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UK HMI slow motion question

hmi magnetic safe fps fps 50hz

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#1 Matt Rozier

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Posted 04 December 2018 - 08:04 AM

Hey everyone, I'm wondering if I were to use a 1.2k HMI with a magnetic ballast in the UK on wall power (50hz) - if keeping my shutter angle at 180, will I get flicker at 50fps or 100fps or is it only likely to happen when I move off these frame rates?  I know magnetic ballasts can be a little more finicky for flicker. 


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#2 Phil Rhodes

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Posted 04 December 2018 - 09:43 AM

Most likely, you'll probably be mostly OK, usually, at 50fps.

 

I've shot stuff under 50Hz mag-ballast lighting at 100fps with the plan of dropping every other frame in post to get either the "brighter" or "darker" part of the flicker, depending how we felt about it.

 

The problem with any approach is that the mains frequency and the camera will slowly drift against one another. As a result, the lighting can change over the course of the shoot. Power cycle the camera for "different" results.

 

In short: dangerous, but interesting.

 

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#3 Matt Rozier

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Posted 04 December 2018 - 11:43 AM

Thanks Phil!

So you found that when you shot 100fps there was def flicker?  

I'm just weighing up the options of buying an older but western made HMI with a magnetic ballast or a Chinese copy with an electric ballast. 

I only really tend to shoot at 25 and 50fps and am based in the UK, so no need for unusual frame rates but I've just heard that I may get flicker at 50p. 

It sounds like I may be safer going the chinese copy route.  Renting is fine, but there's periods on lower budget stuff where that isn't always possible and buying a genuine arri with electric ballast is out of my price range :-(   


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#4 Phil Rhodes

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Posted 04 December 2018 - 11:51 AM

So you found that when you shot 100fps there was def flicker?  

 

Sure, it couldn't really be any other way. Every alternate frame will be bright then dark at 100fps on a 50Hz discharge light source.

 

You can do 50p, though watch out for flicker at off shutter speeds.

 

The chinese copies are often flicker free - in fact that's sort of the problem. Most non-movie-oriented HMI ballasts are effectively stuck in what an Arri ballast calls "flicker free mode," where they drive the lamp with a square wave. It's easier to build and also produces more constant (thought not completely constant) light output. It also makes the lamps buzz or whine, so that type of ballast is often more suitable for high speed work than it is for anything else. If you want to record sound in the same scene with the same lighting, though, you are likely to have problems.

 

I have a 575W HMI PAR which uses a Chinese ballast - not actually an Arri knockoff, but one made rather cheaply in China - and it has these characteristics.

 

Renting is great if you're just adding one thing to an existing rental package. If you're having to set up a whole rental just for that one thing, it becomes a nonstarter, I know what you mean.

 

Buying high-priced western-made ballasts, at least for small HMIs, is a dubious proposition at this point since LEDs are likely to start to overtake small HMIs over the next few years. You'd need to amortise that investment quickly, I think. For bigger stuff - 2.5K and up, say - it may be longer before LEDs get that big.

 

Where are you, roughly?

 

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#5 Matt Rozier

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Posted 04 December 2018 - 12:06 PM

Hi Phil !

 

Man that advice is golddust, I'm in High Wycombe, so just on the outskirts of London.  And I'm having exactly the problem you state with renting.  Oftentimes I'm just renting one or two things - so it can get quite complicated with drop off and pick up times etc. 

I've got a few lights (tungsten and LEDs) which I get by on in most situations, but every so often I need something with a bit more juice.  I was looking at the Chinese 1.2k HMIs and the Chinese M18 knock offs.  I'd mainly be using it as an exterior source but I also do a lot of music stuff too and it'd be handy to be able to use it through diff etc, so a noisy bulb might become a problem.

 

Tricky.  The magnetic ballast is handily quieter.  And if I can get 50p out of it when needed then it'd probably do the job.  But a Chinese m18 would give me a fair bit more fire power when I need it.  LEDs are great and they're certainly getting there, but they're not quite there yet in terms of the amount of power I need - pushing light through windows etc. 


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#6 Phil Rhodes

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Posted 05 December 2018 - 06:09 AM

I'm in Essex, I'd say come borrow some of my stuff and see what you think, but it's a bit of a hike. Couldn't agree more about the power level on LEDs. Everyone talks about this stuff as if it's the second coming - at 100W a pop. It's just not enough, sometimes, and especially for high speed.

 

I have some ceramic metal halide stuff built out of ETC Source Four PARs and commercial CMH ballasts, which is flicker free all the way to 900fps and is silent. Common examples are only 150W a lamp, and don't hot restart, though I do use it quite a bit, it does come in sizes up to 600W that I've not played with much. I have a couple of units under construction which use clusters of nine, for a total of 1350W, to solve exactly the sort of problem you describe. That's a lot of metalwork, though, and it depends how dedicated you are to homebrew.

 

People report mixed results with the chinese stuff. I have seen ballasts where all the flicker-free control does is illuminate a different LED; it's not even connected to the actual ballast hardware (yes, literally, that is a thing.) I think you need to be willing to pull out a tool kit and a multimeter (and a PAT tester) and investigate thoroughly, as some of them have been shown to have a standard of wiring that you would not want to use in public. That said they do generally work at some level. Anticipate replacing the bulb it comes with, or at least anticipate replacing it soon.

 

A much better option, if you can possibly stretch to the money, is something like the Photon Beard Platinum Blonde. It's a 1200W HMI in a blonde housing, which, being open-faced, is extremely efficient and powerful if rather a blunt object in terms of control of the light. Photon Beard use Power to Light ballasts which are of very high quality and work very nicely. It's not cheap. But neither is it that expensive.

 

Also, 1200W Arri ballasts are about the cheapest thing to buy because they come up secondhand quite a bit. But their flicker performance for overcranking varies with their age. I almost bought a little 575 ballast from ebay a while ago, with the hope of quieting my HMI PAR, but wasn't quite convinced it'd cut the mustard.

 

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#7 Matt Rozier

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Posted 11 December 2018 - 08:57 AM

Thanks Phil ! 

That's all really useful info !! Wow 1350W of LED power is huge ! You should market it ! 

The Photon Beard is a really interesting option too - I wasn't aware of that, so I'll check it out :-) 

Thanks again Phil - really appreciate your help :-) 


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#8 Phil Rhodes

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Posted 12 December 2018 - 06:01 AM

It's not LED, it's ceramic metal halide. Wish it was.

 

I could do it in LED, probably, but it'd be pretty expensive. And fan-cooled.

 

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