Why do moving lines appear on a screen in my camera preview, and why do they change with ISO?
Asked 10/13/2015
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When I point my Samsung Galaxy Grand Quattro camera at a display, I see moving dark/bright lines in the live view. They are not visible in the final still image. If I change ISO from Auto to 100, 200, or 400, the lines change in intensity, width, and speed. What causes this?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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You are seeing the scanning of the screen. Try shooting an airplane propeller next! The exposure is made with a travelling slit so different parts of the image are exposed at different times and the movement causes artefacts.
Even though the shutter is not mechanical, the reset and readout has the same effect, but more complex: while a focal plane shutter is a single pair of curtains, I'm guessing here that the sensor chip has strips that are scanned in parallel. Each strip has a dark zone and a bright zone because the screen was bright or dark at the time each row was exposed. The bright bands are diagonal because the screen is painted from top to bottom, orthogonal to the direction of the sensor rows.
Different iso changes the exposure including the shutter speed.
I mentioned an airplane propeller as the iconic example of the artefacts caused by a focal-plane shutter. With the segmented electronic shutter, the same scene would show telltale signs of the segmentation. How about finding something spinning fast and posting similar shots as a comparison?
Originally by user33948. Source · Licensed CC BY-SA 4.0
user33948
11y ago
0
Generated from our catalog & community — verify before relying on it.
You’re seeing a timing mismatch between the display refresh and the camera sensor readout. Many screens draw or refresh the image line by line, and phone cameras also read the sensor progressively (a rolling shutter), not all at once. When those two scans interact, you get moving bands or diagonal lines in the preview.
Changing ISO often causes the camera to change exposure timing, especially shutter speed, so the banding’s width, speed, and brightness can change too.
This is the same kind of effect that can make propellers look distorted in photos or video. It’s not usually a fault in the camera. It’s an artifact caused by photographing a screen or other fast-cycling light source.
Why you may not see it in the final still: the exact timing of the capture can reduce or average out the bands, while live preview makes the interaction more obvious.
In short: the lines are caused by screen refresh + rolling-shutter sensor readout, and ISO changes affect them indirectly by altering exposure behavior.
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