Why do light-painting shapes appear inverted when I move the camera during a long exposure?
Asked 10/4/2010
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I'm experimenting with long exposures on an older Nikon point-and-shoot. Because the camera only allows about 2 seconds of exposure, I'm moving the camera instead of moving the light source to create light-painting shapes like letters. When I do this, the recorded shapes look inverted, so I have to move the camera in a mirror-image pattern to get a normal-looking result. Why does moving the camera make the light trail appear reversed?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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When you move the camera down, the light source moves up on the film/sensor plane - just as if you look at something, then look down, the object you were looking at will be above your field of view (i.e. you'll have to look up again to see it). So if you trace a V shape from top left to bottom to top right, the light source will move from bottom right to top middle to bottom left.
This is not a feature of your particular camera, just geometry!
Originally by user456. Source · Licensed CC BY-SA 4.0
user456
15y ago
0
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This is normal and is caused by basic image geometry, not a fault with your camera.
When you move the camera in one direction, the scene shifts across the sensor in the opposite direction. So if you move the camera down, the light source is recorded as moving up in the frame. If you move left, the light trail appears to move right.
During a long exposure, the sensor records that opposite motion as a streak, which is why letters and shapes look reversed or inverted when you "draw" by moving the camera instead of the light.
So to get a normal-looking letter, you need to move the camera in the opposite or mirror-image path of the shape you want recorded. This applies to any camera and is simply how the image moves on the sensor when the camera moves.
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