Why can rolling-shutter skew appear opposite to the mechanical shutter direction?
Asked 7/12/2016
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I understand that a camera lens projects an inverted image onto the sensor, so if a mechanical shutter travels top to bottom across the sensor, the scene should effectively be recorded bottom to top. But in a video frame showing rolling-shutter distortion, the skew seems reversed: the cork’s shadow appears in a way that suggests the upper part of the scene was recorded earlier than the lower part. How can that happen if the shutter normally moves downward?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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You are correct that the image is inverted as it is projected on the sensor and that the mechanical shutter reveals the bottom of the scene before the top of the scene. What you have missed is that the image (with the shadow of the cork falling on the red shirt at a later time than the the actual cork is seen flying through the air) is not a still frame taken using the mechanical shutter - it is a frame capture from a video recording.
During video recording the mechanical shutter stays open. It doesn't cycle between frames. The sensor itself is read out electronically in the opposite direction from the direction that the mechanical shutter normally moves - the sensor is read starting from the bottom of the sensor as it sits in the camera which is the top of the scene in front of the camera due to the lens' inversion. As the host in the video that your image is from explains, the photo in your question is a frame grab from a video recorded with the camera, not a still image recorded using the mechanical shutter.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
10y ago
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Because that example is from video, not a still photo made by the mechanical shutter. In video, the mechanical shutter is typically just open and does not sweep for each frame. Instead, the sensor is read electronically line by line.
That electronic readout can happen in the opposite direction to the mechanical shutter’s travel. So even though a still image might be exposed according to the mechanical shutter movement, a video frame with rolling-shutter distortion is governed by the sensor’s electronic scan direction.
In the example discussed, the sensor data is being read in a direction that makes the top of the scene be recorded before the bottom, which is why the skew looks opposite to what you expected from the shutter animation.
So the key point is: rolling-shutter artifacts in video are caused by sequential sensor readout, not by the mechanical shutter curtains.
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