HMI

[Andrew Greenan]

Hi,

Old topic, i'm sure.

 

HMI's running at 50Hz, won't flicker at 25fps 180-deg shutter; nor at 24fps 172.8-deg shutter. that right? regardless of whether they're flicker-free HMI's or not.

 

Flicker in HMI's isn't a constant strobe effect, but gives a flash frame every 24. Is that right?

 

i'd love to, but i can't do tests on all this.

 

Andrew

[David Mullen ASC]

Correct, assuming you are running crystal-sync 24 fps and the 50 hz power for the HMI's is within the frequency tolerances.

 

It's not a pulse per frame, more like 2 pulses( a 50 hz HMI on a magnetic ballast is pulsing 100 times a second, I think).

 

What causes flicker is uneven exposure where some frames get 2 pulses and some get something else like one and a half, let's say. That's why your camera has to be running crystal-sync.

[Manu Anand]

Dear Andrew

David is correct when he says its two flashes per cycle

so an HMI operating on 50 hz flashes 100 times/sec and one operating on 60 hz flashes 120 times/sec

Your camera has to be crystal controlled and a frequency meter should be keeping an eye on the AC supply so that it stays as static as possible.

a lil research to refresh my memory -

So a sub multiple of 100 frames per second must be exposed to get a whole number of shorter than 1/100th of a second flashes and that would give you even exposure from one frame to the next.so the safe speeds that you can shoot on are therefore 100,50,33 1/3,25,20,16 2/3, 14 2/7,etc etc (dividing 100 by 1,2,3,4,5,6,7,8,9,10 etc upto 25) to give you a slowest speed of 4 fps without a flicker problem.

 

Manu

New Delhi

[Andrew Greenan]

Thanks guys,

Andrew