What’s Up With “Flash Duration”?

Flash Duration?

A topic of great discussion around the studio lately has been flash duration.  What is it exactly and what’s all the hype?  Two things have become clear – it can’t be had without compromise, and there are a lot of terms on the chalk board.

In simple terms, studio strobes function by filling slowly with a bucket of electricity and pouring it quickly out through flash tubes.  The blink we see is one quick splash of light, but the reality is a burst of light that builds quickly and then dies away over time, a power graph shaped like a reverse skateboard ramp that is anything but click on/click off (the artwork above was inspired by studying countless graphs of luminance over time).

So there are challenges to describing the length of a typical studio strobe pulse.  Indeed, the complete total time would be somewhat unimportant a measurement, because the metered time would include the fading tail end of a curve while capacitors and tubes smolder off the last drops of power, with some portion of the total flash time affecting little the final exposure.  Indeed a cutoff is needed.  The real questions is: for how long is a given strobe putting out a useful amount of light?

t0.1 and t0.5

Most distributors and manufacturers adhere to one of two standards for reporting flash duration stats.  “t0.1” is a measurement of the time during which a flash pulse is at or above 10% of peak intensity.  “to.5” is a measurement of the time during which a strobe pulse is at or above 50% of peak intensity.  The chosen cutoff point, whether 10% or 50% (or occasionally 30%) is designed to limit duration numbers to an accurately measured, repeatable figure that describes the length of time a strobe is near peak power and gives a workable idea of the brevity of a pulse.

Certainly both t0.1 and t0.5 reporting have advantages.  t0.5 numbers look faster on paper, and it is a closer measurement of the still briefer time a flash is at full power (not containing the same length of trailing fall-off).  t0.1 has the distinct advantage of being a more accurate measurement of the actual length of a pulse, though a portion of that length includes a relatively low amount of actual light.

The rule of three

In practice t0.5 numbers are roughly three times faster than t0.1 numbers.  Broncolor, as one example, lists the duration numbers of its Senso A2 1200 (a pack we reviewed here) as “up to 1/600 s (t 0.5), 1/180 s (t 0.1)”.  For the same pack listed by two different cut-off points, then, the t0.5 value is roughly three times faster.

Editor’s Note – the rule of three does not apply as easily to Broncolor’s current crop of high end packs.  In units like the Grafit and Scoro, a digital cut off circuit can be engaged, trimming flash duration (along with power).  As the trimmed power curve is more plateau-like, with a less tapering fall-off, t0.1 and t0.5 numbers are much closer.

Studio Strobe Exposure Readings, by Shutter Speed

The above chart tracks a falling metered strobe exposure as shutter speed increases.  The max sync speed of contemporary cameras can range from 1/125 (Mamiya 645DF without LS, Pentax 645D) to 1/200 (most Canon DSLRs) to 1/250 (most Nikon DSLRs) to 1/500 (Hasselblad H cameras and Leica S CS lenses) to 1/1600 (Mamiya/Phase One’s Schneider-Kreuznach leaf shutter lenses).  As illustrated here, though, actual power available during a brief exposure might be less than expected, especially at high power.  The various lines are measurements from different power settings, showing that lower power settings suffer least from problems caused by flash duration.  The test pack, in this case, was a Speedotron 1205CX.

Duration vs. Power

The kicker is the “up to” prefix to any duration measurement, including the most exotic of high end power packs.  Indeed, governed by the simple physics of charged capacitors dumping electricity, duration is always inversely related to power.  Every flash on the market, from the $120  Nikon SB-400 to the $12,000 Profoto Pro 8a Air 2400, offers quickest duration at minimal power and longest duration at maximum power.  When Speedotron tells customers a pack can deliver up to 1/1250 second flash duration (as they did with the new 1005 pack, reviewed here) such speed is only possible with the strobe set for minimum power.

The traditional method, then, of getting snappy strobes to freeze spinning fashion models, flying water droplets, bouncing children, and other crazy action, is to use large, powerful strobe packs, and to turn them way way down.  Many strobe makers, therefore, offer heads with multiple power inputs.  Bi-tube or even quad tube heads, can suck the juice from multiple dialed-down packs to crack off fast pops with actual power.  Check out the Profoto Pro 7 Twin, the Broncolor Pulso Twin, or the mighty Speedotron 105 Quad Tube for examples of multi-tube heads.

Max power duration and shutter speed – At the other end of the pack power spectrum, duration stats are worth noting for a different reason.  At full power, many flash systems are slow enough to have their light output clipped by quick camera shutter speeds, affecting exposure.  The above chart shows the relationship between shutter speed and recorded strobe light.  Notice the readings fall off dramatically for high power settings.  This would be a major consideration for photographers using cameras capable of syncing at 1/500 s or higher, but even at 1/250 s, not an uncommon max camera sync speed, the pack tested here is incapable of delivering its full power.

In conclusion, a few tips

It’s a strange game of compromise, then: interpreting the stats strobe makers give us, choosing the gear we use, and dialing in the right blend of speed and power.

General rules include these:

always determine the units being presented.  Are stats given in t0.1, t0.5, or t0.3?  Is the total range of flash duration given or just a maximum?  As a general rule, a t0.1 duration is three times a t0.5 duration, and fastest speed given is only available at minimum power.

In selecting studio strobes, it’s important to decide if duration is important, because, along with recycle time it’s a costly stat.  Photographers dealing with subjects that are often in motion, will want to pay close attention to duration numbers.  Long flash duration could lead to blurred images or subjects with odd trails, though the payoff is high cost and bulky gear.

Once a photographer is on set, shooting, it’s important to remember strobes are at their quickest when power is at its lowest settings.  If images are falling victim to motion blur (blurred details, trailing edges, or less-than snappy objects in motion) dial packs down.  The difference will have to be made up with aperture, ISO, or light placement, and a close eye will have to be kept on ambient light, but the strobes will be working at their fastest.  Need to freeze really speedy subjects?  Consider buying, or renting a big pack that both has impressive duration numbers and power to spare, since you might be often relying on a very low power setting.

It’s been fun to delve in to the many issues surrounding flash duration, and I’m happy to say our efforts at Photo Arts Monthly won’t end here.  In upcoming strobe reviews, we’ll be adding our own duration measurements, taking a close look, that is, at what these pricey boxes are really putting out!  I look forward to it!

A special thanks to the following industry experts who were interviewed for this editorial: Colin King of Broncolor, Mark Rezzonico of Profoto/Mac Group, Randy Hoffman of Keeble & Shuchat (a Northern California Profoto dealer), and Tim at Pro Camera, San Francisco’s premier photo gear rental house.

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by Matt Beardsley

 

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