As I wrap up my writing about the annual Hollywood Professional Association’s Tech Retreat, I’d like to touch on new technology that I hinted at last time. It deals with motion in cinema capture.
The motion I refer to comes from either moving the camera—panning, tilting, dollying and handholding—or from objects that move within the scene. Because of the typical 24-frames-per-second frame rate used in cinema, the capture of the movement can be tricky.
The problem is referred to as motion judder. If you move the camera too quickly, the image appears to have what’s often called stutter or strobing. The same artifact may also occur when the camera doesn’t move but something in the scene—a vehicle, for instance—moves too fast.
What’s too fast? The picture at the top of this article shows a page from my copy of the ASC’s American Cinematography Manual. There are several charts in this book that address this specific problem. The image above shows a chart listing panning speeds for 35mm cameras at specific focal lengths. Using the data in these charts, a camera operator can determine how fast they can pan a camera with a lens of a given focal length and not get motion judder.
I bet you can guess how many people pull out their ASC manual when setting up a shot! And even if they did, the director would probably tell them that it doesn’t matter what the chart says, “I need the camera to move this way!” or “I need that car at that speed!”
In fact, most of the time people shoot with a 180-degree shutter (in photographic terms: a shutter speed twice the frame rate). This leads to motion blur. And motion blur helps minimize the appearance of motion judder.
However, as we move to HDR the problems of motion judder are more obvious. High contrast images really magnify judder artifacts. During the breakfast roundtables at the HPA Tech Retreat, post professionals described some scenes as almost unwatchable in HDR because of motion judder.
There’s software that can help. The software is operated by someone that might be called a motion grader. Next time, how this new position can help with motion judder.