Most modern CMOS RGB color cameras are rolling shutter cameras. This happens because the camera sensor’s processor reads row pixels in sequence. This is done because nowadays the sensors have a lot of pixels, 12MP to 32MP cameras are common on smartphones now. 12MP means 12 million pixels. Also cameras are capturing frames at pretty high framerate from 90fps to even higher. 90fps gives around 11 milli seconds to the camera processor to process a frame. Doing this for 12 million or more pixels is a difficult task. To distribute this task overtime to keep the manufacturing costs lower and required engineering behind it less computationally intensive, rolling shutter technique is implemented. In this, first pixel rows are exposed and read before the last pixel rows of the camera sensor. This is represented in figure B below:
This rolling shutter technique exposes the camera sensor rows at different times. This produces a special effect in fast moving subject pictures taken with a rolling shutter camera.
The above is rolling shutter effect observed on the following setup:
This effect can easily be generated at home experiments with regular smartphones or DSLR cameras. Be sure to reduce the shutter time/shutter speed to a very low value as otherwise the picture won’t be sharp and there will be a lot of blur. For this test I used a shutter speed of 1/12000 which is 83 micro seconds of exposure per frame on an iPhone 12 camera. I don’t know if the hardware supports such low shutter speeds but the app says this number and pictures look crisp, so maybe it does :). Another thing you’ll see with very low exposure times is that the picture is very dark. So be sure to prop up the ISO value as well as provide your setup a lot of light.
Rolling shutter effect video, also showing aliasing:
How to know rolling shutter direction of your camera?
Another information we have here is the direction of rotation of the wheel. The wheel is rotating counter clock wise. Going in a bit deeper, we can know the rolling shutter direction of the camera sensor. If the direction of rolling shutter is top to bottom, we will see convex leaf facing left direction on a counter clockwise rotating wheel as explained below:
While, if the direction of rolling shutter is bottom to top, we will see a convex leaf facing right direction when wheel is rotating in counter clockwise direction as explained below:
Since, we observe convex leaf facing left direction with a counter clockwise rotating wheel, the rolling shutter direction of my camera is top to bottom.
This was a fun experiment and arriving at this setup was fun. Non-working setups made to observe this effect included a hair blower to rotate a paper cup :). It did not work because the speed of the wheel was not fast enough as well as camera shutter speed not low enough.
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