Posts Tagged ‘Milos’

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Rolling Shutter vs. Global Shutter: Just Learned Something New About My GoPro

Sunday, July 28th, 2013

On this rainy Sunday morning I was just sorting through images I shot during my recent trip to Greece, and I learned something new (and surprising) about my GoPro camera: even when shooting stills, its electronic shutter is rolling, not global. If you just thought “huh?”, then read on.

On a short flight from the island of Milos back to the Athens airport I set up the GoPro on a suction cup mount pointing out the airplane window to do little timelapse of the flight. Looking at one of the still frames from the timelapse, I saw this:

GoPro still frame shot out the airplane window. Well, I guess that answers that question!

The above image is not Photoshopped in any way (other than the watermark). “What in God’s name is going on with that propeller, the blades are split in pieces!” you might say. Photographers, videographers and some others knowledgeable about electronics will know immediately what is happening here. The propeller blades did not, in fact, break into pieces during my flight (thankfully); the blades appear this way in this image as a result of a curious side effect of the way certain digital camera sensors work called “rolling shutter.”

Digital cameras’ sensors are composed of millions of individual pixels arrayed in a grid of rows and columns. DSLR cameras have a physical, mechanical shutter that starts and stops the collection of light hitting the sensor, but other types of cameras (point & shoots, cellphone cameras, and cameras like GoPros) do not have a mechanical shutter, and instead start and stop the process of collecting light on the sensor electronically rather than mechanically; essentially the circuitry in the cameras tells the pixels in the sensor when to start and stop “looking.” On many cameras, the pixels start and stop “looking” the way you’d expect: all at the same time. Cameras that behave this way are referred to as “global shutter,” because the “shutter” (which is in fact just an electrical signal) acts globally, on the entire sensor at once. Some other cameras though (notably, most digital video cameras) use what is referred to as a “rolling shutter” in that instead of reading the entire grid of the sensor’s pixels at once, they read the pixels one row at a time: the camera records what the first (top) row of pixels “sees,” then the row below it, then the row below that and so on, on a “rolling” basis until it reaches the bottom row. This process happens in a fraction of a second, so it is usually fast enough that it isn’t relevant to the image, especially when recording individual, still frames instead of video.

But when photographing something that’s moving very, very quickly (like an airplane propeller at full throttle!) that fraction of a second during which the camera moves from recording the top of the image to the bottom matters: by the time the camera gets around to recording the bottom of the image, the subject recorded at the top of the image has already moved, so in the finished image you’re seeing the same subject recorded in different positions. With a spinning propeller, that results in images like the one above, distorted by the passage of time.

Many cameras use a global shutter when recording still images (as opposed to video) and I thought my GoPro did too. I knew the GoPro used a rolling shutter during video recording (that is quite common on all but the highest-end professional video cameras), but it turns out that it uses a rolling shutter during still frame shooting as well.

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