My last post considered extracting stills from 4K video. A caveat I didn’t mention for many cameras─aside from the small file size─is rolling shutter.

Background: To cope with the demands of video, cameras use an electronic, rather than mechanical, shutter. Shooting 4K, the electronic shutters produces frame rates of 24 or 30 fps, faster than the fastest mechanical-shutter from high-end dSLR’s. Electronic shutters are silent, as well, so in situations like a close-up of a birds nest, you’re disturbance is minimized.

Most electronic shutters scan the image data by row from top to bottom, and therein lies an Achilles heel. Even if your shutter speed is high, the scan rate is slow. This leads to weird blur effects on moving subjects like the ones shown below.

1080p video is less prone to rolling shutter because the demands are less. You’re typically scanning half or less lines. My Sony a6300 has pronounced rolling shutter with 4K, but I rarely see it at 1080. At 4K the a6300 scan takes about 34ms (1/30 sec.). For 1080 it is less than half that.

Below are subject movements exemplifying the horrors of stills extracted from 4K.

Rolling shutter with fast movement: weird blurring is caused by the rapid wing downbeat of this Red-breasted Sapsucker and the slow 4K top to bottom scan (1/30 sec) of the Sony a6300.

Hairy Woodpecker leaving nest. We can see the ghost of both wings (one very faint) at the top of the scan, and then one wing matching the scan rate so it blurs from top to bottom, while the other only gets picked up again at the bottom of the stroke. Bizarre.

Here we see the top of the wings flared for landing, but farther down the scan the wings have already contracted.

At first I liked this eye-almost-sharp image of a female Hairy Woodpecker leaving the nest. But the wings are distorted, moving forward with the scan, and even the body seems a little bent.

This image looks pretty normal because the wings aren’t moving much and the scan went quickly through the bird.

Keep on Truckin’: I laughed when I saw this, looks like dad’s been emaciated from all the work. But again, it’s the top-down scan, here stretching out his body, distorting the angle of his approach.

Bottom line: If you’ve noticed that your 4K video bends vertical lines or behaves like jelly with camera movement, or, as shown above, if your stills extractions look strange or bizarre, your camera has a slow readout speed, so beware.

Gary