The radio trigger paradigm – Broncolor RFS2.1 & Phase One Flash Sync issues
The Broncolor Move Flash Pack system, and the RFS 2.1 radio trigger seemed a great idea combined with the Phase One 645DF+ camera/IQ160 and leaf shutter lenses. A great combo, allowing flash sync speeds of up to 1/1600 second. And yet there are often surprises.
Why use flash sync at high speed in the first place ?
Working in full sunlight, with objects against a bright background requires special measures.
First of all we need to adjust the exposure of the subject and secondly we want the keep the background brightness under control. In other words, the objects requires more light, where as the background will require less light.
The sensor should be exposed in such a way that the object is properly lit while keeping the bright background under control. Who does not know those picture where a person stands in front of a bright sky, and appears like a silhouette without details.
The only way out is to use a fill-in flash, lighting the subject. But even then the ambient light (bright sky, background) will can be blown out. The reason is simple, the flash duration is very short versus the CDD exposure time (shutter speed). Ambient light will continue to hit the sensor during the shutter open time. If that light is bright it will burn out those area’s despite the fact that the subject is now properly lit. One could increase the shutter speed to reduce the effect of the ambient light, or we could stop down the aperture.
Neither one will do the job; first the shutter speed is limited when using a flash, since the flash sync is typical between 1/64 to 1/250 second and that is still to slow allowing too much ambient light to fall onto the CCD. Stopping down the aperture is an alternative, however it destroys the shallow depth of field.
There is however a solution, using a camera that is capable of shooting at high shutter speeds and normal flash sync. Not to be confused with hyper sync or high speed sync (see my other write up). Flash sync is defined as the time the CCD is fully exposed with a shutter (curtains) that are fully open.
Phase One 645DF+ Camera’s allow for flash sync speeds of 1/800 second for the focal plane shutter and an additional 1/800 seconds in combination with a leaf shutter lens. Providing whopping 1/1600-second shutter speed sync.
Now we can reduce the shutter open time to reduce the ambient light while retaining a shallow depth of field, right?
Yes and no, when I tried the Flash sync with the Broncolor Move pack and the Phase One 645DF+, I noticed that while I was speeding up the shutter, the pictures were losing exposure. The flashback was untouched and set a 600 Watt/s. A faster shutter setting (below or equal to 1/1600sec) affected the exposure of the picture more and more.
It took me a while before I realized what was happening:
The Broncolor Move is triggered with a radio transmitter fitted on the hot shoe of the Phase One Camera. It is the RFS2.1. This specific transmitter has a delay of 425 microseconds. Could that affect the loss if exposure?
Indeed , a 1/1600 shutter flash sync speed exposes the full CCD to the light for 625 micro seconds. As soon as the CCD is fully exposed (shutters fully open) the hot shoe creates a short . Flagging that a flash is needed. The RFS2.1 Broncolor transmitter has a transmission delay of the flash to 425 microseconds. That leaves a very small window for the actual flash pulse (200microseconds). The Flash pulse must fit within the remaining 200 microseconds. After that the shutter starts to close . If the flash pulse is exceeding the 200 micro seconds, then that light is lost.
Broncolor Move packs have a flash duration that varies with the selected power as indicated below (figures for eco mode):
- 1200 watt/s = 2667 microseconds
- 600 watt/s = 1143 microseconds
- 300 watt/s =763 microseconds
- 150 watt/s = 465 microseconds
- 75 watt/s = 382 microseconds
- 38 watt/s = 333 microseconds
- 18 watt/s = 267 microseconds
- 9 watt/s = 190 microseconds
- 5 watt/s = 143 microseconds
As can be seen, a power setting bigger then 9 Watt/s will result in the flash pulse to fall partial outside the 200 microsecond window. Hence the shutter will start to close before the flash is fully completed. The result is an incorrect exposure.
Going through the same calculation without a radio delay , and using a flash-cord creates a flash window of 625 microseconds. Hence power settings on the pack above 150 Watts will start to have some exposure loss.
Of course at lower flash sync speeds the problem is far less of an issue. The only solution is to use either the flash cord or the build-in profoto radio trigger.
The V-grip of the 645DF+, contains the radio trigger and can be set to “fast” mode. In the fast mode the trigger/transmitter in the V-grip is constant on and as such does not incur a transmission delay like the Broncolor trigger RFS 2.1 .
Shooting with other pack types and radio triggers will have a similar problem, the best thing is always to check the transmitter delay and knowing your flash pulse behavior as outlined in this text.
Thanks for reading,