Lighting for high frame rate cameras

Martin came to me with a customer problem earlier this week.  The customer bought the "low cost" Phantom Miro camera to do high frame rate photography.  Cool bit of kit that I hope to play with some time in the near future (*ahem*, Martin...).  The problem is providing enough light for the camera at such fast shutter speeds.  The customer already owns a tungsten 6k light, but even that's not enough.

I'm assuming that the customer bought the new Miro M320S which maxes out at about 1540fps in HD and has a native ISO of 1100 - oddball ISO rating, huh?  Given these specs, which lights does the customer need to buy?  That's actually fairly easy to calculate.  A while back M. David Mullen, ASC published a simple formula which he uses to order up lights by using the foot-candle photometric data of the fixture.  Good thing I published this to my personal blog just as a way to keep track of the formula!

foot-candles (approximately) = 25 x (T-stop)^2 / (ISO * shutter time)

So let's say the customer wants to operate the lens at a typical T/2.8.  We're using cine lenses here; none of that f/# funny business!  I'll assume the camera can operate with a near 360-degree shutter speed, but in truth I have no clue since I don't have any experience with the camera.

FC (approximately) = 25 x (2.8)^2/(1100* (1/1540)) = 267 foot-candles

Let's say the customer wants to use a more comfortable to focus T/4 with a typical 180-degree shutter angle.

FC (approximately) = 25 x (4)^2/(1100* (1/3080)) = 1120 foot-candles

From here it's pretty easy to know what lights to order since most vendors, like Arri, publish photometric data for their fixtures.  The Arri 200W HMI provides approximately enough light to do what the customer needs, maybe even with diffusion involved, however that assumes a short throw distance.  The customer will have to do his own research with photometric data when the light-to-subject distance is better understood.  A Film Gear or Arri 575W HMI might be a safer, more versatile purchase.

The only real variable is how much light loss comes from different types of diffusion and bounce.  That's something for a future blog article.

There is an easier way to do a back-of-the-envelope calculation when you need something quick and dirty.  The rule of thumb is ISO 100, 180-degree shutter angle at 24p, T/2.8 equates to 100 foot-candles of light for proper exposure.  So for instance if you have an Arri Alexa with a native ISO of 800 then you only need 1/8th that amount of light, or 12.5 foot-candles.  If you want to operate the lens at T/4, then you need 200 foot-candles because that's half the amount of light/one stop less making it's way through the lens.  Easy, yes?

Video Lighting for High Frame Rate Video

There was a question posted on Cinematography.com recently about lighting for high frame rate video.  The forum member wanted to know which lights to use.  That got me thinking, which lights are appropriate for the FS-700 we have in rental stock?  The FS-700 can shoot anywhere from 24fps up to 960fps, but as we all know, the footage is only really usable for film/video work up to 240fps.

On the forum, M. David Mullen - who I think is going for a world record commenting on the most cinematography web sites - commented that he tends to use 5k tungsten fixtures when shooting high speed video.  I was wondering if that was overkill or was it really true that you need that sort of massive bulb?  The amount of flicker a tungsten light produces is mainly related to the thermal mass (i.e. physical size) of the bulb.  The bigger it is, the longer it takes to warm and cool between 60Hz cycles from AC wall power.  So all of you DIY-ers out there can forget about using household 100W bulbs.  You need some real video-purposed lights here.  For HMI and LED, the flicker is related to the ballast design.

I grabbed four lights off the show room floor and lit up a Macbeth chart.  The lights used were...

• Arri 300W Tungsten
• Desisti 1kW Tungsten
• Ushio Pro Panel LED
• Film Gear 575W HMI

I purposely set the Ushio LED panel at 100% on it's internal dimmer since it's output is fairly dim compared to the tungsten Fresnels I was using.  Dimming at different levels wasn't tested.  I also set the HMI ballast to flicker-free mode because that was the best performance the light could achieve for this test.  My goal wasn't to defeat the lights, just to show what they do under their best performance and what we can recommend to customers that need to shoot high-speed video.

As expected, the Arri 300W light shows significant flicker at all frame rates.  What surprised me is that the Desisti 1k didn't really show much flicker, if any.  The Ushio and Film Gear lights were both flicker free at all frame rates.

In the video below the top half of the frame is a static frame grab.  The bottom half of the frame is the continuous video capture.  I wanted to make sure there was a consistent frame of reference to see the flicker effects easier.

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Comparison of Video Lights for High Frame Rate Video from Video Gear on Vimeo.

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As always, if you have questions please call the shop and we'll help you pick the best lights to use for any video capture situation.  Salvador would love to talk lights with you, especially after the new Mole-Richardson LED lights came through the shop!  When the customer left with the M-R LEDs I'm pretty sure Salvador had to hold back a tear, but don't tell him I wrote that.