Friday, 28 May 2010

High Speed Sync: Nikon Options (Revisited)

If you've ever needed to kill ambient midday or required to freeze action, then you'll know that insane shutter speeds are your friend. However, less friendly comes when flash is thrown into the mix.

With Nikon bodies and SB units you've got 3 options to acheive high speed sync (HSS) or auto FP sync (as per Nikon literature): CLS, radio triggers and optical slaves.

A little background first: The reason why insane shutter speeds (well, anything over the body's native max sync speed) is problematic is down to the way the sensor is exposed by the focal plane shutter curtains. Simply put, no mechanical dual curtain shutters can travel at those high speeds.

To overcome this, HSS closes rear curtain whilst the front curtain is still moving across the sensor and an auto FP SB unit (SB600,800,900) fires pulses synchronised with the travelling curtain slit which forms the final exposure.

Great stuff. However, with great things theres always a catch: for the camera body to enable auto FP mode (moving shutter curtains at high speed) an auto FP compatible SB unit must be mounted on the hotshoe OR CLS must be used. If the body does not detect a HSS SB unit on the hotshoe and high shutter speeds are used with flash, the rear curtain will be visible in your frame.


This is relatively straight forward with the higher end Nikon bodies that have in-build commanders or with the optional SU-800 unit. The body uses the popup flash as the commander and the remote auto FP SB unit pulses in sync with the shutter.

So why not just use CLS for HSS? Well, its the same reasons why people select wireless radio triggers for non HSS, coupled with the fact that a lot of off camera SB units are older/non auto FP enabled (ie SB80dx and older).


Now, this is where it gets a little interesting. As mentioned, CLS will instruct the SB unit to pulse several times over the exposure but for the other options this isn't the case. During the exposure, non-CLS SB units will flash once as per usual. However, the important thing to note is the flash duration - that is the length of the flash pulse. When the bodies a sync'ing at slower speeds such as 1/250, the exposure captures the entire flash pulse, from the start, ramping up to full pulse and then tailing off: as power output increases, the flash pulse duration gets slower - this is a good thing for acheiving HSS.

To put this into context, the SB80dx (approx) flash durations as spec'd in its manual:
  • M 1/1: 1/1050
  • M 1/2: 1/1100
  • M 1/4: 1/2700
  • M 1/8: 1/5900
  • M 1/16: 1/10900
  • ...
  • M 1/128: 1/141600
The list ignores the flash durations faster than 1/11000 of a second because the Nikon bodies, all the way up to and including the professional D3 series, only go to 1/8000 shutter speeds. Why is this important?

For the purpose of HSS we can almost consider the flash pulse as a continuous light source. This means that flash pulse that is faster than the fastest shutter speed is no use to us. In practical usage, its only flash durations slower than 1/4000, when shotting at 1/8000, that interest us due to the time taken to get to full pulse ouptut.

So how does this apply to our usual wireless radio triggering tools?

Wireless: PW PlusII

We have already said that the camera needs to forced into auto FP sync mode: that will be at least a SB600 on the camera hotshoe. But we rarely, if ever want on camera flash to be part of the final exposure - this means we set the output of the on camera SB unit to be at minimum output and most likely pointed away from the subject (back over the photographers head for instance).

With the hotshoe occupied, the PW unit needs to be connected via the camera body's sync port or the SB units sync port (if using SB800/900) to the CAMERA/FLASH socket (not the FLASH socket).

The remote flashes are connected as normal to their PW units but with the caveat of power ouptut's flash duration being slower than the selected shutter speed.

Wireless: Remote Optical Slave

Same story goes here. The only issue is that the on camera SB unit must be visible to the slave unit. However, given that the most likely use for HSS is to sync outside in midday sun, optical units may have trouble getting slaved. This of course is the same with normal sync speeds.

Science Over: Application

As stated, this technique is useful for matching the sun, allowing for wider apertures and less power required. Remember that the sunny 16 rule holds, this means that we can get away from shooting 1/250 f/16 ISO200 ambient only exposure. A SB80dx in a shoot through umbrealla 3ft away at full power gives around a f/11 exposure.

So with our umbrella'd SB unit, 1/500 f/11 will match ambient, but we can then bring thing down a few more stops, and away from full SB output for shorter recyle times.

1/1000 f/8 requires 1/2 flash output, matching ambient, but to keep the skies bluer will require another stop down so we'd be going at 1/2000 f/8 ISO200, SB @ 1/2 power.

The limiting factor is the flash pulse duration and not so much the power output available. We need to remember to make sure that we stay at the hight SB outputs to ensure that the flash pulse forms part of the exposure.

Key References

Strobist flickr group discussion topic
Jerry PH blogpost detailing his usage

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