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Tuesday, October 4, 2011

Solutions: Rolling Shutter


Discover this little-known artifact that’s generated from some digital cameras when the conditions are exactly right

This Article Features Photo Zoom

A CMOS sensor like this is the cause of artifacts like wobble, skew, smear and split exposure. This type of sensor captures image data by line scan so there's a slight delay as data is scanned across the frame. The delay is miniscule, but in very fast-moving subjects, you can sometimes see strange results. This doesn't mean that CMOS sensors are somehow inferior, but they do exhibit this quirk. Even high-end, full-frame DSLRs like the popular Canon EOS 5D Mark II and the Nikon D700 aren't immune.

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.

The Jello effect is seen in video when the whole frame seems to wobble slightly. It's caused by the camera vibrating while the video is being shot. The Jello effect is most strongly seen in scenes taken with long telephoto lenses, which tend to magnify any camera vibration.

The skew effect is more visible in still images than motion. It's seen when the subject moves quickly from one side of the frame to the other. As the line scan works its way across the frame, a fast-moving subject can be recorded over an extended period, and therefore over more lines, if it's moving in the same direction as the scan and it's moving fast enough. On the other hand, if the subject is moving against the line scan, the opposite skew happens because the subject is being recorded for a shorter period and over fewer lines than it should be.


This photo was taken with a camera phone. You can see the distortion of the propeller blades quite clearly. In fact, one blade doesn't even seem to be attached to the hub.
Image smear is usually illustrated by such subjects as the moving propeller that we mentioned at the beginning of this article. The distortion renders the propeller blades on one side of the frame thinly, and on the other side of the frame, they're rendered fat. Also, a blade can appear to be bent at an extreme if the line scan and the propeller speed are synced up correctly. We've heard some photographers say that they've seen a similar effect when photographing hummingbirds in flight, although it's not a common effect.

One sees partial-exposure issues in still photography only. This happens because the flash duration isn't long enough for the line scan. You see an analogous effect if you shoot a scene at a shutter speed that's faster than your camera's X sync. Unless your flash fires for a very short period, you won't see partial-exposure problems due to rolling shutter. On the other hand, if you shoot with a shutter speed that's faster than your X sync, you'll almost always have partial-exposure issues.

How do you defeat rolling-shutter issues? Some are easy to avoid, while others are just a consequence of digital photography. Smear and skew are difficult to mitigate because the effects are caused by the relative speed of the line scan and the subject, neither of which can be controlled. The Jello effect is easier to deal with. A sturdy shooting platform is the first step in getting wobble-free images. Avoid shooting from inside a car where the engine vibrations can contribute to wobble. Also, because extreme telephoto lenses exacerbate the effect, be sure that anytime you're using a long lens, you also have a very stable platform. While the Jello effect is most visible in video, it does show up in still frames under the right conditions.

The next time you're on a propeller plane, try shooting a picture out the window that has the prop in it. It's an interesting effect, and while the images aren't likely to make the cover of OP, you can have some fun showing the shots to friends.

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