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Video basics3 min read

LESSON 90

Sensor readout and rolling shutter

Why fast pans bend straight lines.

IN THIS LESSON

  1. How a sensor reads a frame
  2. The artefacts you will see
  3. Why readout speed matters
  4. Practice: living with it

How a sensor reads a frame

Most video cameras today use CMOS sensors, and most CMOS sensors in consumer and prosumer cameras read the picture line by line rather than all at once. This is called a rolling shutter: the top of the frame is captured a fraction of a second before the bottom, yet on playback the whole frame is shown as if it were one instant. A global shutter, by contrast, captures every pixel at the same moment. Some CMOS sensors do have global shutters, and older CCD sensors usually did, but they are less common because they cost more or give up some sensitivity.

The artefacts you will see

Because rows are read at slightly different times, fast movement can distort. When the camera vibrates, as in handheld telephoto shots or from a car, the picture can wobble like jelly. When the camera or subject swings sideways, verticals lean, which is called skew. A spinning propeller can turn into curved, disconnected blades. A camera flash or a lightning strike may light only part of a frame, so the top third is bright and the rest is dark. Flickering lights and strobes can cause similar bands across the picture.

Why readout speed matters

The faster a sensor reads from top to bottom, the smaller these effects become. Readout speed depends on the sensor design and also on the mode you choose: many cameras read faster in a cropped or lower-resolution mode, and slower when they use every pixel of the sensor. This is working knowledge rather than a fixed rule, so check your own camera's figures, which some makers and testers publish as a readout time in milliseconds. A smaller number means less skew.

Practice: living with it

This is working practice. Pan smoothly rather than whipping the camera, and use a tripod head or gimbal for fast moves. Avoid strong vibration at long focal lengths, or add a stabiliser. When you film under flickering lamps, set the shutter to match the mains rhythm, for example 1/50 in a 50 hertz country, so bands stay still or vanish. If you film cars, sport or propellers often, look for a camera with a fast readout or a global shutter, and test it before the job.

Try it today

  1. 01Film a doorframe while panning left and right fast.
  2. 02Repeat slowly, then compare how much the verticals lean.
  3. 03Film a desk fan or propeller at two shutter speeds.
  4. 04If your camera has a crop mode, repeat the fast pan in it.
  5. 05Note which mode skews least, and use it for action.