The Swiss Fast Lane (top speed 1/8000th)3
neil posted in Photos, Neil, phototech, 5Dmk2 on January 9th, 2011
This post has been knocking around for awhile in the dark pipework behind this blog and in the dusty parts of my mind. It is unashamedly technical and really only useful for the folks who have access to an Elinchrom Ranger with an S type head, a pocketwizard TTL for Canon and a 5D (mk2). Other equipment will almost certainly work, but I can’t tell you how well!
About a year ago, Alex Ray from The Flash Center came through Edinburgh riding herd on David Hobby. He mentioned that with a certain Elinchrom Ranger head and the new “tweakable” pocketwizard TTL (mini TT1, Flex TT5) transmitter units there was a possibility of synchronising the flash burst and the open shutter far beyond the normal flash synch speed of a camera. This wasn’t the incremental increase found when using speedlights, but the ability to synch at any shutter speed, no matter how fast. Purely by coincidence I had the very equipment to do this already. All that was needed was to play with the HyperSynch delay on the transmitter.
For the stopping of fast objects with artificial light, the norm is to use a strobe with a short duration in a dark area and use the duration of the flash to act like a very fast shutter. This is limiting for a “location” photographer who only has a single cheap, slow, flash head.
The Pocketwizard and Elinchrom trick ensures the shutter will sample some part of the flash output. The shutter is open for less time than the flash is switched on for. At normal speeds, the flash waveform will be shorter than the time the shutter is open for. This means the flash exposure is not affected by the shutter speed at all. At the high speeds involved in this trick the shutter grabs a portion of the flash waveform. This means that the flash will be less powerful than normal and the flash exposure determined by the power of the flash (in two different ways), ISO, aperture and unusually, the shutter speed.
This is a touch unusual as shutter speed does not affect flash exposure in the normal (sub 1/200th) realm of flash photography. However in this case, it determines how much of the flash output is seen by the sensor. It also affects the size of any unexposed or dark bands, where the shutter is open but no flash is present, or where the flash waveform is rising or quenching.
The flash power not only affects the maximum light output of the flash, but also its duration (and thus shape). At full flash power it is possible to get a solid exposure of all but the very lowest part of the frame. At lower powers the flash exposure drops steadily with shutterspeed and the top of the frame sees a dimmer flash output while the lower part of the frame is cut off. It seems that this trick works at its best with the Ranger dumping its maximum output into your subject, your sensor will only see a small portion of this. That said, an Elinchrom Ranger has 1200Ws of power to burn, so with a bare head it is perfectly possible to work with the lights at a distance while outside.
The fact that it works in the presence of ambient light is probably the biggest advantage of this trick for me. Indeed, when working with a single head, it seemed the more ambient light present, the better. Working at very high shutter speeds drastically limits the contribution of the ambient light. In order to include ambient light with a shutter speed of around 1/8000th of a second, at a reasonable aperture, I was normally forced to abuse the high ISO abilities of the 5Dmk2.
The high shutter speeds means a wide aperture is possible in broad daylight. To make the big strobe look like daylight and to dump most of the full power, I tend to use the flash far away without any reflector. Big lights, far away is the opposite of the normal Strobist thinking but I find the results appear more natural at first glance. The light affects the entire scene very evenly, much like a low sun, however it is completely under the control of the photographer. I borrowed a 50mm f/1.2 lens and made some photos with it set wide open.
The ability to freeze fleeting details that are normally invisibly fast can add additional layer of interest to a photograph. I was struck by the ability of the flash to freeze translucent substances such as sheets of water. This worked best with the flash providing a backlight with fill being povided by the ambient. The reverse works equally well but it can be easier to arrange the artifical light behind a subject and it is easier to control the shape of the flash light with modifiers.
The magic delay value set in the Pocketwizard transmitter seemed to be -2500, the maximum allowed by the software. The reciever was a standard plus 2 Pocketwizard. It would be interesting to see what the optimum delay value was if Pocketwizard allowed a larger range to be set. The drawback is that at this delay value the use of normal hot shoe flashes is limited. A dark band at the top of the frame is observed from 1/250th with the exposed portion of the frame being relegated to the center half of the frame by 1/1000th of a second and closing to a tiny slit at higher speeds.