Why? BECAUSE REASONS, THAT’S WHY!
Rather than spinning the GoPro camera itself in his earlier bullet-time rig (which also used a fan and two flashlights), Mark Rober and another engineer friend from NASA JPL designed a newer revision (the latest of at least FIFTEEN revisions), where the rig uses a high-end Phantom camera and a scooter motor to spin two mirrors on aluminum arms. This way, the team was able to get that coveted rotating-around-an-object perspective, while the super-expensive camera is safe, sound and stationary on the ground below, shooting at an insanely awesome 7,200 frames per second. Check out that image on the left; it’s really pretty amazing. Check the link below for the video.
It may not resemble the razor-sharp footage we’re used to seeing from Phantom cameras, but this is still very amazing, and to meet the bullet-time results, the image bounces off two mirrors. In their first attempt, they used mirrors from the hardware store, but ended up losing about 30% of light each time it passes through a mirror. With the high shutter speed of Phantom cameras, this means that the team needed a LOT of a light. They ended up using acrylic, front-surface mirrors that lost only about 4% of light, but the surfaces of those mirrors aren’t perfectly flat, which resulted in some warping. Enter problem #2: those mirrors make the camera seem to be far away from the subject. After trying a 105mm lens (which didn’t let enough light in — even in the best, bright light), they used a 50mm lens that allowed plenty of light, but had a wider point of view, making the subject seem further away and smaller. This yielded a comparatively small square of useable footage — as you can see in the GIF above from the raw footage — which needed to be cropped and stretched to fit into the 720p frame of the last video, causing further pixel degradation.