It looks like the Sony A7III mirrorless camera looks to have a slight problem when shooting 4K in full frame. According to testing done by a A7III shooters on YouTube, when shooting 4K in full frame at 24 fps, the A7III picks up a slight stretch of the image along the x axis.
The Sony A7 III has a very odd problem when shooting 4K and is squeezing/stretching your video! When you shoot 4K, 24fps in full frame mode on the A7 III, your video will be stretched vertically! – Caleb Pike, DSLR Video Shooter
The slight flutter was picked up by DSLR Video Shooter Caleb Pike, who was doing a comparison test of the Sony A7III in both full frame and Super 35mm modes. The Sony A7III has a 6K image sensor which then scales down the image to 4K when shooting in full frame, so you get the best possible image.
Consequently, Pike would then crop down the full frame image to fit the aspect ratio of the Super 35mm frame, in order to overlay them, and discovered was that the full frame 4K image, shot at 24 frames per second, slightly stretches the frame, making it difficult to overlay the images accurately.
Pike probably wouldn’t have picked up on it, had he not been using a complicated test chart which is designed to line up cropped camera modes for comparison. Not sure he was actually seeing what he was seeing, Pike duplicated his test and got the same results, and then added 1080HD to see if the stretch would be more pronounced. It was.
Then he contacted a few Sony A7III shooters and asked them to perform the same comparison test. The results were the same. Shooting in full frame 4K at 24 fps, the 6K image sensor of the A7III then scales down the final image to 4K with a very slight stretch along the X Axis. Pike suspects it’s just an error in the math that the A7III algorithm uses to scale down to 4K, and should be easily fixed in a future firmware update.
Meanwhile, Pike said there are a few things that A7III shooters can do. First, you can ignore it. Pike admits that most people won’t pick up the stretch because it’s very slight. But if you need to switch from an APS-C camera to a full frame camera, viewers may pick up that something is amiss. To deal with it, Pike recommends taking the X Axis of the image and increasing it by 1% solves the issue. Moreover, the other option is to shoot in a different frame rate, like 30fps. The issue doesn’t present itself in any other mode, but then you don’t have the best possible image shooting in full frame 4K at 24p.
You can bet that Sony will do some quick recalculating in their next firmware update and solve the problem. But the good news is, that the issue is hard to notice and easy to fix in post.