HMI flicker

[FilmUnited]

Hi David,

I have an upcoming shoot involving a lot of daylight interiors that will be augmented by HMIs. The production is unable to afford the extra $100 per light to get flicker free ballasts.

When using the magnetic ballasts, I am under the impression that the HMIs pulse twice per cycle, or 120 times per second at 60 hertz. I have been told conflicting information from different sources about shooting with magnetic ballasts. One camp tells me that any frame rate that divides straight into 120 will be acceptable at any shutter angle. Thus 24fps at a normal 180 shutter would be acceptable.

However, I have also heard that using a 144 degree shutter at 24 fps is the way to go. At least in my head, this seems to make more sense because 144 degree shutter at 24 fps yeilds an exposure time of 1/60 of a second, allowing two even pulses from the HMI.

However, many websites that I have found, including the Panavision website, clearly state that speeds of 24, 30, 40, and 60 fps are safe at "ANY shutter angle". Can you clarify?

Thanks,
Corey

 

[Kevin_Zanit]

The information below in your post is correct.

Those are some "safe speeds" at a 180 shutter, to do different shutter speeds than those require a different shutter angle.

I honestly don't remember how the math works out (I am sure David can chime in here ), but I have always just referred to tables when in doubt.

The question to ask yourself is if you are confident that your camera is running at those speeds solidly. In other words, is it not wavering at all in it's speed? (most modern cameras don't have an issue)

Also, is your power source running at a solid 60 Hz? (wall power should be very solid).


Kevin Zanit

 

[FilmUnited]

the location is an old house. hope the circutry works well.

and yes, the camera should run at exactly the frame rate set.

thanks,
Corey

 

[David Mullen ASC]

While technically, 144 degrees is the optimal shutter angle shooting 60 Hz AC discharge lamps at 24 fps crystal, 180 degrees is within a window of tolerance.