6 Q&As on Sony’s global shutter

The Sony Alpha 9 III is causing a huge stir in the world of photography. Why? Because it’s the first full-frame CMOS camera to have a global shutter. So what does that entail? What does it do and what does it mean for the future? Here are the answers to the six most pressing questions.

1. What’s a global shutter?

It’s a process that reads all pixels of the camera sensor simultaneously. The term «shutter» is actually a bit of an outdated term, but it’s important to understand it.

Classic cameras have a mechanical focal plane shutter that opens and allows light to fall on an analogue film or digital sensor. It then closes again after a specified time. How? With the help of two shutter curtains, with the second one following the first. For fast shutter speeds, the focal plane shutter never fully exposes the entire sensor. What happens is that the second shutter curtain begins to cover the sensor again before that happens.

For SLR cameras with an optical viewfinder, the mirror must first be folded out of the way for this process. More recent camera models no longer need to do this, as they have an electronic viewfinder that takes the image directly from the sensor. This has paved the way for a different type of «shutter»: the electronic kind.

With an electronic shutter, the sensor’s permanently exposed. When you press the shutter release, the camera reads out the image at that very moment and saves it. Previous CMOS sensors do this line by line. The camera reads the top row of pixels first and works its way down to the bottom. This takes a certain amount of time, which varies depending on the sensor. Take the Sony Alpha 7R V, for example. It has a back-illuminated sensor and 61 megapixels and takes around 1/30 of a second. The Nikon Z 8, with its stacked sensor and 45 megapixels, needs just 1/270 of a second.

The new Sony Alpha 9 III is the first to do things completely different. The camera fully activates the sensor immediately and reads all pixels simultaneously – a genuine technological breakthrough.

2. What are the pros?

The global shutter eliminates several problems that other shutter systems struggle with. It also opens up new possibilities:

• No distortion: The image content may change in the time classic sensors need for line-by-line image capturing. This poses a problem with fast movements. Tak a golf club in full swing, for example. It travels quite far in just a fraction of a second. While the top line of the sensor’s exposed or being read, the golf club will have moved from where it was in the last line. That’s why it looks warped on the photo. With global shutter, it stays straight.

Can there be distortion with a mechanical shutter?

Yes, theoretically there can. A classic mechanical shutter works at high shutter speeds according to the same principle as the line-by-line reading of the sensor. Which means, the light doesn’t enter everywhere at the same time, but is only let in through the shutter, which moves from top to bottom. However, because a mechanical shutter only takes around 1/400 of a second to do this, the distortion is minimal and only visible in extreme cases. Only the expensive mechanical central shutter doesn’t have this issue. And the global shutter is its electronic equivalent

• No banding: LED light sources cause similar problems for conventional sensors as fast movements. The light they emit isn’t constant, but flickers at a high frequency. In conventional shutter systems, the long readout time leads to «banding», which results in irregular exposure within one image and looks like bands. The global shutter gets rid of this problem. The exposure between different images can continue to vary at short shutter speeds.

• Higher continuous shooting speed: Because the readout speed limit has been eliminated with the global shutter, extremely high continuous shooting speeds are possible. The bottleneck is now the processor, which has to process the images. The Sony Alpha 9 III can take 120 pictures per second in RAW format. That’s a quantum leap compared to previous cameras.
• Improved flash synchronisation: With a mechanical shutter, you can usually only synchronise a flash up to 1/200 second. With shorter exposure times, the entire sensor is never exposed, only a very small part of it. The flash has no chance to expose the entire image. With regular electronic shutters, the opposite is the case. The flash firing time is shorter than the readout speed of the sensor. This, again, translates to the image being unevenly exposed by the flash. These issues are gone with the global shutter. In theory, a flash can be synchronised up to 1/80 000 second with the Sony Alpha 9 III.

3. Who’s it for?

The advantages of the global shutter are particularly noticeable in three cases:

• Sports photography: The high continuous shooting speed is ideal for capturing that perfect moment. Even the fastest movements no longer distort.
• Video: The rolling shutter effect is also completely eliminated in videos. Camera pans produce vertical lines that remain dead straight.
• Flash on location: If you want to compete with ambient light using flash, you’ll benefit greatly from the high synchronisation times that are possible. If you increase your shutter speed, for example from 1/200 to 1/1000 second, the ambient light exposes the image 80 per cent less. The strength of the flash, however, doesn’t change (I’ll come to the limitations later). This allows you to darken the background with less powerful flash units. And you can use open apertures in sunlight without the need for lossy high-speed flash modes.

