You know it, because it happens to you. You’re taking still pictures with your phone and you get some weird distortion like on the blades of the helicopter in the image above. It can actually make some things look slanted or warped while other things look just fine. It’s a nuisance, and it makes you wish you had a better camera with you.
We call this effect “rolling shutter” because that’s what it was, originally. This classic photo shows the same effect:
and it’s actually due to the same problem. As the camera’s shutter rolled across the film, the image moved, so different parts of the image were exposed with the subject in a different position. 100 years ago this was because of a real, mechanical shutter which rolled across the surface of the film, and in fact mechanical shutters are still found in some cameras although they work much better.
Today’s rolling shutters are caused by too much information traveling from the camera’s chip to the phone’s memory. Remember, in a phone every milligram is precious and power consumption has to be kept at a minimum. So, cell phones won’t use a power-hungry, wide-bandwidth interface between camera and memory. The picture is captured one line at a time, and if something in the picture is moving fast enough, it’s going to be in a different position when the bottom of the image gets into the phone’s memory.
Rolling shutter is very obvious in low-quality video and it’s even easy to see with expensive video rigs if you’re a fan of celebrities. As they get out of their cars at night and flashes go off, you’ll see the video showing horizontal bars of white instead of the whole image going white. This is due to the rolling shutter effect of video cameras, and it’s rare to see it unless you’re capturing something as quick as a camera flash.
As cell phone components get less expensive and more efficient, you see less of the rolling shutter effect but then, you have situations like the latest crop of phones that capture 4K video… more pixels means more information and that means more possibility of rolling shutter.
There isn’t much you can do combat rolling shutter effects. There are some filters that can be applied, and you can try shooting in a bright environment so that the image is captured as quickly as the phone can store it. But really it’s just a nuisance we have to live with.