By Ellis Vener
Shortly after turning in my review of the PocketWizard ControlTL system, I took delivery of a PocketWizard PowerST4 receiver and had a chance to try it out with an Elinchrom Style RX300 monolight, courtesy of the rental department at Professional Photographic Resources in Atlanta.
Before we proceed, I want to quickly recap the difference is between the two ISO standards for flash duration measurement and why these numbers are important to you, especially if you want to use a high-power flash at settings faster than your camera’s X-Sync limit.
An electronic flash works by releasing stored electrical energy into light and heat during a very brief period of time, but unless it is an IGBT-controlled flash (most hot-shoe-mounted speedlights are IGBT controlled, as is the Paul C. Buff Einstein 640, the Photogenic Solaires, and the Broncolor Grafit and Scoro pack and head systems), the light intensity varies varies during the time the flash is firing. The rise and dropoff of the energy release resembles the cross section of a powerful wave: there’s a near vertical upward slope rising to the peak output level, followed by a trailing tail of declining force. With IGBT-controlled lights at any setting below full power, the light cuts off sharply, depending on the flash's programming.
When a flash manufacturer advertises or provides specs for its lights, they can use either of two ISO standards for measuring the flash duration. The most commonly used standard is t0.5, which measures the length of time the flash is producing light at or above 50 percent of peak intensity at a given power setting. The other standard is t0.1, and it tells you how long the flash is emitting light at or above 10 percent of peak intensity. Another way to think about this is to think of the t0.1 measurement as being equivalent to a shutter speed setting’s ability to freeze motion. If you want to know how well a flash can freeze motion, unless t0.1 is specified, multiply the advertised F.D. by 3. In other words if the advertised F.D. is 1/900, the t0.1 will be close to 1/300.
Although a t0.5 measurement is around three times shorter than the t0.1 measurement with non-IGBT flash gear, unless your camera’s sync speed is equal to or shorter than the t0.5 duration you’ll still be getting a lot of photographically significant light. Depending on the scale of subject movement relative to the photo’s framing you can get a combination of frozen motion with a blurred edge.
While you might expect a full power setting to take the most amount of time to discharge, due to the way they are engineered, reducing energy levels on most monolights actually lengthens the flash duration no matter which standard you measure it with.
Take the Elinchrom Style RX 300 monolight for example. While it generally has short flash duration, at full power (300 watt-seconds) Elinchrom says the t0.5 F.D. is only 1/2,850, but at the minimum energy setting of 9.375WS (1/32nd), the t0.5 flash duration more than doubles to 1/1,350. The same is true for the similar-power AlienBees B800 monolight: 1/3,300 at full but 1/1,650 at 1/32nd, and Profoto D1 250 monolights: 1/3,700 at full and 1/1,400 at minimum (1/64th).
Why is this particularly relevant to the ControlTL system? If you are shooting at your camera’s x-sync speed or below, it isn’t, but when you start using the ControlTL system’s ability to let you use non-smart speedlight flash at shutter speeds higher than a camera’s x-sync, the length of flash duration sets an upper limit on the shutter speed range above X-Sync that you can use. While in my book, short flash duration is generally a good thing, it can become a limiting factor.
For the portrait of Kirk Tuck in the main ControlTL review (Professional Photographer, September 2010) I used a battery powered Profoto AcuteB 600 as the main light and a Canon 580 EX II as a kicker from the opposite angle. At full power the AcuteB 600 has a t0.5 of 1/1,000 (meaning the t0.1 is about 1/333). This meant that the flash was emitting light for a long enough time that at shutter speeds up to 1/2,500 I could evenly expose the entire frame even though only a narrow section of the sensor in the Canon EOS 1D Mark IV was being exposed at any given instant.
But using the same camera and MiniTT1 with a Elinchrom Style RX 300 at full power, with either a PowerST4 receiver connected to the Skyport, or a FlexTT5 connected to the regular sync connection with the same camera, the highest full-frame sync speed I could get on the same camera was 1/500th, and that was only after much experimentation finding the best HSS setting for the MiniTT1. While going to minimum power would have substantially increased the flash duration allowing for very short shutter speeds it would have made a whole lot less light available. Changes in camera firmware can also affect the timing of the sync pulse when the camera thinks it is in Hyper Sync Speed or FP mode.
Moral of the story: Know your light!