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PAL 25fps lighting with HMI magnetic ballast


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#1 Christophe Collette

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Posted 22 January 2007 - 03:36 PM

Hi, I am shooting a video for french TV, therefore I am shooting 25fps, I want to make that nothing flicks on my set, I will have practicals, which are 60 hz and HMI with magnetic ballast, what should I do? I know I should be shooting at 150 degrees shutter angle, but will that work with the practicals in the house which are 60hz...

Thanks for letting me know, anything else I should be careful of when shooting for Pal at 25 fps??? What are the flicker free speeds, 50 and 75 are but what are the others?


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#2 Stuart Brereton

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Posted 22 January 2007 - 05:03 PM

If your practicals are tungsten they shouldn't flicker at any speed. As long as you have your shutter angle correct, PAL safe speeds are 25, 50, 75, 100, 125, 150 - in short anything divisible by 25
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#3 Matthew Buick

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Posted 22 January 2007 - 05:15 PM

I will have practicals, which are 60 hz and HMI with magnetic ballast


Do you mind if I ask what on earth that sentence meant?
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#4 Stuart Brereton

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Posted 22 January 2007 - 05:38 PM

Christophe is in Canada, where the mains power frequency is 60 hertz, as opposed to 50 hertz in the UK. This means that there may be visible flicker when shooting at 25fps.

In practice, this is highly unlikely with tungsten lamps, but with HMI lamps (where flicker is part of the design) and magnetic ballasts, rather than 'Flicker free' ballasts, it is a problem.
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#5 Dominic Case

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Posted 22 January 2007 - 05:50 PM

Do you mind if I ask what on earth that sentence meant?

Firstly: (a) it's good to ask if you don't know - but (B) are you sure you can call yourself a director of photography (which is what you do in your details) if you don't know what the sentence means?

In respect of (a):

I will have practicals,
- light sources that are visible in the frame (ie are props as well as being lighting)

which are 60 hz
- operate at US mains frequency 60Hz - not UK/Europe which is 50Hz

and HMI
- HMI stands for Mercury (chemical symbol Hg) Medium-arc Iodide - a type of approximately daylight-balanced light which - like flourescent lights - flickers at the mains frequency

with magnetic ballast
- voltage controller that drives the HMI light. Magnetic ballasts do result in flicker, whereas other fancier types of ballast eliminate it.

When your lights flicker at something slightly different from the frame rate of the camera (or from a multiple of it), you get a slower interference-type of flicker in your image.
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#6 timHealy

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Posted 22 January 2007 - 06:33 PM

Hi, I am shooting a video for french TV, therefore I am shooting 25fps, I want to make that nothing flicks on my set, I will have practicals, which are 60 hz and HMI with magnetic ballast, what should I do? I know I should be shooting at 150 degrees shutter angle, but will that work with the practicals in the house which are 60hz...

Thanks for letting me know, anything else I should be careful of when shooting for Pal at 25 fps??? What are the flicker free speeds, 50 and 75 are but what are the others?
Christophe


Like Stuart said practicals will be fine, but perhaps you should get flicker free ballasts for your HMI's to be sure you'll be OK.

Best

Tim
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#7 Christophe Collette

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Posted 22 January 2007 - 10:12 PM

Thanks a lot guys!! I was also wondering something, you might think this is a dumb question but I will ask anyway since I do not know for sure the answer, for you guys in Europe, are your cameras preset to 150 degrees shutter angle? Should I set mine to 150 degrees when shooting at 25 fps, I think not but I am really confused with the maths and theory of flickers. In fact, my real question is about the relation of shutter angle to flickers, does a different shutter angle affects flickering? Do you have to have degrees that perfectly divide ( when shooting 25fps) in shutter angle as well? example 50 degrees, 100 degrees, 150degrees or is that not a matter of concerns... My guess is no but I'd rather ask!


Very confused...

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#8 Stuart Brereton

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Posted 23 January 2007 - 08:49 AM

In the UK, we shoot 25fps with a 180 deg shutter as standard. This gives a 1/50 sec exposure, which syncs with the 50hz mains power that we have here. To get a 1/60 sec exposure (which you need for 60hz power) you need a 150 deg shutter.
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#9 Christophe Collette

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Posted 23 January 2007 - 10:31 AM

Thanks a lot Stuart!
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Media Blackout - Custom Cables and AKS

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Rig Wheels Passport

Broadcast Solutions Inc

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Technodolly

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Ritter Battery

Glidecam

Willys Widgets

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