How does a camera sensor form a long-exposure image?
Asked 8/21/2016
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When using a very slow shutter speed, does the camera keep collecting light for the entire time the shutter is open? How is the final image created on the sensor? For example, in long-exposure night photos I may see only light trails from traffic, but not the cars themselves. What determines what appears in the final image during a long exposure?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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So, If I set the shutter speed to say, an hour. Will the camera keep collecting light as long as the shutter stays opened?
Yes. But an hour is quite a bit longer than you'd typically want unless you take steps to drastically limit the amount of light entering the camera, like using a very dark neutral density filter.
How does it create the final image?
Same as for any other exposure: the sensor is a large array of photosites, where each photosite corresponds to a pixel in the final image, and each photosite accumulates a voltage as photons strike it. After the exposure, the voltage at each site is read, and that set of values is the image.
I don't see any traffic at all in the final image but the lights only. So, what factors decide that image?
You don't see cars because cars don't emit any light, aside from their headlights and tail lights (which you do see). Cars often do reflect light from other sources, but there aren't any light sources in the image that are close enough to be reflected by the cars.
Originally by user4262. Source · Licensed CC BY-SA 4.0
user4262
9y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. During a long exposure, the sensor keeps collecting light for the entire time the shutter is open. Each photosite on the sensor gathers incoming photons and builds up a signal; when the exposure ends, the camera reads all those values and turns them into the final image.
A long exposure is simply an accumulation of light over time. If too little light is gathered, the image is underexposed; too much and it becomes overexposed. That’s why very long exposures often require a small aperture, low ISO, or a neutral density filter to avoid too much light.
In traffic scenes, bright lights from headlights and taillights register strongly as they move, creating visible streaks. The cars themselves may be much darker and spend only a short time in any one spot in the frame, so they may not build up enough exposure to show clearly. What appears in the final image depends on brightness, how long something stays in one position, and the overall exposure settings.
Many cameras allow only limited shutter times directly, often up to 30 seconds, with longer exposures available in Bulb mode.
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