Mark wrote:
>>That's interesting. I don't see anything I can relate to the
high-speed (10-20 kHz) flashing of the super FP flash; the strange
lines are just artifacts of the shutter, as he explains.
<<
Having disagreed with Winsor on this, I now think you and he may be right. My
original thought was the curved shape was due to focal plane shutter (as
indicated on website) and the increased number of spokes was due to flash
strobe effects (aliasing/like wagon wheel spoke effects on movie). If you
make some assumptions: 30000RPM = 500RPS. So in 1/4000 sec the disk rotates 1/8
turn and during that time the flash fires 2.5 to 5 times. (assumes 10-20kHz
like F280). At top and bottom of frame the spoke should then be widened by ~1/8
of disk circumference. If we assume this is at least a 1/125 sec sync capable
vertical shutter, then shutter travel time is less than 8mS. During shutter
travel time the disk then rotates ~4 times. To confuse things more the 4 spoke
pattern is symmetrical so this could potentially cause additional harmonic
patterns at various harmonics of rotational speed. It would be interesting
to see the same test with a single radial line, rather than 4
radial
lines, or better with 4 different colored lines to confirm what we think is
happening. You would have to take a few shots to get the different
orientations. Also as you suggest repeat with natural light as a sanity check.
Regards,
Tim Hughes
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