A global shutter lets you use flash at a much wider range of shutter speeds: so you can use a short exposure to prevent a sunset blowing out, but still light your subject with a strobe.

Sony FE 135mm F1.8 | 1/4000 sec | F2.8 | ISO 400
Photo: Richard Butler

The Sony a9 III is the first full-frame mirrorless camera to be built around a global shutter CMOS sensor, and we doubt it'll be the last. So what is global shutter and what does it mean for photographers?

Progressive shutters

A global shutter is one that captures all its pixels simultaneously. To understand the potential benefits this brings, it's worth understanding the alternative: the progressive shutters used in the majority of large sensor cameras.

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Most cameras use a mechanical shutter mechanism to end their exposures, and the majority use one to start them, too. Mechanical shutters are typically a series of thin blades that cover the sensor to prevent it from receiving any additional light, outside the time you want to capture an image. But, while they move very quickly to start and end the exposure, they're not instant. This means that the top of the photo is captured a fraction of a second before the end of the photo. In most instances, this makes little or no difference, but there are circumstances in which it affects the image, and this is where global shutters have an advantage.

Global shutters

The Sony a9 III is the first ILC to utilize a Stacked CMOS sensor with a global shutter.

There are three main benefits to global shutter, all of which relate to situations where the scene changes incredibly quickly.

The most obvious of these is flash photography, where the flash itself lights up the scene for only a tiny fraction of a second. Even the fastest mechanical shutter will take around 1/250th of a second to travel across the sensor, so any exposure shorter than this requires the second curtain of the shutter to start closing before the first one has fully opened. This means there’s no point in time where the entire sensor is exposed, so a single flash of light can’t light up the whole image. This is the flash sync speed.

With a global shutter this isn’t a problem: the whole sensor is captured at exactly the same time, so the whole image will be illuminated by the flash, even for the shortest exposure. This means you can use strobes in their most powerful, single flash mode, rather than having to resort to high-speed sync, which tries to pulse or extend the duration of the flash.

In practice, this means a global shutter camera can use the shutter speed to adjust the background brightness of a flash image, even if with short exposures. The flash level and aperture will control the exposure of the foreground, but you can darken everything else in the image, or prevent the background from blowing out using the shutter speed.

Interestingly though, at very short exposures you risk the opposite of the problem progressive shutters have: instead of having to extend the duration of the flash to accommodate a slow shutter, you risk the exposure being too short to capture all of the flash's output.

Banding

Even with the fastest of progressive shutters, there's a risk of banding appearing if you shoot an image with a flickering LED panel in it. This banding is harder-edged and therefore more prominent with a fully electronic shutter, such as the one in the Nikon Z9, shown here. Though, as professional sports shooter Mark Pain highlights: "99% of viewers don’t notice [it] anyway."

Photo: Mark Pain

The second situation where a global shutter comes in handy is another one where you have short pulses of light such as in LED lighting and displays. LED lights often turn on and off very quickly to control their perceived brightness, while LED displays flicker to show different colors or brightnesses, or refresh to display a different image. With a progressive shutter it’s possible to accidentally capture this flickering pattern because each part of the image is capturing a slightly different slice of time. This doesn’t happen with a global shutter because it captures a single instant. There’s still a risk of the exposure changing, shot-to-shot as the LEDs flicker on and off, but you won’t get distracting bands in your images.

With very short exposures there’s a chance that LED display panels will appear the wrong color though. LED displays control which color they appear by making their different colored elements flicker at different rates, so there’s a chance you’ll capture an image at a moment when some of the elements aren’t illuminated, or where you only captured a part of their flicker pattern.

Distortion

Rolling shutter distortion is rarely a problem in stills photography but can result in slightly misshapen objects if they move across the frame quickly enough. It's video where distorted motion tends to be more of a distraction.

Photo: Dale Baskin

The third benefit of a global shutter is that you can’t get distorted images because your subject has moved while the shutter was opening and closing. You’ll still get motion blur if your exposure is too long, but there's no risk of movement occurring during the start and end of the exposure. This is rarely a problem with mechanical shutters: in principle, a global shutter will avoid distortion with even the faster moving subjects such as moving rotor blades or a golf swing, but it’s pretty uncommon to take a photo with a mechanical shutter and see noticeably distorted movement.

It matters much more in video, where cameras typically use electronic shutters that read out progressively. Some of the latest cameras are very fast, but there are plenty of 4K-capable cameras where each frame’s exposure takes more than five times longer to start and end than with a good mechanical shutter. This means there are more types of subject motion or camera movement that generate visible distortion. So it’s video shooters who are most likely to appreciate the elimination of the rolling shutter effect that an instantaneous global shutter brings.