This is not strictly related only to digital cameras. Older film based cameras often feature a focal plane curtain shutter, which "rolls" across the film plane and exposes the film horizontally or vertically. A very famous example of this is the racing car picture taken in 1913 by Jacques Henri Lartigue using a 4x5 Speed Graphic camera.
The shutter moves relatively slowly on this camera, when compared with modern SLRs, which gives the distortion of the racing car. The distortion is especially visible in the wheels, which appear to be leaning forward. This effect was later copied by cartoonists when they wanted to give the impression of speed.
Two years ago, I video recorded a spinning wheel to evaluate the rolling shutter artefacts of the GH2 compared with the GH1, and found that they were comparable.
Now, I have done the same experiment again with the Panasonic GH2 and GH3.
While this spinning wheel test might not be very realistic for what you are going to be video recording yourself, it does serve to compare the rolling shutter artefacts generated by the two cameras. Here is the test and the results:
Both videos were recorded at the same settings: 1080p@25fps, f/2, 1/500s, ISO 1600. I used the Olympus 50mm f/2 1:2 macro lens.
And here are two pairs of still images from the two video streams:
Based on this test, it is clear to me that the GH3 improves upon the GH2 in terms of rolling shutter artefacts. I can see this because the rotating propeller is less distorted in the GH3 video, even though it rotates at the same speed. If the camera had a perfect global shutter, the blades would be perfectly straight, albeit still a bit blurred due to the rotation.
The improvement is significant enough to show up in this test, and hence, it will probably also mean an improvement in real life video use. Of course, both cameras still use a rolling shutter when video recording, but the speed of the sensor readout is faster with the GH3.
I have not yet tested the Panasonic GH3 a lot, but I expect that this is just one out of several areas where the GH3 improves upon the GH2.
Still images, mechanical shutter
The mechanical shutter found in both cameras, for still images, is also a rolling shutter type. However, it moves much quicker than the electronic readout, and hence, rolling shutter artefacts are very rarely seen with still images.
The flash sync speed is the same on both cameras, 1/160s, hence I would expect that their mechanical shutters travel at the same speed.
This is a still image extracted from a video stream recorded with the Panasonic GH2:
As you can see, the blades are bent due to the rolling shutter effect. When using the mechanical shutter for still images, though, there is virtually no rolling shutter effect:
Both the GH2 and GH3 feature electronic shutters for still images. These can be enabled at the user's control. With the GH2, though, you only get 4MP output images with the electronic shutter, so the feature is not that useful. The GH3, on the other hand, gives you a full 16MP still image output.
In addition to recording the videos, I also took some pictures using the electronic shutter with both cameras. Here are the comparisons. Note that the GH3 image was scaled down relatively more than the GH2 image, since the GH3 image was much larger initially. The lower image was taken with the mechanical shutter:
I think these results are very interesting. Previously, people have speculated that the GH2 electronic shutter images were restricted to 4MP since Panasonic wanted to clearly distinguish between the mechanical shutter images with no rolling shutter artefacts, and the electronic shutter image prone to rolling shutter. However, I think this study shows that the reason is that the GH2 uses the video output to make the electronic shutter images. And the video sensor output is restricted to some smaller subsample of the image, otherwise it would require too heavy processing.
On the other hand, the GH3 has a completely different sensor readout for electronic shutter, as compared with the video readout. While the video output has been improved over the GH2, the full image readout takes more time, and gives much more artefacts.
The lower image, taken with the mechanical shutter, still shows some very marginal rolling shutter artefacts, however, they are barely noticeable.
Using the electronic shutter mode of the GH3, it is again possible to take images like the one I opened the article with:
This picture was taken at f/2.8, 1/800s, ISO 800. Setting a faster shutter speed would not have helped, as the problem here is not the shutter speed, but rather the speed of the sequential sensor readout, which stays constant.
If the car was driving from right to left in the example, it would have been leaning forwards, rather than backwards.
The conclusion appears to be that the GH3 has less rolling shutter artefacts during videos, which is certainly good. On the other hand, it has much more rolling shutter artefacts with the electronic shutter. But then again, the GH2 had no high resolution electronic shutter at all, so this is still a step in the right direction.