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[OM] Re: Super FP artifacts

Subject: [OM] Re: Super FP artifacts
From: Tim Hughes <timhughes@xxxxxxxx>
Date: Sat, 28 Feb 2004 17:27:33 -0800 (PST)
Cc: markml@xxxxxxx, wincros@xxxxxxxxxxx
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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