HMI Flicker?

[richiel]

Does anyone know safe frame rates when shooting with old magnetic type HMIs? Shot yesterday at 23.98 , shutter at 48, and noticed very slight slow dark band rolling from the bottom of the screen.
Thanks

[Evin Grant]

Any 60hz speed should be fine (1/60) shutter speed.

[Shawn Nelson]

In my test, 1/40 is also okay

[Nick Gardner]

Hi,

Been shooting all week with HMIs, 180 shutter 23.98, no problems. Check the power to your light. Other stuff on the same leg (coffe machine, curling iron) can make stuff turn spooky too.

Nick

[Kenn Christenson]

If you're shooting with magnetic ballasts, you need to have your shutter at 1/60 or 1/40 - unless you're shooting with electronic - flicker free ballasts. Sounds like you're shooting on mag ballasts, though.

[Nick Gardner]

If you're shooting with magnetic ballasts, you need to have your shutter at 1/60 or 1/40

That's simply not the case. 24 fps 180 degree shutter (1/48th) is a totally safe speed.

[Shawn Nelson]

That's simply not the case. 24 fps 180 degree shutter (1/48th) is a totally safe speed.

In my tests, mag ballast at 1/48 still produces rolling when you look at a white wall

[Nick Gardner]

In my tests, mag ballast at 1/48 still produces rolling when you look at a white wall

Not saying you are wrong, but did you attempt to fix this with the light, or just assume it was the camera? What was the power source for the HMI? What wattage?

I have been shooting all week at 180 and 24, with HMI's from 575 to 4ks, flicker free and magnetic ballasts, with zero issues. We had flicker issues with a flicker free 4k ballast, it was a power issue. Used a different leg and the problem was solved.

I find it hard to believe that Red would build a camera that can't shoot under discharge lighting at the most universal shutter and FPS settings.

As far as I can tell, all the old rules apply, with the exception of some (very few) off speed rolling shutter issues.

Nick

[James Mathers]

You can find a couple of handy charts for Flicker Free Camera Speed/Shutter Angle combinations on the Cinematography Electronics website; (one for 60Hz and one for 50Hz):

http://www.cinematographyelectronics...ickerfree.html

In the days before Square Wave HMI Flicker Free ballasts, this was always a major concern when shooting off-speed, and Cinematographers memorized the safe combinations. Even nowadays, if you are working with Flicker Free HMI's, it can still be a concern if you're out in world shooting practical lights in the background of your night exteriors; many will flicker if you stray from the safe combinations. However, at 23.98, or 24fps you should not experience flicker in a 60Hz environment at any shutter angle. I would check the HMI and the power it is being fed.

James Mathers
Cinematographer
Owner RED #30X
President of the Digital Cinema Society
Studio City, CA

[Richard Andrewski]

I've posted this before but here it is again. There is a formula for figuring out all this with 50 or 60hz low frequency lighting and is applicable to both HMI and fluorescent (magnetic ballasts usually but there are actually electronic ballasts that operate at this lower frequency too):

Safe camera speeds with 60hz lighting :

Camera speed in fps = shutter angle / 3 / LPEP

Where LPEP = number of light peaks that you want to capture per exposure
period (typically 2 per cycle, or hz).

Safe shutter angles with 60hz lighting :

Shutter angle = speed in fps * 3 * LPEP

For 50hz power replace 3 with 3.6 in both formulas.

What all this means is that if you don't capture at least two cycles of light output during your exposure then you start to get flicker.

So for example in 50hz situations and assuming you want 30fps it would be as follows to figure out shutter adjustment:

30*3.6*2 = 216

Here's a bit of explanation. Each shutter moment is a moment where we're capturing light. If you're light is flickering that means that its not refreshing at a very high rate. 50 to 60 times per second is not so much! You are practically guaranteed to see flickering if you're shuttering is open at a moment the light is refreshing and not fully lit. The faster your shutter the more likely you are to be open at such a moment. Many feel that you need at least two refreshes of your light per shutter moment to be safe and not see flicker. Of course electronic ballasts like those in a fluorescent ballast refresh at 40,000 times per second and 75 to 150 times per second is common for metal halide / hmi types.