We recently received an email from a contributor who had taken a photograph out of the window of a small airplane. The contributor, inspired by a couple of recent articles on iPhone photography, was using a smartphone to take the photograph, and he got an interesting effect. The propeller blades are seriously distorted, but it's not just blur from a slow shutter. The distortion is different in various parts of the frame. The photograph was taken with an iPhone, but you'll see the same effect in images taken from many cameras, including high-end DSLRs. So what's going on?
The phenomenon is due to the way CMOS sensors record images. Think back to film. The camera shutter opens, allowing light to hit a piece of light-sensitive film more or less all at once (there are exceptions with fast shutter speeds and focal plane shutters). With CMOS sensors, the image is recorded through line scan, which also is called rolling shutter. This means that the image is being made across the sensor over a period of time. It's a very short period of time, but it's definitely not all at once.
As a nature photographer, this is a phenomenon that you're unlikely to run into frequently. You won't see it with landscapes or even fast-moving wildlife, but it does come into play. When you're shooting HD video with your DSLR, you're likely to see some of the artifacts caused by rolling shutter. The most common issues are the Jello effect, skewing images, smeared images and partial exposure.