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  • Hey all, just changed over the backend after 15 years I figured time to give it a bit of an update, its probably gonna be a bit weird for most of you and i am sure there is a few bugs to work out but it should kinda work the same as before... hopefully :)

Is there a flicker issue using magnetic-ballast HMIs with the RED?

Stephen Pruitt

Stephen Pruitt

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Hi all. . .

I see some HMIs on Ebay just now, but they are older Arrisun units with magnetic ballasts. Is there a flicker issue with these units when shooting 23.9 or 24fps? Are there any other issues with the magnetics that I should be aware of?

Thanks very much.

Stephen
 
Yes, you don't want to use magnetic HMIs with a rolling-shutter CMOS camera unless you're extremely careful about your shutter speeds. You won't be doing frame ramping with a magnetic HMI.

It doesn't "flicker" like a film camera does, instead the CMOS camera will get multiple rolling orange bands scrolling through the image. That doesn't happen with a high-frequency electronic ballast.

If you insist on trying it, don't use 1/48 shutter. That's just plain not allowed. You will probably be safe at 1/40 or 1/60 though.
 
Well, that's very good to know, Barry. . .

Geez, these HMIs are OBSCENELY expensive! Like $5500 EACH for a 1200W Arri from BHPhoto. Are there any options that are a lot less expensive???

Thank you very much.

Stephen
 
I just shot a ton this weekend with a mag HMI, I have no idea what my Red was at (I was purely directing, yay!). Is there a way for me to find out shutter just from the R3D or the QTs?
 
Cinewalt said:
Barry, where have you been all my life?
Click to expand...
Over on DVXUser... :)

Here is the sample footage I posted, the "rolling pulses" we now get to play with in post: http://www.rent-a-red.com/WavesSampleH264.mov
Click to expand...

Yep. It'll happen with any rolling-shutter camera. People have to educate themselves to the fact that the rolling shutter works differently from conventional video and film cameras, and you have to know what it will and won't do to your footage. For most circumstances it's fine, but for some it just plain isn't, and the sooner you find out about it and work around it the better off you'll be.
 
Stephen Pruitt said:
Geez, these HMIs are OBSCENELY expensive! Like $5500 EACH for a 1200W Arri from BHPhoto. Are there any options that are a lot less expensive???
Click to expand...
Renting. New electronic-ballast HMIs are not inexpensive!

Cool Lights is making a series of inexpensive HMI/metal halide lights which work well. They're not Arri-class gear, they're inexpensive Chinese imports, but they're less expensive. I have one of the 150w units, cost me $390 and tested it with the Red and it worked fine unless you really cranked the shutter speed up to 1/1000 or so.
 
Weird, so I just got word from my DP that my footage was shot 23.98 and 1/48 and yet my footage looks great, no rolling stuff.
 
Whatever frame rate you shoot you have to adjust your shutter speed, this is not only in Red, it is with Varicam, HVX-200, and many, many more...

I have magnetic Ballasts with my 1200's and they work fine when I shoot certain shutter speeds, which does limit me...
 
Shawn Nelson said:
Weird, so I just got word from my DP that my footage was shot 23.98 and 1/48 and yet my footage looks great, no rolling stuff.
Click to expand...

In film... There are "windows" with HMI's where you are in phase with the pulse of (North America) 60HZ and 24 frame-ish and 180 degree is sort of on the edge - 24 fps and 144 degree shutter with magnetic ballast is safer. They are not hard on/off windows they are sweet spots in the windows. 24 fps and 1/48th (180 degree) shutter are on the edge but usually safe. I would think that even with the rolling shutter it would be safe. Consult an ASC manual for this type of info, at least for a film based opinion. The good thing about RED is that it is WYSIWYG you can play back the reference QT movie and see what you have before you move on to the next scene.

I shot a film a while back with a Red at 23.98 and a Phantom at up 500 frames a second and beyond and there is no way to get even a square wave (electronic) ballast not to flutter at those frame rates. We were using 18k HMI's to get maximum light for high speed shooting. A talented editor had a plug-in that removed the strobing effects. The promo looked great no strobing after post. It is a matter of knowing your tools.

briferg
 
Ooh, I must find the no-strobe plugin!
 
briferg said:
We were using 18k HMI's to get maximum light for high speed shooting. A talented editor had a plug-in that removed the strobing effects. The promo looked great no strobing after post. It is a matter of knowing your tools.

briferg
Click to expand...

which plug in....? for which software application?
could be useful.
 
