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What is global shutter?

global shutter

What is global shutter?

What is a global shutter in a camera? This question has been recently the most asked in photography circles. Since the introduction of this technology by the camera manufacturer Sony, there has been a lot of confusion going around. People who even thought much about how shutters work have joined the queue in the confusion.
To make it simple for everyone, let’s reduce to the simplest answer: global shutter is a technology built by Sony that allows all sensor pixels to be read simultaneously.
global shutter vs rolling shutter

This picture was taken using Rolling Shutter

Is global shutter better than rolling?

If we want to objectively answer the question above “Is global shutter better than rolling?” we need to listen to the very respected experts in the domain such as Photo Metrics. On their website, they put both shutter mechanisms face to face so that it is easier for the readers to understand and make their minds up. So, according to photometrics.com

Global Shutter

After a global shutter camera has been exposed to a signal from a sample, all sensor pixels are read out simultaneously, hence the term global shutter. This means that images obtained from cameras with these sensors are snapshots of a single point in time. This is advantageous when synchronizing camera exposure to the light source activation using hardware triggers (explored in the triggering article), as the exposure happens at the same time across the sensor. CCD and EMCCD cameras typically use global shutters and can operate as ‘full frame’ imaging devices.

However, while this shutter mode has no time difference across the image, the readout is typically slow, especially when using CCD/EMCCD with global shutters as these technologies are limited in their readout rate due to only having one analog-to-digital converter (ADC). The more pixels on the sensor to transfer, the slower the total frame rate, even if the whole frame can be captured at once. In addition, a global shutter can result in increased read noise, limited framerates, and longer duty cycles for the camera.

Rolling Shutter

While global shutter reads out the entire sensor at the same time when exposed, some camera sensors readout row by row when exposed, with the readout ‘rolling’ down the camera sensor rows, which is why this method is known as the rolling shutter. Each row takes a certain amount of time to read out (e.g. 10 μs), known as the ‘line time’, meaning that the resulting image features a small time delay between each row. If a camera with a rolling shutter has 2048 rows (as in a typical 2048 x 2048 CMOS sensor), it means the top row is read out at time 0, while the bottom row is read out 2.048 ms later (10 μs multiplied across 2048 rows), this is known as the ‘frame time’.

Advantages of Global Shutter

  1. Distortion-Free Images: The most significant advantage of the global shutter is its ability to freeze fast-moving objects without distortion, making it ideal for sports photography, high-speed videography, and scientific research.
  2. Improved Video Quality: For video, the global shutter eradicates the jello effect commonly seen in footage captured with a rolling shutter, ensuring smoother and more consistent quality.
  3. Better Performance in Low Light: Global shutters are typically more efficient in low light conditions, offering better noise performance and image quality.

What are the cons of global shutter?

The known cons of Global Shutter today is the price. Understand that the only camera with global shutter technology is the Sony A9 III. And imagine the price: 6 999,99$ in Canada.

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