Camera
What is global shutter: 3 ways it can change photography
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.
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
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| 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
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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
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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.
Camera
Pentax K-1 and K-1 II firmware updates include astrophotography features (depending on where you live)
Yesterday, Ricoh quietly released firmware 2.50 for its Pentax K-1 and K-1 II DSLRs. However, the features you can expect to gain from this update may depend on your geography.
Ricoh’s English-language firmware pages for the K-1 and K-1 II state that firmware 2.50 delivers “Improved stability for general performance.”
However, astute Pentax users noted that Ricoh’s Japanese-language firmware pages (translation) indicate that the update also includes a limited feature called “Astronomical Photo Assist,” a collection of three new features designed for astrophotography: Star AF, remote control focus fine adjustment, and astronomical image processing.
Star AF is intended to automate focusing on stars when using autofocus lenses. Rather than manually focusing on a bright star and changing your composition, it promises to let you compose your shot and let the camera focus.
Remote control fine adjustment allows users to adjust focus without touching the lens and requires Pentax’s optional O-RC1 remote. Astronomical image processing will enable users to make in-camera adjustments to astrophotography images, including shading correction, fogging correction, background darkness, star brightness, celestial clarity, and fringe correction.
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| Astronomical image processing on the K-1 and K-1 II will enable users to make in-camera adjustments to astrophotography images, including shading correction, fogging correction, background darkness, star brightness, celestial clarity, and fringe correction. |
According to Ricoh, Astronomical Photo Assist is a premium feature that must be purchased and costs ￥11,000 for an activation key (about $70 at current exchange rates).
Although these astrophotography features appear to be Japan-only for now, a Ricoh representative tells us, “Ricoh Imaging Americas confirmed that the premium firmware features for the PENTAX K-1 and PENTAX K-1 Mark II will eventually be available to US customers.”
Firmware update 2.50 for both the K-1 and K-1 II is available for download from Ricoh’s website.
As part of our twenty fifth anniversary, we’re looking back at some of the most significant cameras launched and reviewed during that period. Today’s pick was launched seven years ago today* and yet we’re only quite recently stepping out of its shadow.
The Nikon D850 is likely to be remembered as the high watermark of DSLR technology. We may yet still see impressive developments from Ricoh in the future (we’d love to see a significantly upgraded Pentax K-1 III), but the D850 was perhaps the green flash as the sun set on the DSLR as the dominant technology in the market.
Click here to read our Nikon D850 review
Why do we think it was such a big deal? Because it got just about everything right. Its 45MP sensor brought dual conversion gain to high pixel count sensors, meaning excellent dynamic range at base ISO and lower noise at high ISOs. Its autofocus system was one of the best we’ve ever seen on a DSLR: easy to use and highly dependable, with a good level of coverage. And then there was a body and user interface honed by years of iterative refinement, that made it easy to get the most out of the camera.
None of this is meant as a slight towards the other late-period DSLRs but the likes of Canon’s EOS 5DS and 5DSR didn’t present quite such a complete package of AF tracking, daylight DR and low-light quality as the Nikon did. With its ability to shoot at up to 9fps (if you used the optional battery grip), the D850 started to chip away at the idea that high megapixel cameras were specialized landscape and studio tools that would struggle with movement or less-than-perfect lighting. And that’s without even considering its 4K video capabilities.
In the seven years since the D850 was launched, mirrorless cameras have eclipsed most areas in which DSLRs once held the advantage. For example, the Z8 can shoot faster, autofocus more with more accuracy and precision, across a wider area of the frame and do so while shooting at much faster rates.
But, even though it outshines the D850 in most regards, the Z8 is still based around what we believe is a (significant) evolution of the same sensor, and its reputation still looms large enough for Nikon to explicitly market the Z8 as its “true successor.”
Nikon D850 sample gallery
*Actually seven years ago yesterday: we had to delay this article for a day to focus on the publishing the Z6III studio scene: the latest cameras taking precedence over our anniversary content.
| Photo: Richard Butler |
We’ve just received a production Nikon Z6III and took it into our studio immediately to get a sense for how the sensor really performs.
Dynamic range tests have already been conducted, but these only give a limited insight into the image quality as a whole. As expected, our Exposure Latitude test – which mimics the effect of reducing exposure to capture a bright sunrise or sunset, then making use of the deep shadows – shows a difference if you use the very deepest shadows, just as the numerical DR tests imply.
Likewise, our ISO Invariance test shows there’s more of a benefit to be had from applying more amplification by raising the ISO setting to overcome the read noise, than there was in the Z6 II. This means there’s a bigger improvement when you move up to the higher gain step of the dual conversion gain sensor but, as with the Z6 II, little more to be gained beyond that.
These are pushing at the extreme of the sensor’s performance though. For most everyday photography, you don’t use the deepest shadows of the Raw files, so differences in read noise between sensors don’t play much of a role. In most of the tones of an image, sensor size plays a huge role, along with any (pretty rare) differences in light capturing efficiency.
As expected, the standard exposures look identical to those of the Z6 II. There are similar (or better) levels of detail at low ISO, in both JPEG and Raw. At higher ISO, the Z6III still looks essentially the same as the Z6II. Its fractionally higher level of read noise finally comes back to have an impact at very, very high ISO settings.
Overall, then, there is a read noise price to be paid for the camera’s faster sensor, in a way that slightly blunts the ultimate flexibility of the Raw files at low ISO and that results in fractionally more noise at ultra-high ISOs. But we suspect most people will more than happily pay this small price in return for a big boost in performance.
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