Magnetic or Electronic?

Head / ballast noise with electronic solutions is one major issue to think about. Can you live with a bit of noise because on some electronic ignitor / ballast solutions there is a bit more noise due to the high frequency the bulb is being driven at. I would categorize it as an electronic "whine". The nice thing about this higher frequency drive is the lack of flicker at most commonly used shutter speeds. Not all electronic ballast HMIs make "head noise", but some do. In most cases, if the head is at a normal position from the subjects and from where the microphones are, you'll never record or hear the sound.

Magnetic ballasts do not have this problem as they drive the bulb at line frequency. But therein lies the issue being discussed here. They have less noise but flicker is definitely an issue at some shutter speeds. In addition, in my experience, magnetic ballasts do not give the same bulb lumen output that a good and efficient electronic ballast does. Put equivalent wattage magnetic and electronic driven systems side by side and compare brightness of output to see what I mean.

So, each one has its own benefits and inconveniences that dictate which one is best for you. For instance, if the HMI, the actor and microphone are all right next to each other, you may decide to use a magnetic ballast for that scene, otherwise, electronic ones will do fine for you with the good separation of the microphone / subject and HMI head.

I didn't notice if anyone else posted the HMI flicker formula but I'll show it here anyway. Apologies if its a repeat. This applies to anything with a 50 / 60 hz frequency such as a magnetic fluorescent or HMI ballast.

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

Many may think its better to run at a faster shutter speed but it's the opposite. Think about it like this: 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 that much (a good fluorescent electronic ballast is refreshing at 20,000 to 40,000 times per second and a good HMI electronic ballast at up to about 150 times per second). You are practically guaranteed to see flickering if you're shutter 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.
 
Thank you very much, Richard! Very informative. . .

. . . and also depressing.

:-(

Stephen
 
Yep. . . they are degrees. . . or, put another way, at 24fps, 144 degrees is the equivalent of a 1/60th exposure, whereas 216 degrees is 1/40th.

Now, exactly how much the look will deviate from using 144 or 216 relative to 180 degrees is something that I cannot address. I'm not a DP. Is there a noticeable difference?

Stephen
 
Hi Guys...I'm the lister on ebay with the HMI 1200 pars. I thought they were getting way above what they were worth...more than I paid for them used a few years ago. A bidder asked a question if they were "flicker less" and i responded by saying that they were NOT, as described in the listings. So, I began to worry that maybe people were not reading the listings, and did a google for HMI's and Flicker, and up came this very timely thread (I didn't happen to be on Reduser yesterday ot today...unusual for me)

So, this cleared it up a bit for me with regard to the Red camera and others...Thanks for all the information on this and as usual, the rest of the site. This is a really great forum to share and learn from.

The listings I have on ebay are Magnetic HMI's...so you need to shoot with a shutter spead that works...just like film cameras...in case anyone from here is bidding on them.

Thanks a lot

Larry
 
I have used magnetic ballast HMIs with the red with no problems. Can you randomly shoot whatever frame rates and shutter angles you want with no problems? NO. You can't do that with ANY camera. There are pros and cons to both Magnetic and electronic ballasts. Mag ballast are way less finicky than electronic ones. Electronic ones are smaller, lighter and you can do speed ramps. As long as you play by the rules, there should be no problems with magnetic ballasts.

Nick
 
How about a whole bank of 100 compact fluorescent bulbs?

How about a whole bank of 100 compact fluorescent bulbs?

Hi all. . .

There must be some reason (except effort and "aesthetics") why this isn't done:

Say we take a 4x4 piece of plywood. Say we mount 100 "bare bulb" sockets to the plywood (10 strips of 10 sockets). Say we put a switch on each strip. Say we put 100 14W compact fluorescent bulbs in the sockets. Bulbs are available in several different temperatures.

What have we created? A 100 bulb, 1400W fluorescent "fixture" with the light output of 6000 watts of tungsten? Or a worthless, unusable piece of experimental junk?

What say we?

Thanks much.

Stephen
 
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