The frame rate research that The Pickfair Institute conducts centers around manipulating three major frame rate artifacts: flicker fusion, motion fusion and motion blur.
Flicker fusion is the speed at at which intermittent light becomes completely steady to the human observer. Film cannot be projected continuously; the light must be blocked for a period in order to hide the transition between frames. The flashing intermittent light from film projection is why the movies have been dubbed “the flicks.” In a 24 fps film the audience is actually seeing 48 flashes of light per second. As you can see in the clip below, rotating shutters are added in order to interrupt the image beam. One shutter masks the switch between frames and an additional shutter is added simply to increase the “flicker” frequency from 24 Hz to 48 Hz.
Even in the days of silent 16 fps movies the flicker rate was 48 Hz, because they inserted a third blade into the projector shutter, thus yielding the same 48 flashes per second.
48 Hz flicker is desired in order to achieve flicker fusion. A faster flash rate allows intermittent light to appear completely steady. At 24 Hz the changing of frames could be perceived as a juddery strobe, but at 48 Hz the light flashes fast enough for the flicker to be imperceptible to the untrained eye. Of course, we all know there’s no film moving through a digital projector so there’s no requirement for any flicker at all, either from the chip itself, or from the images it projects. The question for us is; does the human brain desire to have these interruptions?
Motion fusion is the point at which individual frames in succession become apparent movement. The important factors which govern motion fusion are speed of the subject and the size of the camera shutter. Slow subjects and larger angle camera shutters yield smooth movement, while fast subjects and smaller camera shutters yield judder and strobe artifacts.
A conventional film camera cannot capture light continuously; camera shutters are added to film cameras in order to block the light when switching between frames. This is not to be confused with the projector shutters discussed above. Camera shuttering blocks the light being recorded onto film, not the light being projected into a theater. Standard 24fps films have a 180 degree camera shutter, which means that light entering the lens is only being recorded half of the time. Shutters of all sizes are used and each has a unique cinematic feel.
As you can see in the above clips, smaller shutters result in less motion blur, more judder and increased strobing. The 360 degree camera shutter above should achieve the smoothest motion fusion; the sticks form a continuously moving blur. The 36 degree example illustrates poor motion fusion; the sticks have very little motion blur and their movement is juddery. This may come as a surprise for some of our HFR advocates, but even at 120 fps strobing can occur if there is a small camera shutter and the action is sufficiently kinetic.
Motion blur is effected by the amount of time that the film (or sensor) is exposed to light. Slower frame rates, and smaller (more open) camera shutters result in longer periods of exposure which shows more motion blur. Our base camera footage is shot at 240fps with a 360 degree camera shutter. You can view the source footage in slow motion below. The clip demonstrates the absence of any shuttering, either camera or projector.
The action is demonstrably smooth, continuous and devoid, or almost devoid, of any artifacts such as judder or streak. Motion blur becomes much more pronounced at lower frame rates. Motion blur not only effects our perception of motion fusion, but the ambiance and feel of a scene.
The video above is a quad screen showing, 2, 24, 60, and 120 fps material but slowed down to a refresh rate of 24fps. We can see immediately a lot of motion blur occurring at the lower rates that diminishes the higher the frame rate goes.
This is Carrie doing the Tai Chi with far more restrained motion. As you can see, the amount of motion blur is also function of how much movement there was in the scene.
Now is a good time to reiterate that there is no shutter angle or frame rate that is “best”. At each step up in frame-rate, we’re effectively changing a “filter” much as we would add a diffusion filter. There are many such filters in a cameraman’s bag. Frame rate manipulations in movies like The Matrix and Seven Samurai are likewise “filters” just as the choice to use black and white, or color are filters. These filters represent choice, and choice is the very essence of art.