Rolling shutter effect

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Animation to illustrate the development of the rolling shutter effect

The rolling shutter effect (for example: “rolling shutter”) is a positional error that can occur in photos or video recordings of moving objects.

Explanation

At first it seems natural that the exposure of all points of the light-sensitive element (film or sensor) of a camera begins at exactly the same point in time. However, there are cameras for which this does not apply to the entire surface due to their design; Rather, one has to imagine a "line of simultaneous exposure starts " that wanders over the element either line by line or column by column, for which it takes a short, but not negligible time.

As long as the subject is still, all image points are exposed at their correct position in such cameras , regardless of when the corresponding points were exposed. If, however, pictures of a moving subject, or with a moving camera, are taken with consecutively exposed lines, objects are shown at their current location - and since the mapping is done line by line, the object has already moved so far from one line to the next that when it is combined of all lines an object is displayed that was not shown as a whole at once, but line by line at different times. A straight line of the subject can be shown crooked or distorted when the camera is panned or a moving subject (corresponding to the successive individual lines).

The rolling shutter effect occurs when:

  • Focal- plane cameras . The effect is particularly evident when the subject is illuminated with a stroboscope .
  • Digital cameras with image converters in CMOS sensor technology. These sensors read the image line by line or column by column. This takes a comparatively long time, especially with professional sensors. If the affected (mostly mirrorless) system cameras use an electronic shutter instead of a focal plane shutter - in the absence of one or optionally for noise reduction - this is correspondingly slow - even slower than the focal plane shutter. Problems are then already caused by certain common artificial light sources and photography with flash , which is then either not possible at all or only possible with a long flash sync speed that is even unusable during the day .

High-resolution camcorders and video-compatible digital cameras are increasingly being equipped with CMOS image converters, which replace the CCD sensors that were previously used . The CMOS image converter can cause the rolling shutter effect to occur in these devices. As in photography , this effect occurs through diagonal distortion when recording fast-moving subjects; in addition, stronger vibrations of the recording device, such as those that occur when taking pictures from a vehicle, lead to a strong distortion of the entire image, which causes the recordings may become unusable.

  • Examples of rolling shutter recordings

  • The car driving past a camera with a focal plane shutter is shown diagonally distorted. The wheels appear oval instead of round. The motionless picture elements in the background, on the other hand, remain undistorted.

  • Rolling shutter effect on a propeller

  • Rolling shutter effect of a digital CMOS image sensor. The exterior mirror, which is at rest relative to the camera, appears undistorted.

Avoidance

The rolling shutter effect can be avoided with so-called global shutter CMOS sensors, which do not record the image in rows and columns, but rather completely. Due to their different mode of operation, CCD sensors do not show a rolling shutter effect.

Examples for illustration

Shooting stars

Shooting star of the Perseids recorded during a ten-second long-term exposure with a CMOS image sensor with Bayer mosaic . The brightly shining head of the shooting star was captured 17 times by individual color pixels during the rasterized exposures .

Shooting stars can be taken in the dark during a long exposure . With many CMOS image sensors, these exposures, which usually last several seconds, are recorded at several images per second, which are then added to form a total recording. If a single readout run by the image sensor is shorter than the duration of the falling star, only the brightly shining current location of the meteorite is recorded. All other pixels along the light trail only ever capture the comparatively weak afterglow of the falling star.

simulation

In the following images, a camera panning to the left (the subject moves to the right) is shown schematically when a vertical, green line is recorded with three lines following each other from bottom to top:

Rolling shutter effect
Recording with the camera stationary
Recording of line 1 at time t
Recording of line 2 at time t + Δt
Recording of line 3 at time t + 2 · Δt
Total recording composed of the three line recordings with rolling shutter effect

With a higher number of lines, the steps and jumps are correspondingly smaller, so that in the borderline case a continuous sloping line would arise.

Flatbed scanner

The following two images were taken one after the other with the same flatbed scanner with line sensor . In the first picture, the paper template was motionless, in the second picture the template was moved at a constant speed perpendicular to the direction of movement of the scan unit, so that the rolling shutter effect becomes visible.

Rolling shutter effect in the line scanner
Image with motionless paper
Shot with moving paper - the scan was done from right to left while the original was pulled down.

Individual evidence

  1. ↑ Hands-on with new Sigma 'fp' - a compact, full-frame, L-mount mirrorless camera , DPreview, July 11, 2019.
  2. ↑ Advantages and disadvantages of electronic, slot and central locking , digitalcamera.de, October 17, 2016.

Web links

Commons : Rolling Shutter Effect  - collection of images, videos and audio files