Why Motion Pictures Still Can't Move
Why Motion Pictures Still Can't Move
(Editor’s Note—Much has been written lately about high film frame rate—with several famous filmmakers exploring this new frontier, including: Peter Jackson, James Cameron and Douglas Trumbull.)
By Dale Johnson, Technology Editor
“Huh?” “Whaaaa?” “What are you talking about?” you may say. “I watch moving pictures every day on TV, at the cinema, and even on my phone.”
As most of us know, what we see as moving pictures is actually a series of still photographs...24 of them each second in the case of a theatrical film, and 30 of them each second when we’re watching images on television.
It’s only possible to perceive these series of still photographs as moving, because of a phenomenon of the human brain called ‘persistence of vision.’
When an image impinges the retina in the human eye the visual data is then sent to the brain. But once that image has disappeared, the brain will continue to sense the image for about a fifth of a second. This means that when images are changed in rapid succession the eye/brain combination can not detect the interruption between those images.
If the information within each image is quite similar, with only subtle changes from one image to the other, this is perceived as—or interpreted as—motion by the brain.
Edward Muybridge was one of the early experimenters with this characteristic of the mind to see motion where none exists. He was retained by a former governor of California to settle a dispute as to whether all four feet of a galloping horse were off the ground at the same time.
He set up a series of cameras along the edge of a race track so that the shutter of each camera would be triggered by a thread strung across the track as the horse passed by. Muybridge used 12 cameras, and those resulting 12 images seen in succession produced the illusion of motion.
This was all done to prove a bet about a running horses’ position when in motion, but it set off, or at least bolstered, a burgeoning interest in propelling the fairly new invention of photography to the next level: pictures in motion.
As development moved toward machines that would use a long strip of film to record a series of pictures, it was determined that Muybridge’s 12 images in one second did indeed provide the sense of movement. But there was an annoying awareness of flicker at that frame rate that was a distraction to viewers.
Eventually, experimenters in this new field of motion photography settled on 16 images per second as the minimum number to reduce the ‘flicker’ to an unnoticeable level. This worked well for the era of ‘silents’—the era of motion pictures without sound.
When sound on film arrived in 1926, it was soon determined that 16 frames per second was too slow for adequate audio play-back quality. So additional experimenting settled on 24 fps—90 feet per minute with 35mm film—to render decent quality sound.
That frame rate also reduced the noticeable flicker even more, so that it is imperceptible to virtually every human, even though there are now experiments with 48 fps* and 60 fps going back to the 1960’s with Doug Trumbull’s ‘Showscan.’ Mr. Trumbull is now experimenting with shooting video at 120 fps and then reducing that back to 60 fps and then to 24 fps. The final 24 fps scene may have a mixture of imagery being shown in different frame rates within it, achieved digitally.
The idea here is to capture even the most rapid movement with the same sharpness as in static portions of the frame by using this high frame rate. Once that’s accomplished, then the playback rate is reduced to 24 fps again.
This is ironic since animated films are produced with every frame perfectly sharp. Motion blurring is then sometimes applied to certain scenes with fast movement to induce a heightened sense of realism.
In television and other forms of video, 30 fps is the proper frame rate. That came about because of the electrical current we use: 60 hertz (cycles per second) alternating current.
In the early days of television, it was found that if a picture...a frame of film...was scanned by a cathode ray tube it would form an image on the face of the tube. This is what formed the TV screen.
But if that picture was scanned by the tube 60 times in one second, by the time the scan line reached the bottom of the image, the top of the image was already fading.
The solution was to scan every other line, the odd numbered lines, from the top to bottom, then go back and fill in the even numbered lines from top to bottom.
This produced a new element...a field. A field is the set of odd numbered scan lines of an image, or the even numbered scan lines. There are two fields to make a completed image.
Since television operates on this need for 30 frames per second, and motion pictures (both theatrical and documentary) were recorded at 24 fps, a different problem had to be resolved in order to display films in a TV medium.
The solution for this was what is called “3-2 pulldown.” Twenty-four pictures in each second needed to be changed to 30 pictures in each second.
There are a couple of schemes for doing this. Remember that there are two fields now for each of the 24 pictures in a second of film. Every third frame will have one of it’s fields repeated and interlaced with one field of the next frame to produce a new frame of picture. This happens six times per second, every third frame,resulting in a set of 30 pictures in a second, which can now be televised at 30 fps.
There is now some controversy among small producers and documentary producers as to whether it’s better to acquire your footage at 24 fps or 30 fps. Some of the thinking is that 24 fps is more ‘movie’ or more ‘cinematic,’ meaning more ‘Hollywood,’ I guess.
The real difference is that 24 fps is acquired with no fields—every frame is a complete picture with no interlacing. There’s no scanning of alternate lines in the frame.
The 30 fps is usually acquired with interlacing—although not always the case. Often this is designated as 60i, which means 60 fields to produce 30 completed images in each second.
I have always used 30 fps (actually 29.97 fps) because of another complexity we won’t go into here. Thirty fps has always worked well for both projection onto a theater screen and for DVDs intended to be displayed on a TV screen.
I just don’t see much advantage to the slower frame rate, even though there are some ardent and vociferous proponents for it. But in the light of new experiments in higher frame rates by Hollywood itself, I’m even more hesitant to consider 24 fps.
*Peter Jackson’s new “Hobbit.”