Global shutter cameras are not a new thing. They have been around for a very long time. The Sony Z750 is 2.5 years old and has had a global shutter since day 1. There are also the HDC-3200 and F5500 4K global shutter cameras.
The PMW-F55 had a global shutter and CCD cameras had global shutters.
And now Sony have announced the new A9 MKIII stills camera that also has a global shutter:
And now Sony have announced the new A9 MKIII stills camera that also has a global shutter:
So, given that it’s not really a new thing – why doesn’t every camera have a global shutter?
The main reason is noise – and in particular fixed pattern noise that will show up in blacks and deep shadow areas if you try to lift the shadows or use high levels of gain. With a global shutter the signal from every pixel is globally shifted into a memory cell at the end of each exposure period and then those memory cells are read out while the next frame is being capture. Each memory cell will have a slightly different very tiny signal offset and as the arrangement of the memory cells never changes these offsets get added to the signal and appear in the output as a fixed noise pattern. It can be harder to eliminate this fixed pattern noise in post production compared to random noise and it can look very ugly, not at all like film grain.
In addition the readout can be delayed by up to 1 frame more than a rolling shutter sensor as the readout from the sensor to the image processor must wait until after the frame has been captured and shifted from the pixels to the memory cells. This adds additional latency to the monitoring (not really an issue in a photo camera, but more of a problem in a video camera).
IQ IS A BALANCING ACT.
Image quality is never about one single factor. It is about the balance between noise, readout speed, DR, colour, artefacts. But when one issue, such as fixed pattern noise overwhelms any other benefits it tends to become a problem. The F55 was well know for it’s fixed pattern noise, so a good bright exposure was always desirable to avoid the noise. An under exposed F55 was ugly and generally you would always try to shoot 1 or 2 stops brighter than the cameras base ISO. Early tests of the A9 III appear to indicate that it is a bit noisier that other similar rolling shutter cameras and the limited ISO range suggests that the sensors DR is also a bit more limited – this shouldn’t really be a surprise as noise limits the shadow DR. Plus this is a single ISO camera, no dual ISO goodness with the A9 III.
So, a high end global shutter camera may well be good to have, but are you willing to give up dual ISO, exceptional low light performance or low noise? Given the A9 III sensor appears to have a native ISO of 250, what about needing to use an EI of 250 to get the best performance out of your S-Log3 or raw video camera when everything else can now be rated at 800 without issue? The F55 was 1250 ISO, but you needed to shoot at around 320-640 EI to get an image as clean as we can now get at 800EI with the newer cameras and there was no way you would want to shoot at 4000ISO/EI with an F55 but now we take for granted the ability to shoot at high ISOs without excessive noise.
I have no doubt that the A9 III is a great photo camera and that it’s global shutter can bring some benefits such as eliminating the need for a mechanical shutter and very high speed flash synchronisation. But these benefits are not essential for a video camera. In the future maybe all cameras will have global shutters, but we are not yet at the point where a global shutter doesn’t have any downsides. The extra memory cells, the extra transistors used to control the movement of the tiny signals on the sensor all add a little extra noise. The sensor might run hotter too especially if used for video. Plus the sensor is probably more expensive to make. So, while I think the A9 III is a welcome addition I don’t think it makes our rolling shutter video cameras obsolete. The majority of films shot on film had a small small amount of rolling shutter caused by the sweep of the cameras rotary shutter across the film.
I have no doubt that the A9 III is a great photo camera and that it’s global shutter can bring some benefits such as eliminating the need for a mechanical shutter and very high speed flash synchronisation. But these benefits are not essential for a video camera. In the future maybe all cameras will have global shutters, but we are not yet at the point where a global shutter doesn’t have any downsides. The extra memory cells, the extra transistors used to control the movement of the tiny signals on the sensor all add a little extra noise. The sensor might run hotter too especially if used for video. Plus the sensor is probably more expensive to make. So, while I think the A9 III is a welcome addition I don’t think it makes our rolling shutter video cameras obsolete. The majority of films shot on film had a small small amount of rolling shutter caused by the sweep of the cameras rotary shutter across the film.
