Frame Rate

Dealing with Frame Rate really makes you feel the reality of the motion picture medium you’re working in. A changing image is what makes film, movies, TV and animation all time-based experiences. It’s still a surprise to some people that movies are not moving images, but in fact a series of stills shown in rapid succession.

History

There is a threshold at which still images flashing one after another appear to be continuous motion. It depends on persistence of vision, beta movement, and the other optical and neural processes (explained briefly in the video.) It turns out to be around 15 frames per second.

Early films were silent, and were often shot with hand-cranked cameras at 16 to 18 frames per second. Projectionists matched those frame rates only approximately. Their priority may have been to make sure the film didn’t catch fire in the projector, rather than to reproduce the original motion accurately.

When filmmakers added synchronized sound to film in the late 1920s, they were forced to lock down a consistent frame rate so that the audio ran smoothly. They settled on 24 frames per second.

Most projectors are multi-bladed, and project each frame two or three times before advancing to the next frame, making flicker less noticeable. For 24fps film, this makes the film flicker at 48 or 72 times per second.

Video historically has run at 30fps (NTSC) in North America and 25fps (PAL) in Europe. Those correspond to the cycle frequencies of A/C power (60Hz in N.A., 50Hz in Europe.) Most lights, including some used in filmmaking, flicker with the alternating current. When video is captured exactly in sync with the current fluctuation, illumination appears constant. (NTSC video is actually 29.97fps for signal broadcasting reasons.)

Interlacing

I don’t like interlacing, but I’ll mention a couple of quick notes about it.

Interlacing is a video technology. It means that images are captured in two fields: one field is every other line of the image, the second field is the remaining alternating lines. It looks like the image below.

It was a cheat to reduce flicker in early TV signals while keeping the bandwidth relatively low. The advantage is that at 30fps, there are actually 60 interlaced fields. That picks up some of the motion that would otherwise be missed in between full frames, so the video flows more smoothly. It’s especially noticeable with moving objects and fast camera pans.

But today we have the ability to shoot 60 frames progressive (full frames, not fields) if we want smooth video. My beef with interlacing is that it often causes technical glitches in post production when we convert between different resolutions and standards of film, video, broadcast, the web and mobile devices. Film, computers, mobile platforms, gaming systems, streaming, and web video are usually all progressive. Interlacing is clearly on its way out. Finally.

The Future of Frame Rate

There are many reasons to get video (forget film for a second) up to higher frame rates. Fast movement and some camera moves cause flicker when shot and viewed at 30fps or lower. And screen sizes are changing. When Optimus Prime flies across our new big-screen TVs, his position in our field of view changes a lot from frame to frame. That makes it harder for our brain to link the individual frames, and he might look flickery. Until recently, TVs couldn’t do the multi-blade projector trick that film theaters can do.

Now though, TVs can interpolate video and break it into more frames per second to artificially create sharper, smoother motion. Look for the 120Hz or 240Hz labels in the store.

Digital video projectors in theaters are moving to 48fps and 60fps projection. Movies are now being shot, edited, and delivered at these faster frame rates. A new movie standard may emerge in the next few years to catch up.

ESPN has been broadcasting sports at 60fps for years, and the action is noticeably sharper and easier to watch at that rate. Cable, Satellite, HDTV transmission, and video streaming standards will probably eventually reach 60fps.

Like the renewed surge in 3D movies, some of these ideas work and some are a disaster. Personally, I like 24p. The little bit of motion blur makes it seem warmer and more natural to me. But maybe I’m just nostalgic for something familiar.

The Larger World of Frame Rate

But remember, frame rate is not just about competing video standards and reducing flicker. Manipulating frame rate is the ticket to slow motion, time lapse, and other effects that are becoming standard tools for all filmmakers. Anyone with a still camera and a tripod can shoot time lapse movies, and with plugins like Twixtor and Timewarp, carefully shot video footage can be slowed down considerably in post. Have fun exploring. And thanks for tuning in.