If you begin to dabble in DSLR videography, one of the first controversies you run into is the optimal frame rate for video capture and playback. Legend has it that you should capture the video using a shutter speed twice as fast as the frame rate you desire for the final playback (e.g. 1/50th for a 24fps movie). But how do you decide what the final frame rate should be? Traditional films were shot and played back at 24fps and that “filmic” look continues to be the standard. 24fps looks just artsy and unrealistic enough to remind ourselves that we are watching a movie. But with digital technology, movie makers are finally beginning to explore higher frame rates. An interesting question then becomes, what is the highest frame rate that a human can perceive?
Patrick J. Mineault, a PhD student in neuroscience has summarized the surprisingly complicated answer on his blog. The phenomena behind human frame rate perception is flicker fusion - the point at which a flickering image appears stationary. Flicker fusion depends on three variables:
Stimulus Luminance – The brighter the image, the faster the frame rate needed to saturate human perception. Even the brightest stimuli were undistinguishable above 50fps.
Stimulus Area – the larger the stimulus area, the faster the frame rate needed to saturate human perception. Human perception response drops off at 60fps. The area concept directly applies to screen size, meaning that higher frame rates may be more useful on a theater screen than a small monitor.
Stimulus location – the farther away the stimuli is from your center of vision, the less likely you are to perceive it.
So it would seem from this data that 60fps is approaching the upper limits of human perception. But movie frame rate perception is a harder effect to quantify since the images aren’t really flickering – they are smoothly transitioned from one frame to the next. Nevertheless, it makes sense then that Peter Jackson is shooting the new Hobbit movie at 48fps. Other directors are also investigating creative uses of higher frame rate movie capture. In the segment below, Douglas Trumbull, of Blade Runner fame, shows how high frame rate shots can be used selectively to enhance action shots.
Still not convinced? Go ahead, crank up that frame rate and try some experimentation of your own.
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