fluorescent light ballast attachment?

chrismagicc

chrismagicc

Well-known member
i bought a six bank fluorescent light on eBay a while back
i noticed that at high shutter speeds, the light would flicker and band.

is there a cheap solution? a ballast attachment of sorts?
cheap is key haha

thanks guys
 
Sounds like your fluorescent fixture uses the old-style conventional magnetic ballasts that run on power mains frequency (60Hz)
That is why fluorescents that are used for production (film/video) use the newer electronic ballasts that run at a much higher frequency so they don't "strobe" and flicker.

How heavy is the ballast(s)? Can you hear them humming? A heavy and /or humming ballast is a sign of a magnetic unit.

The solution is to remove the magnetic ballast(s) and replace with electronic. Whether this is "cheap" depends on how carefully you shop for replacement ballasts and whether you can DIY the replacement or have to pay someone to do it for you.
 
chrismagicc said:
here is the link to the light:
http://cgi.ebay.com/1650w-Studio-Vi...531?pt=LH_DefaultDomain_0&hash=item4cf3fd55b3

i am using a t2i and i want to get the shutter to at least 320 for twixtor slowmotion purposes
Click to expand...

I didn't see any mention of what sort of ballast they use... so I'd sort of assume it was the 60 Hz variety... Don't know... does the T2i have a 1/240 or 1/480 th shutter speed... Since I don't do this sort of thing, I don't really know whether this would work. But one of the problems with increasing shutter speed is... needing an increase in light... really fast frame rates, and attendant shutter speeds need much more light...
 
Way cool effect. http://theawesomer.com/twixtor-slow-motion/70121/

We don't know what ballast is in your lights but I agree it could be a magnetic ballast, (though I doubt this since it is apparently new). The reason I hedge is because while I know that Kino Flos have frequencies of 20,000 Hz or more, I don't know what the frequencies of ordinary electronic ballasts are. They could be much less. Richard Andrewski undoubtedly does know and I hope he drops in and gives us a tutorial. Why that matters should become apparent in a moment.

But let's assume you have a magnetic ballast. You are in the USA so alternating current from the power company is 60Hz. That means it goes through one complete cycle 60 times a second. But each cycle has a positive voltage peak and a negative one. Because the light pulses for each peak in your fluorescent, (it doesn't care whether the current is flowing one direction or the other), you get 120 light pulses per second and each pulse lasts 1/120 or 0.008333 (3s repeat) seconds.*

Now think about what is going on in your camera. Say you are shooting 24P. Twenty four times a second (okay, almost) a frame is exposed. If you set the shutter speed to 1/60 that is 0.01666 (6s repeat) the shutter opens and light pours onto the imager for that long. While the shutter is open 2 (0.01666/0.008333) complete pulses of light hit the imager. Each frame will be lit by exactly the same amount of light so you will not see any artifact.

If you use a high shutter speed and one that is not evenly divisible by 60 you won't get an even number of light pulses while the shutter is open. In fact, above 1/120 you won't get even a single full pulse of light. Getting less than a full pulse of light guarantees trouble because the light in each pulse increases and decreases within the pulse due to the sine wave shape of the varying voltage. You can see that if one frame is exposed to a barely starting fraction of a pulse and another gets the brightest peak and another gets something less, then each frame will be exposed differently and you will get the flicker, rolling bright or dark bands or rhythmically shifting color artifacts.

Kino Flos and other pulsating lights designed for cinematography have high frequencies so instead of say 2 pulses per frame they will have hundreds. A few pulses more or less per frame will not make a perceptible difference so they are called flicker free ballasts.

There is another problem. The t2i's shutter is only fully open for shutter speeds of 1/200 or less. We know this because that is the highest speed for which it has X sync, which is the ability for a speed light (strobe) whose duration is extremely short, to fill the entire frame. Above 1/200 the camera uses moving "curtains" as in old-fashioned focal plane shutters to paint a strip of light across the imager. This strip is narrower and narrower as shutter speed increases. A Kino Flo or any very high frequency light helps you here, but you will always get only a portion of the frame exposed above the X sync speed and fewer flashes reaching any part of the imager.

And, as pointed out you are going to get into problems having enough light with artificial light and higher shutter speeds. There is a good reason the sample video was shot with daylight.

I'm a gaffer not a cinematographer so I might have gotten something wrong here and if so would be happy to stand corrected, but I don't think so. I could go on and on but then this would become an article :)

Be careful, the Web has a lot of wrong information complete with incorrect diagrams about the causes of flicker and the accurate information is often about rotating shutters in movie film cameras not the innards of a DSLR.

Good luck and be sure to share back what you learn elsewhere and from experiments.

*I'm going to express the fractions as repeating decimals instead of ratios because you might be able to see what is going on better that way even though the decimal fractions are not exact and the ratios are.
 
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You don't have to have an magnetic ballast to cause flickering. It just takes an electronic ballast that's not functioning well or one without a high output driving frequency. My tests show you really shouldn't use anything under around 18khz to have good results. Most good ballasts however like the ones we use drive at 40Khz or so and there is never an issue with one like that. So you either have one or more malfunctioning ballasts in your unit or the maker just didn't do much testing above certain frame rates/shutter speeds and they have a ballast with a relatively low driving frequency.
 
That's a 240 volt only ballast. It won't work in the USA or Canada if you are in North America. In addition it only drives one bulb. You would probably be replacing a ballast that drives two bulbs.
 
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