (or able) to hold still for several minutes while their phone takes a single photo. So they cap the exposure time at some fairly short duration with the tradeoff of not being able to capture photos of very dark environments. It might take a photo in 1/500th of a second in a bright environment, and 1/40th of a second in a dark environment.
But if you are willing to wait for a longer time and are able to keep the camera still, you can bump up the exposure time to multiple seconds, minutes, or even hours. Then you can take pictures in very dark environments and see things that you can’t even see with your eyes.
The length of the exposure is how long the sensor/film on a camera is exposed to light for.
To record a photo, a camera sensor wants to ‘see’ a certain amount of light – too little light and there is nothing recorded, to much light and it overloads the sensor.
On a bright sunny day there is a lot of light, so to collect the right amount of light can take a tiny fraction of a second.
At night however there is very little light, so the camera will take a lot longer to collect enough light to record a clear picture.
As an analogy, try filling up a glass of water from a tap. The glass will hold a certain amount of water – to little and you have nothing to drink, but too much and your glad over fills and water spills everywhere. A bright sunny day is like turning the tap on full, where it will fill the glass very quickly. A photo at night would be only letting a trickle of water out of the tap, so it will take a lot longer to fill the glass up. In the middle you could turn the tap on a reasonable amount, which fills the glass up reasonably quickly, but not as quick as turning the tap on full – like taking a photo on a full day compared to a really bright one.
So when someone says they used a 12 minute exposure, that means the lighting was very dim, so they needed to set the camera up and let it take 12 minutes to record the photo (which means you need a tripod to hold the camera steady, as if it moves at all during those 12 minutes the photo will be all blurry).
See it as a sponge in a bucket of water and compare it to light.
If you have a bucket with water and you dip in the sponge like that, the sponge is immediately wet. If you take a camera and point it immediately at the sun, you’ll have a white picture.
If you dip it in really slowly, the sponge will gradually become wetter, until your sponge is sufficiently wet for you to use. If you lower the light intake by the camera, or exposure, you’ll get a picture of the sky with a white/yellow dot on it.
If you have a very dry sponge and a bottom of water, the sponge won’t become fully wet immediately but will take some time to get fully saturated when it has finally absorbed all of the water. This is the same as shooting a picture of the stars. The light of the stars is super dim, so your camera needs more time to absorb the light to show a proper picture.
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