# HMI Flicker with different frame rates

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### #1 Scott B. Jolley

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Posted 13 April 2006 - 10:24 PM

When shooting with the different frames rates on the varicam and using HMI lighting, are there sync or flicker problems? Are electronic ballest needed? What is the math to figure it out?
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### #2 Michael Nash

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Posted 13 April 2006 - 10:51 PM

Same rules apply as for film, except you adjust shutter speed instead of shutter angle.

You can follow the "HMI safe" frame rates derived from the "twice the frequency rule." Any number that divides evenly into 2x the power cycle is a flicker-free frame rate at any shutterspeed. That's 120/x for 60Hz. countries giving you: 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 20, 24, 30, 40, 60.

If you want to shoot at some other frame rate, you need to set your shutterspeed (through clearscan or synchroscan) to a number that matches or divides evenly into the power cycle. For example, 1/60 sec., or 60.0Hz. For the most natural-looking motion blur, pick the denominator or Hz. that's closest to double the frame rate.
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### #3 Scott B. Jolley

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Posted 14 April 2006 - 08:48 AM

Same rules apply as for film, except you adjust shutter speed instead of shutter angle.

You can follow the "HMI safe" frame rates derived from the "twice the frequency rule." Any number that divides evenly into 2x the power cycle is a flicker-free frame rate at any shutterspeed. That's 120/x for 60Hz. countries giving you: 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 20, 24, 30, 40, 60.

If you want to shoot at some other frame rate, you need to set your shutterspeed (through clearscan or synchroscan) to a number that matches or divides evenly into the power cycle. For example, 1/60 sec., or 60.0Hz. For the most natural-looking motion blur, pick the denominator or Hz. that's closest to double the frame rate.

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### #4 Jeremy

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Posted 18 April 2006 - 03:58 PM

HMIs actually have very strange color warping when you play with a shutter speed that catches the lights at a different frequency. By standard settings on the Varicam, this usually happens with 1/250 and up.
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