Pick the Right Video Frame Rate

Pick the Right Video Frame Rate

One of my readers wrote in recently asking how to decide what video frame rate to shoot. So, in the interests of contributing to the discussion, here are my thoughts.

BASIC THEORY

Here’s my take: You pick the video frame rate that yields the image “sharpness” that you want.

By this, I mean that the slower the frame rate, the greater the motion blur on all moving objects. The greater the motion blur, the harder it is to see the edges of things.

A TRIP BACK IN TIME

Projected film first appeared between 1877 and 1880. And, in the earliest days of recording, film was shot on cameras that were hand-cranked. In fact, the mark of a good camera operator was not their framing, but how steadily they could turn the crank to keep the recording speed of the film constant.

Film was new enough in those days that anything that moved would draw a crowd. But, while hand-cranking film cameras was fine for comedy, because the speed changes enhanced the comedic effect; hand-cranking cameras played hob with dramatic scenes.

Gradually, by 1920, producers and studios replaced hand-cranked cameras with motor-driven cameras. However, there was no standard for frame rate. While it was generally agreed that a frame rate of 16-18 frames per second was necessary to give the illusion of movement, industry practice varied frame rates between 12 and 26 frames per second. And there was no guarantee that the projectionist would display the film at the frame rate at which it was shot.

Producers would often select slow speed frame rates to save on film costs, while theaters would often project films at faster frame rates so they could fit more movies into an evening (i.e. sell more tickets). Since there was no audio synced to picture, no one really knew what speed anything needed to be shot or played at.

This situation changed instantly with the advent of The Jazz Singer, arguably the first “talking” picture. Suddenly, the sound track was on the same strip of film as the picture. Any changes in film speed would be immediately noticed in the audio.

The industry huddled to answer the question: “What is the slowest possible frame rate we can use (to reduce film stock costs) that yields the highest quality audio?” And the answer became 24 frames per second.

Not necessarily because it looks “filmic,” but because it provided the best balance between limiting film stock expenses and providing acceptable audio.

NOTE: A sidebar to the frame rate issue is that in order to minimize frame flicker from projecting 24 fps film, which is a strobing effect seen by the audience when frame rates get too slow, the actual projected image needs to be quite dark. As total projected illumination increases, so does the perception of frame flicker. This is one reason movie theaters are so dark.

FAST FORWARD TO TODAY

There is a lot of debate on the new “high-frame rate” method of shooting film at 48 frames per second. This first rose to the level of public consciousness with the release of “The Hobbit.”

Long advocated by Roger Ebert, and others, the benefit to faster frame rates is that images contain much less motion blur, which results in much crisper images. These higher frame rate images can also be projected using much brighter lamps, without displaying frame flicker.

UPDATE

My background is video, not film. Some characteristics of film, like shutter angle and shutter speed, I have not completely figured out. So, I’m happy to add the following comments supplied by Philip Hodgetts.

[Larry, let me add]: the higher frame rate the higher the shutter speed by necessity, which is why the image is sharper. (In film this would be called “shutter angle.”) Some film and video has a super sharp (too much imnsho) image where the shutter speed is way faster than the “normal” shutter speed for that frame rate. Each frame rate has a default shutter speed, roughly [half] the frame duration. At 24 fps the frame duration is 1/24th of a second, so the default shutter speed is 1/48th second. (This gives the default looks that Larry talks about in the article.) At 24 fps and 1/200th or even higher shutter speed the image will be super sharp, but the motion will be much less smoother. To me it’s a very disturbing look, but it’s a creative decision if you want to over-ride that.)

Another interesting addition is that film is rarely projected at 24 fps. In perfect conditions flicker fades from perception at about 18fps, which is the speed (on average) of most silent films.  24 fps is the minimal acceptable speed for sound-on-film. Even in a very dark theater, with that big screen, 24 fps projection is going to flicker, which is why each frame is projected twice (or even 3 times) for an effective “flicker” rate of 48 or 72 flickers-per-second, [making it] much less perceptible.

Thanks, Philip. Always pleased to learn new stuff.

HOW TO CHOOSE

For me, the choice comes down to motion and effects. Higher frame rates will yield cleaner motion with the ability to create better effects when chroma-keying or rotoscoping because edges are sharper and contain much less motion blur. (It is far easier to add motion blur in post production, than to remove it.)

Everything I shoot is at 60 frames per second.

For projects going directly to the web, the frame rate doesn’t make any difference, YouTube and the rest can easily handle whatever frame rate you shoot.

When going to cable or broadcast, 60 frames per second in North America, and 50 frames per second in the rest of the world, are standard.

For theatrical release, your choices are more constrained. Film output still needs to be 24 fps. However, for digital projection, you have, essentially, the same frame rate options as you do for the web. With the shift from film to digital, the old constraints of 24 fps (and its cousin 23.976 fps) become less and less important.

As with all projects, always verify what your deliverables need to be before you start shooting. It is FAR easier to shoot at the correct frame rate than it is to convert to it after all shooting is complete.

 

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