Magnetic HMI ballast with HVX200a

M

mrbrycel

Well-known member
I got my Arri 1.2k HMI Par with magnetic ballasr, fully aware of the flicker issues. But now I'm a little confused...

I read that 24fps at 180 degree shutter (1/48th) was flicker free, but that 23.98 at 180 degree shutter was unsafe..

So first I test the Canon 7d at 24p (which is really 23.98 right?) at 1/50th, and sure enough, rolling bars. 1/30th, 1/40th, and 1/60th looked fine.

Then I tested my HVX200a, 24p (23.98 as well right?) at 180 degree shutter, and it from what I could see it looked completely fine. I know it has a global shutter so I wasn't expecting rolling bars, but it wasn't flickering at all either.

Why is this? Is 1/48th a flicker free shutterspeed with the HVX? Or am I not seeing a very subtle flicker? Does the HVX actually shoot 24fps, then convert to 23.98?
 
"flicker free" refers to film, not video. The flicker usually only happens on film when film is shot at a "non-HMI-safe" frame rate.

With video, you have two potential issues, but they have nothing to do with frame rate, they have to do with shutter speed.

With a CMOS sensor, you'll get the scrolling bars.

With a CCD sensor, you'll usually get a very slow color shift, like as if your white balance was hunting continuously. It's nowhere near as noticeable as the bars, so people don't usually comment on it, but ... try it with a 1/250th shutter and you'll see it, especially if you play the footage back at high speed.
 
Barry_Green said:
"flicker free" refers to film, not video. The flicker usually only happens on film when film is shot at a "non-HMI-safe" frame rate.

With video, you have two potential issues, but they have nothing to do with frame rate, they have to do with shutter speed.

With a CMOS sensor, you'll get the scrolling bars.

With a CCD sensor, you'll usually get a very slow color shift, like as if your white balance was hunting continuously. It's nowhere near as noticeable as the bars, so people don't usually comment on it, but ... try it with a 1/250th shutter and you'll see it, especially if you play the footage back at high speed.
Click to expand...

Does the color shift happen at any shutterspeed? I didn't notice anything at 180 degrees (24pN), but when I started decreasing the shutter angle I could see the slow shift. At 60fps, from 180 degrees up to 350 didn't seem to have any problem from what I could see, are all of those shutter angles safe?

Is it safer to go with the slowest acceptable shutterspeed, or is it the same with trying to use shutterspeeds that can evenly go into 120 (1/30th, 1/40th, 1/60th)?
 
Safest are those that are dividable into 120 (1/120, 2/120, 3/120, 4/120, 5/120). With any other shutter speed, the further it varies off-cycle (and especially the faster) the more color shift you'll see. Minor variation (i.e., 1/59 instead of 1/60) will mean very minor color shift, probably imperceptible. Major variation (1/1000 instead of 1/60th) will mean massive color shift.
 
Barry_Green said:
Safest are those that are dividable into 120 (1/120, 2/120, 3/120, 4/120, 5/120). With any other shutter speed, the further it varies off-cycle (and especially the faster) the more color shift you'll see. Minor variation (i.e., 1/59 instead of 1/60) will mean very minor color shift, probably imperceptible. Major variation (1/1000 instead of 1/60th) will mean massive color shift.
Click to expand...

Ok that makes sense.

Now with CMOS sensors like on the 7D, will
I still be safe at 1/30th, 1/40th, and 1/60th? When I tested it, I didn't see
rolling bars on the footage so I'm assuming it's no
problem. I have a job coming up shooting on the 7D, and I want to make sure I can use my magnetic ballast with no problems.
 
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