how does long exposure shots on cameras work?

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How does the camera capture a light source a trail of light instead of just a point.

In: Technology
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Think of it like all the frames in 2 seconds of a movie combined into a single image, except it’s live so it is continuous time being recorded in that 2 seconds not divided up moments in time. A long exposure captures everything over the span of time the shutter is open. Photography literally means “light writing.” The light is like a brush that paints on the film or sensor for exactly how long you set it. The longer it exposes the more writing is done by the light on the film/sensor.

Some background first. In the film days cameras captures an image by focusing light from the world with the camera’s lens system. The most basic being a “pinhole” camera where the lens is just a tiny hole.

The light from the lens is directed to focus on film (and with a digital camera it’s a sensor array made from a silicon chip).

A mechanism called a shutter normally blocks the light from hitting the film and it set to open for a specific amount of time, this is what, “exposes” the film to light and results in changes to it’s chemistry which is how the image is captured. In digital the silicon sensor is able to measure the total amount of light that hits that part of the sensor (made up of sub pixels).

A further mechanism called an aperature is able to modify the amount of light hitting the film/sensor.

And last the sensitivity of the film (or a setting to the digital sensor) which is called “iso”. All these things must be balanced so the picture is visible. Too much light and you end up with a picture that is all white. Too little and it’s all black.

Ok, now how is a long exposure made? A long exposure means the camera collects light for a fairly long time. Maybe a few seconds to several hours. The other settings must be adjusted based on the lighting conditions so that the exposure works out. In a bright scene that might mean a low iso (low sensitivity) and a high aperature (reduce the light passing thru the lens). There are additional filters one could use to further reduce the light if necessary too.

The result is capturing the image over a fairly long period such that the image shows streaking car headlights, or stars, or individual cars replaced with steaks of light.

Exactly the same way as any other shot. There’s no such thing as “long exposure” in reality. Any photo whatsoever can have a trail of light in it, because any photo happens over a time period. It’s just a question of how bright it is and how fast it moves in the image.

A 1/60 shot of a fast moving car will be blurred. A 60s exposure of a rock will be sharp.