Apart from that, the global shutter will make a mechanical shutter superfluous in the future. This eliminates a complex component. Cameras could be designed to be more compact – and more affordable. However, the price reduction’s only likely to happen in the long term. Like any new technology, the global shutter will remain expensive for some years to come.

Despite its advantages, the Sony Alpha 9 III is the only camera of its kind that’s currently only of interest to a very small target group. If you don’t take pictures of fast sports, you’ve probably never had a problem with distorted images. Not even animal photography benefits greatly from the global shutter. The rolling shutter effect regularly occurs when filming with regular sensors, but these are already well under control with new cameras. And if you tend to use your flash in the studio and not outdoors, you’ll hardly ever need fast shutter speeds anyway.

4. Can I really synchronise my flash with any shutter speed?

Yes, but at under 1/1000 of a second you won’t get much out of it. Sony advertises synchronisation times of up to 1/80,000 second for the Alpha 9 III. Although this is possible, it’s also pointless. Why? Because at very fast shutter speeds, the flash unit becomes a bottleneck. A flash tube needs a certain amount of time to emit its light. This is referred to as burn-off time. If the global shutter exposes the sensor for less long than this burn-off time, it no longer picks up part of the flash light. As with continuous light.

Here’s a little deep dive if you want to know exactly how long typical burn-off times are and what the specifications mean. If you don’t care, just scroll down to the next point.

The light intensity of a flash tube isn’t linear, but a curve that rises quickly and falls slowly. The stronger you set the flash to, the longer the burn-off time. It’s specified as T.5 or T.1. T.5 indicates the time that the flash shines at more than 50 per cent intensity. T.1 indicates the time it shines for at more than 10 per cent intensity.

T.1 is the relevant specification to estimate the shutter speed at which a global shutter no longer captures all the light. T.5 is misleading, because the flash has usually only emitted 60 per cent of its light at this point. Unfortunately, many manufacturers only specify T.5 because it looks better. Now that you’re armed with this knowledge, here are a few examples:

As you can see, the T.1 burn-off time of most flashes at full power is a good 1/300 of a second. Even if you accept a light loss of around 40 per cent, you’ll hardly achieve more than 1/1000 of a second. Faster shutter speeds only make sense if you don’t set the flash to full power – but then you have no additional advantage in the fight against the sun.

5. What are the cons?

A sensor with a global shutter requires an extremely large number of circuits in a very small space. After all, the signals from all pixels must be transmitted simultaneously. This is only possible in a stacked sensor, as the photodiodes and circuits aren’t arranged on the same level, but one behind the other. A conventional sensor probably wouldn’t have enough space for this.

Not to mention that it’s an expensive construction. This was already the case with regular stacked sensors. With an additional global shutter system, they’re even more difficult to manufacture. That explains why it took Sony so long to develop it. What’s more, the complexity of it could have a negative effect on image quality. Namely:

• less dynamic range
• more image noise
• lower resolution

Sony claims the global shutter in the Alpha 9 III doesn’t lead to poorer image quality. But this will only be verifiable with the final cameras and RAW images. In the first hands-on test with a pre-production model, I didn’t notice anything negative in terms of image noise or dynamic range.

The resolution of the Sony Alpha 9 III is a solid 24 megapixels – which is probably the limit of current production technology. More pixels would mean even more circuits.

6. What’s next for global shutter?

The next camera that would be suitable is the Canon EOS R1. It’s also a sports photography flagship. According to rumours, it will be launched in early 2024. Apart from Sony, Canon is the only manufacturer that builds its own sensors.

However, it’s impossible to guess how quickly Sony will share its technology with other brands. The group supplies sensors to almost the entire industry: Nikon, Fujifilm, Hasselblad, Olympus, Apple. Sooner or later, the global shutter is likely to make its way into other high-end cameras. However, it’ll probably take longer before it’ll be featured in mid-range cameras. Even regular stacked sensors are still reserved for the flagships. The only exceptions so far are the OM System OM-1 and the Fujifilm X-H2S. Both in smaller formats.

Would a global shutter in smartphones also make sense? In theory, yes. After all, they also read their sensors line by line. But the sensors are tiny – no larger than 1 inch (which in reality means only a 0.63-inch diagonal). -Therefore, the readout speed is very high. The iPhone 15 Pro, for example, manages 1/200 of a second.

In practice, the rolling shutter effect is therefore only a problem in extreme situations. Similar to the stacked sensors of the Sony Alpha 1 (1/240 s) or the Nikon Z 8 (1/270 s). Unlike larger cameras, a smartphone will hardly ever be in a situation where distortion becomes visible. Usually, it wont’ have to synchronise with flashes either.