– What exactly is the “exposure” in photography?

Question

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?

In: Technology

Answer 1

“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.