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Friday, March 24, 2023

Increasing Shutter Angles Beyond 360°

The world of cinematography has long held on to the axiom that 24 FPS with a 180° shutter is cinema and any derivation therefrom is sacrilege because of everything from the 'soap opera' effect to props looking fake to inadequate levels of motion blur.

The few challenges to that self-evident truth like The Hobbit are endlessly criticized. The half-baked attempts to mitigate the problems of the low framerate standard, like interpolation, are rebuked by big name film makers who advocate for a 'film maker mode' so you get to see each any every one of those 24 frames and the illustrious chop and flicker between each one.

The force of progress, however, is insuperable and resisting the benefits of higher framerates can only go so far. Filmmakers are able to use high framerate recordings and downsample them in the post production to create a result that is indistinguishable from a low framerate recording but allows them to modify various parameters after filming is complete. This process has gone so far that industry tools not only allow for simple downscaling but also the artificial introduction of literal judder or variance in frame times.

I will not define shutter angle and exposure length in this piece but their definitions are crucial to understanding these concepts.

Obviously when exposure time is infinitely short, downsampling framerate is as simple as discarding frames. Let's say however that you are using a 180° shutter and are filming at 144FPS. That would result in a 1/288th of a second exposure time. Let's also say you wish to downscale to 24FPS with a 180° shutter. That is no longer simple nor even possible to do without causing unnatural artifacts. As the following image shows (if teal indicates time when a frame is exposed), there simply isn't information available in the original recording to reproduce the lower framerate video.

This problem however disappears if we simply use a 360° shutter for the higher framerate source footage. We simply have three frames of exposure for each frame of 24fps 180° shutter angle exposure.

The video linked here shows a professional example of this process: https://vimeo.com/105838602#t=1m51s

The logic of such a process is that by simply combining the images of multiple frames shot with a 360° shutter you are able to create an effective shutter speed multiple times longer than the one is was originally shot with. So with a 1/144th second exposure frame from a 144FPS video one can combine it with both its neighbors to create a 1/48th second exposure theoretically indistinguishable from a 1/48th second exposure created by native 24FPS 180° shutter recordings. 

The natural corollary of that capability is that with a simple rolling average, one is capable of creating a video with an effective shutter angle greater than 360° directly contrary to traditional cinematographic understanding. Even camera manufacturer RED claims unequivocally that: 

"The larger the angle, the slower the shutter speed, all the way up to the limit of 360°, where the shutter speed could become as slow as the frame rate."

They are far from alone in their misconception. The ability to create effective shutter angles greater than 360° allows filmmakers to create nearly arbitrarily high framerate videos without reducing the level of motion blur whatsoever compared to something like 24FPS with a 180° shutter.

Ultimately, the only honest arguments for using 24FPS in modern film making boil down to cheapening out or clinging to an objectively reduced level of fidelity like not unlike SDTV to prevent the viewer from being able see the content clearly and pretending like it's a creative choice.