What Is ‘Global Shutter’?

This post is by Ron Dawson from The Online Photographer

The New Sony top-of-the-line “fast” camera, the A9 Mark III or A9III, is here. It went up on B&H this morning ($6,000).

The camera is a technical tour-de-force, despite remaining surprisingly friendly to the hand and shoulder. The big news is that it has a 24-MP ~24x36mm (full-frame) global shutter, the largest global shutter yet made in any camera. A world first for Sony.

So what, exactly, is global shutter?

The familiar focal-plane shutters from half a century ago and more are mechanical shutters. They’re easy enough to understand: a physical curtain blocks the image-forming light and is moved aside temporarily to allow image-forming light to reach the recording substrate. You probably know about many of the types as the form evolved, from rubberized cloth to titanium, eventually solving the high-shutter-speed problem by opening a smaller and smaller slit which traveled across the image; this is what caused distortion like Jacques Lartigue most famously used in his brilliant and beautiful photograph known as “The Racecar” (I saw a modern print of this at Kathleen Ewing Gallery in Washington, D.C. 30 years that remains memorable). It’s also what caused the “flash sync” headache, because electronic flashes were of extremely short duration and could only register on the substrate if the entire shutter was open when it fired. So the flash sync speed was always slower than the highest possible shutter speed on a mechanical camera because it couldn’t make use of the trick with the moving slit.

With CMOS digital sensors, electronic shutter became an option—instead of the sensor being physically blocked, photosites were turned on and off directly. However, the concept of the “moving [virtual] slit” persisted, because it was much easier to design and build a camera that registered a smaller amount of photosites, but to did so in succession. Starting from one side and progressing to the other like a scanner does. It meant the processor had less to do at once. This sometimes resulted in artifacts in the image that you might have seen: not only distortions with panning or camera movement, but banding, waviness, and other still and video effects known as “rolling shutter.” These included wobble (a.k.a. the jello effect), skew, spatial aliasing, and temporal aliasing*. Some of the most pronounced (and bizarre) examples can be found in pictures of airplane propellers and helicopter rotors. These types of EVFs, along with mechanical shutters that use a traveling slit to effect high shutter speeds, are both known as rolling shutters.

A global shutter is simply an electronic shutter that captures the entire image all at once, instantaneously.

The advantages of this might seem obvious, but the details, in the case of the Sony A9III, are like science fiction—at least if you remember that I’ve been watching the digital transition for 35 years now (not with a front-row seat, but I’ve been in the audience). I remember when the word “megapixel” was a new and unfamiliar term; same with the first (futuristic!) 1-GB memory card. Development right along has been fast and furious….


*From the Wikipedia article “Rolling Shutter.”

Original contents copyright 2023 by Michael C. Johnston and/or the bylined author. All Rights Reserved. Links in this post may be to our affiliates; sales through affiliate links may benefit this site. As an Amazon Associate I earn from qualifying purchases. (To see all the comments, click on the “Comments” link below or on the title of this post.)

Featured Comments from:

Joseph L. Kashi: “The immediate question is the top global shutter speed. With some current cameras, it is actually quite slow, on the order of 1/15 second with attendant subject motion blur. If Sony has been able to decrease the global speed to something on the order of 1/250 or so, that would be very useful.”

Mike replies: The official A9III specs, per Sony, are as follows: “SHUTTER SPEED: Still images: 1/80000 [sic] to 30 s, Bulb. […] FLASH SYNC SPEED: 1/80000 s, 1/500 s.” There is a footnote after that “1/80000 s, 1/500 s” which reads, “With compatible Sony external flash.” I have no idea what “1/80000 s, 1/500 s” as a flash sync speed could mean, and apparently Sony isn’t sympathetic to my ignorance. It might mean 1/500th with mechanical shutter and 1/80,000th with electronic shutter, but…further digging required, because, as written, it doesn’t make sense. We’ll sort it out eventually.

David L.: “Sony has been making amazing developments on the camera SOC processor data speeds. The results are seen in very fast frame rates, high resolution video, and now a global shutter. Their ultra high speed electronics have a lot to do with these technological achievements.”


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