Why can a moving car seem to disappear in a long night exposure?
Asked 1/26/2017
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2 answers
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I photographed cars on a highway at night using a 4-second exposure. In one image, the cars’ headlights are visible as bright streaks. In another, a car was directly in front of the camera when the shutter opened, but the body of the car seems to vanish while the background remains visible. Why does this happen in a long exposure?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
7
Technically, the car doesn't completely disappear. The few photons that were reflected from the dark car during the time it was in the frame were caught and recorded by the camera. It's just that there were a lot more photons reflected by the things that were stationary for the entire exposure time. There were also a LOT more photons captured that were emitted by the much brighter taillights. Those much greater quantities of photons from other things overpower the very few photons reflected by the car as it moved through the frame.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
9y ago
0
Generated from our catalog & community — verify before relying on it.
The car didn’t truly disappear — it just contributed very little light compared with everything else in the frame.
In a 4-second exposure, stationary subjects expose the sensor for the full 4 seconds, so they build up much more signal. A moving car only occupies any one position in the frame for a small fraction of that time, so the light reflected from its dark body is spread out and too weak to stand out.
Its lights are different: headlights and taillights are far brighter than the car’s body, so even during a short time they register strongly and create visible streaks.
So the effect is mainly a combination of:
- the car body being relatively dark
- the car moving quickly through the frame
- the background being static for the whole exposure
- the vehicle lights being much brighter than the body
That’s why the body can appear to vanish while the light trails remain obvious.
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AI9y ago
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