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Rolling vs Global Shutter: Key Differences Explained

Both are electronic shutter methods for image sensors. A rolling shutter exposes each row of pixels slightly later than the previous one and reads them out in order. The pixel circuit is simple and noise can be kept low, but because the top and bottom rows are captured at different times, fast-moving subjects are skewed. A global shutter exposes all pixels at the same time and temporarily holds the charge inside each pixel before readout, so there is no distortion. In exchange, the pixel needs more circuitry and tends to be larger.

AspectRolling shutterGlobal shutter
Exposure timingEach row starts and ends exposure slightly later than the previous oneAll pixels start and end exposure at the same time
Readout sequenceRows are read out in order as each finishes exposureThe charge of all pixels is transferred at once to in-pixel storage, then read out row by row
Capture time offsetThe top and bottom of the frame differ by one frame readout time (a few ms to a few tens of ms)None
Moving subjectsFast-moving objects, or panning the camera, cause skewed distortionNo distortion
Flickering light and flashLighting flicker tends to appear as horizontal bands. A short flash may light up only part of the frameThe whole frame receives light at the same time, so there are no bands, and it is easy to synchronize with pulsed light
Pixel circuitTypically a four-transistor pixelNeeds an extra element to temporarily hold the charge, plus transistors for it
Pixel size and sensitivityEasy to make small pixelsThe storage element tends to make the pixel larger and reduce the light-receiving area
NoiseCorrelated double sampling (CDS), which takes the difference between the reset level and the signal, makes it easy to keep readout noise lowDepending on the method, CDS may be difficult and noise tends to increase. Dark current during storage and light leaking into the storage element are also problems
CostLowHigher
Typical usesSmartphones, digital cameras, video recording, security camerasFactory inspection cameras (machine vision), barcode reading, automotive driver monitoring, distance measurement such as ToF, tracking for robots and AR/VR

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

Rolling shutter
When the subject does not move too fast and small pixels, sensitivity, low noise and cost come first. It is the standard in most cameras.
Global shutter
When you need to capture fast-moving objects with accurate shapes, or precisely align exposure with pulsed light or a flash: inspection, measurement and applications where machines judge images.
Reducing distortion
Even with a rolling shutter, shortening the time to read out one frame reduces distortion. Methods include faster readout with a stacked sensor and combining it with a mechanical shutter.

FAQ

Why do moving objects look distorted with a rolling shutter?

Pixels are exposed and read out one row at a time, so the top and bottom rows of the frame are captured a few ms to a few tens of ms apart. If the subject moves in that time, each row records a different position, and the whole image is skewed.

Is a global shutter always better?

Not necessarily. A global shutter needs storage in each pixel, so pixels tend to be larger, and it is often at a disadvantage in sensitivity, noise and cost. Where motion distortion is not a problem, a rolling shutter is chosen.

Why does lighting flicker appear as bands?

With a rolling shutter, each row is exposed at a different time, so rows exposed while a flickering light is changing brightness differ in brightness from other rows, and this shows up as horizontal bands. With a global shutter, all pixels are exposed at the same time, so there are no bands.

Check your understanding

Why do fast-moving subjects appear distorted with the rolling shutter method of CMOS image sensors?

  1. ABecause exposure and readout timing are offset row by row
  2. BBecause all pixels are exposed at the same time
  3. CBecause of the color filter arrangement
Show answer and explanation

Answer: A. Because exposure and readout timing are offset row by row

A rolling shutter exposes and reads pixels row by row, so top and bottom rows are captured several to tens of milliseconds apart. Fast-moving subjects appear skewed and flickering lights produce bands. A global shutter exposes all pixels simultaneously, avoiding distortion, and is used in industrial inspection and automotive cameras, but it needs in-pixel charge storage, making pixels larger.

In the typical four-transistor pixel of a CMOS image sensor, which transistor moves the charge from the photodiode to the floating diffusion?

  1. ATransfer transistor
  2. BReset transistor
  3. CSelect transistor
Show answer and explanation

Answer: A. Transfer transistor

A four-transistor pixel uses transfer, reset, amplifier (source follower) and select transistors. The transfer gate moves charge from the pinned photodiode to the floating diffusion (FD), whose voltage the amplifier outputs; reset initializes the FD and select chooses the row. This structure enables CDS by sampling before and after transfer, and neighboring pixels often share the FD and readout transistors to save area.

What is the purpose of correlated double sampling (CDS) used in CMOS image sensor readout?

  1. ATo amplify the signal twofold
  2. BTo interpolate color information
  3. CTo take the difference between the reset level and the signal level, removing reset noise and per-pixel offsets
Show answer and explanation

Answer: C. To take the difference between the reset level and the signal level, removing reset noise and per-pixel offsets

CDS samples the floating diffusion's output right after reset and again after the photodiode charge is transferred, taking the difference as the signal. The kTC noise that varies at each reset and the pixel amplifier's offset are common to both samples, so they cancel. Combined with complete transfer from pinned photodiodes, this lowers read noise to a few electrons or less.

  1. What is the main advantage of the scheme used in many CMOS image sensors in which each pixel column has its own ADC for parallel conversion?

Practice Image sensors and optical devices questions

Key terms explained

Rolling shutter
A readout method that exposes and reads pixel rows one after another. It is simple, but fast-moving subjects appear skewed. Exposing all pixels at once is a global shutter.
Global shutter
An electronic shutter method that exposes all pixels at the same time. Unlike a rolling shutter, which reads row by row, it does not distort images of moving objects.
Correlated double sampling (CDS)
Taking the difference between a pixel's voltage just after reset and its voltage when the signal is read. It removes reset noise and per-pixel offsets.
CMOS image sensor
An image sensor made with a CMOS process that has an amplifier in each pixel. It is low-power, can integrate peripheral circuits, and dominates smartphone cameras and more.

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