The Speed of Light

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Or, more importantly, the duration of the light really.  I’m gearing up to do some motion freezing shots using flash.  Now, why do this with flash?  We could just use a really high shutter speed and yes, the shutter on my Nikon D800E will go to 1/8000th of a second, which is fast enough to freeze most action in the “medium sized world”.  However, there are a couple of reasons why this is not a good idea.

First, for shots where we need to the camera to react to a noise, vibration or something breaking a detector beam, there may not be time for all of the mechanical gubbins inside the camera to lumber into action before the event is over:  the mirror has to raise, the whole shutter has to start it’s operation etc.  All of this can add as much as 50ms between the trigger event, and the exposure starting.  The second reason has to do with the amount of light we have to work with:  at 1/8000th of  second, not a lot of light is going to get in.

Now, one of the best things about speedlights is that you can’t alter the power.  Yep read that again 🙂  Speedlights do not work in the same way as conventional studio lights.  Whereas studio lights charge up their capacitors with just the right amount of energy for the light output you want, speedlights are always fully charged.  You control the amount of light by altering the duration of the light.  They have a tap inside – these days this is normally an Insulated Gate Bipolar Transistor or IGBT.  So rather than a trigger voltage discharging the whole cap across the flash-tube, your speedlight can turn the flow of electrical energy to the tube on and off.  The power of your speedlight btw, may amaze you.  Power is the rate or flow of energy (the rate of “doing work” in generic terms).  It is not the amount of energy delivered.  Your average speedlight holds around 80 joules of energy in its capacitors.  To empty them fully takes about 1/900th of a second.  1 watt is 1 joule per second, so dividing 80 by 1/900th gives us 72 kilowatts.  That’s a pretty bright light – for just over a millisecond….

So what happens when we turn the “power” on a speedlight down?  You are actually turning the duration down.  Want half as much light?  The light is on for half as long (roughly).  It’s still a 72kw light though  (actually a bit more as the energy flows out faster at the start of the flash, and when you turn a speedlight down, you skip the slower “tail” portion).  For my Nikon SB-900’s at 1/16th power, the light lasts 1/10000th of a second.  That’s fast enough to freeze most subjects, and even at 1/8th and 1/5000th of a second it’ll freeze most dance/hair flicks etc.  You can see the actual times in this table on the Nikon USA web site.

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Now that’s great n all, but now my light is 1/16th output.  I could turn up the gain on my sensor (the so called “ISO”), and open up the lens however that’s only gonna get me so far.  The solution is to sync many lights all firing at exactly the same time, for exactly the same length of time.    As I already had 4 SB-900’s I’ve opted to lash these together on a Lastolite Quad bracket.  The lights are triggered by one Yongnuo YN-622N transceiver.  The trigger signal goes out of the sync port on the 622 and is carried to the sync ports on my SB-900s via a custom Lastolite 1 to 4 pc-sync cable.  This ensures, that all 4 lights get the signal at the same time.  As they are exactly the same light running the same firmware, they all fire at exactly the same time.  Variations inherent in capacitor manufacture don’t figure at this scale.

I also got a Lastolite “Ezybox II” Ezybox Switch  (so named because it has a zip arrangement allowing you change the shape from almost square to a smaller strip).  Despite the name there is nothing “Ezy” about this soft-box.  I’ve put up conventional pole octaboxes in less time  (and believe me, those things are a wrestling match), and the diffusers do not fit properly  (front one is just a bit too small).  It doesn’t sit on the Ezybox Quad bracket too well either, although I think it will stay on as long as there is no wind at all..

There are other solutions to fast light.  In the studio you can now buy mains powered IGBT lights.  Making lights that work in this way at high power has always be a problem, however it seems several manufacturers have achieved  compromise.  They’re not quite as quick as a small speedlight, but they will reach speeds of 1/4000th or thereabouts.  In the UK Lencarta sell the SF-300 and SF-600 lights at 300 and 600 joules capacity respectively.  You can import a Paul C Buff Einstein, however they are stupidly expensive, and have an obscure proprietary mount for reflectors and soft-boxes.  The Lencarta lights are re-engineered Godox QT lights and as such, take standard Bowens S-fit accessories.

In the field, you could use these lights with a portable power pack such as the Godox LP800X which provides 240v mains power, or use something like the elinchrom Ranger Quadra with the “A” heads.  Again, these are not as fast as the small speedlights.  All of these solutions though, are big, and heavy.  Four SB-900’s and a bracket is very portable.  They don’t need to be expensive lights by the way, any old lights will do as long as they have a sync port (rules out my bucket of YN460-IIs, although I could still use them with a hot-shoe trigger each).

I’ll be using this set-up for some faster dance shots, and other action.



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