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HMI Flicker at 25fps


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#1 John Draus

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Posted 09 November 2009 - 06:52 PM

Any suggestions as to what might be the correct shutter angle to remove flicker when shooting with Magnetic ballast HMIs at 25fps in a 60 htz setting (US)?
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#2 John Sprung

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Posted 10 November 2009 - 01:19 AM

You can use any shutter angle. The important thing is the frame rate. In a 60 Hz. country, you have 120 light pulses per second. Every frame needs to see the same number of light pulses. So, you can use any frame rate that can be multiplied by an integer to get 120:

120 x 1 = 60 x 2 = 40 x 3 = 30 x 4 = 24 x 5 = 20 x 6, etc.

So, the thing to do is shoot at 24 fps, and just post everything 4% fast to integrate into a 25 fps show.





-- J.S.
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#3 Dominic Case

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Posted 10 November 2009 - 04:07 PM

Alternatively, since you want to shoot at 25fps, set your shutter to 150 degrees. You need one shutter opening (in that case, 1/25 x 150/360 seconds = 1/60th sec) to cover a whole number of pulses - which come at 120 per second for 60Hz supply.

Or, alternatively again, a search on Google for something like "shutter angle HMI" should pop up an answer a whole lot quicker than this ;)
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#4 John Draus

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Posted 10 November 2009 - 11:17 PM

Alternatively, since you want to shoot at 25fps, set your shutter to 150 degrees. You need one shutter opening (in that case, 1/25 x 150/360 seconds = 1/60th sec) to cover a whole number of pulses - which come at 120 per second for 60Hz supply.

Or, alternatively again, a search on Google for something like "shutter angle HMI" should pop up an answer a whole lot quicker than this ;)



Thanks, looks like either case would work or they may just end up renting electronic ballasts to be safe.
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