Wednesday, April 11, 2012

High Speed Sync Hack

Sports photographers wanting to use flash photography have one common enemy, camera/flash sync speed.

There are times when the photographer wants to reveal the subject with flash.  The problem is the subject is moving too fast for the very limiting shutter speed that photographers are forced to use when a flash is in use.  The majority of today's dSLRs have maximum sync speeds in the neighbourhood of 1/200th or 1/250th of a second.  These are not what I would call action freezing shutter speeds.

What do you do when you want to blend ambient light and flash when your subject requires faster shutter speeds?  Flash durations can be fast enough, but the ambient light continues to spill into the frame the entire time that the shutter remains open.  The bane of sports photographers everywhere, ghosting.

Continuous light is the answer and top-of-the-line flashes can do it.  Instead of triggering the flash while the shutter is open (x-sync), FP/HSS technology begins pulsing the flash before the shutter opens and continues to do so until the second shutter recovers the sensor.  The result is a form of continuous light.  The trade off is these pulses are very weak.

It's time to introduce the high speed sync hack.  It's not new.  Google it and you'll find links that date back a few years.  What you won't find is a decent explanation on how to use it effectively.  That is why I am writing this piece.

What you need is a flash capable of FP/HSS and a method of connecting a radio trigger to send the early signal to other flashes/strobes off camera.  The FP/HSS capable flash will be in the hotshoe, so either the flash or the camera is going to need to have a PC terminal to connect your trigger to.  The key to the hack is using that early trigger signal and sending it via radio to other flashes that are set to full dump, not wimpy little pulses.  If the flash duration is longer than the shutter speed, you have continuous light!

Here is what I do.  I put my SB-800 or SB-900 in the hotshoe.  I then connect my flash, PC terminal to mini phone jack to Pocket Wizard Plus II.  The result is the flash trigger signal goes out before the shutter begins it's travel across the focal plane.
Nikon SB-900 with Pocket Wizard Plus Transmitter mounted in the 'hack' formation

Off camera I configure my radio triggers according to normal convention, with one exception, the flash must be in manual mode and set to full power.  The necessity is the long flash duration.

The magic behind all of this is that I can now use any fast shutter speed and there will be flash and ambient light for the entire shutter duration.  No ghosting!

As long winded as my introduction has been, this is where I tell you how to use it.  Since your shutter speed is faster than the actual flash duration you are not using all of the light emitted from the flash.  As a result of not using the entire flash pulse, light meters will not produce the proper reading.  All those fancy Guide Number calculators that are built into your flash are also no longer accurate.  You need to build your own GN tables for every shutter speed.

Here is what I did.  I placed my off camera flash at a measured 10' from my grey card.  I set my camera up so that it framed the grey card only.  At each shutter speed, I adjusted the aperture to place that histogram spike as dead centre as possible.  The result was aperture multiplied by 10' equaled the GN for that shutter speed.

At ISO 200 and my flash head zoomed to 85mm, here are my results.  You can find this chart typed into my BlackBerry, it's with me all the time.

1/320th = GN 160
1/400th = GN 140
1/500th = GN 140
1/640th = GN 140
1/800th = GN 130
1/1000th = GN 110
1/1250th = GN 110
1/1600th = GN 100
1/2000th = GN 90
1/2500th = GN 90
1/3200th = GN 80
1/4000th = GN 71
1/5000th = GN 63
1/6400th = GN 56
1/8000th = GN 50

For comparison, full power in FP mode at 1/1600th of a second has only enough output for GN40 instead of GN100 using the hack.  That's huge!

What are you going to do with all that extra power?

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