With fractions in the next frame the preceding (etc) blade takes the place of the current blade. As the two blades look alike (at this resolution) the blade appears to be static.
ADDED BY EDIT
Say disc running at 480 rpm - so that's 8 RPS
3 blades - 24 blade passes per second
One of the standard for FPS is indeed 24 (actually often run as 23.976 FPS).
So, using a Nikon D7000 DSLR at 24fps to shoot a video clip would make a disc running at 480 rpm appear to have stopped still. It would make my 120 disc at 415 rpm (top of the green) look like it's turning backwards. Slowing the FPS to 20.75 would make the 120 disc at 415 rpm appear to have stopped
iPhone 5 video capture runs at 30 FPS.
For a 3-blade machine that's going to need 10 rps to have a blade always at the same point of shot
Hence the 600 rpm calculation above would apply to the crappy iPhone.
Last edited by John R81; 15th Dec 2014 at 15:09.
Reason: Added examples