Nikon D300 User Manual page 626

How the Rolling Shutter Works in D-Movie
Mode
Since video is captured at 24 frames per second (fps), the
D300S has an electronic shutter in addition to the normal
mechanical shutter.
Have you ever used your D300S in Continuous release mode
where you are capturing up to seven or eight still images per
second? The mechanical shutter activation combines with
mirror movement to make this cool "chicka-chicka-chicka"
machinegun sound that causes passersby—with their little
point-and-shoot cameras—to look at you in awe. However,
you don't hear that sound when using the D-Movie mode, do
you? That's because your D300S does not use the mechanical
shutter when shooting movies. If it did you would wear the
camera's shutter out with only an hour or two of video
capture. Remember, the camera captures video at 24 fps. An
hour of video requires 86,400 frames at that speed. You
would quickly exceed the tested lifetime 150,000-image
capacity of the mechanical shutter.
Instead, the camera uses its electronic shutter and turns the
sensor's pixel rows on and off, as needed, in a scan from top
to bottom. In other words, the camera records each video
frame by scanning it—one line at a time—from top to bottom.
This is called a rolling shutter. Not all parts of the image are
recorded at exactly the same time! It can produce a skewed
and/or wobbly video when filming rapidly moving subjects
like a race car or flying bird.
Rolling shutters are used by video cameras that have CMOS
sensors, like the Nikon D300S. Most dedicated video cameras
have CCD sensors, with global shutters that do not scan the
626