Buying Strobes? Do you know the Flash Duration? (How to Stop Action in Photography)

The following question came recently:

"Kevin,
I am a big fan of your work after seeing you on strobist, I am now doing quite a bit of adventure oriented advertising photography. I hope you can answer one question for me.... I am deciding between the Elinchrom Ranger RX Speed AS pack with "S" freelight head or Ranger RX pack with "A" freelight head. CAN THE "S" FREEZE ACTION FAIRLY WELL OR DO I HAVE TO GO WITH THE "A"? I understand you are very busy but hope you can lay a little info on me.
I use Canon 5d II, 16-35 II, 24-105, 70-200 f4. pocketwizard plus ii's
Thanks!"

This is a great question because it addresses one of the most important, yet overlooked items to consider when buying a new set of strobes, especially for sports photography, that of Flash Duration.  The key difference between the two heads in question is the flash duration, the "A" head being twice as fast as the "S" head.  Flash duration, just like shutter speed, is measured in fractions of a second and becomes your action stopping measurement when working in low light or dark (studio) situations. Typically, you want the shortest/fastest flash duration possible, just like you would want the fastest shutter speed possible when shooting and trying to stop action.

To visualize this, find a room in your house that you can turn completely dark (no ambient light leaking in).  Put your camera on a tripod and open your shutter for 30 seconds.  Now, let's say during your exposure, the fastest man in the world, Usain Bolt, runs in front of your lens and just as he passes by, you fire your strobe.  Will he be in focus, blurry or a combination of the two?  Remember, your shutter is open for 30 seconds, so wouldn't he just be a blurry abstract blob in the final image?  The truth of the matter, is that the action freezes based entirely upon your strobe (remember no ambient light) and the accompanying flash duration of that strobe, or in other words, how QUICKLY your strobe goes on and then off.  So if your strobes flash duration is 1/125th of a second, Usain is going to be blurry!  So what is your strobes flash duration?  In the case of an Elinchrom Ranger it is a combination of which head, pack and power setting is used.

If I were shooting Mr. Bolt in a studio with my Ranger AS Speed pack, A head and Full power on the B channel (366 w/s), the absolute best (shortest) flash duration I could achieve would be 1/1700th of a second, plenty fast to stop the action (see my note on using the t.1 rating instead of what the mfg suggested t.5).   However, with the same settings but switching to the "S head" my results would be 1/850th of a second.  Now, what if I needed full power from my pack (1100 w/s)?  Note the chart above with 1 A head @ Full power = 1/2300 s and 1 S head @ Full power = 1/1250 s.  For the A head, my action stopping power has been reduced from 1/1700th to 1/766th and S head from 1/850th to 1/400th, which starts to creep into the blurry zone for action stuff on the S head.  Lower power settings would result in an even slower duration and more chance for motion blur.

Skiing photograph shot during the SLC shootout. Skier: Jason West.  4 strobes used in total: 2 Elinchrom Ranger RX Speed AS Packs, 3 "A" heads and 1 Canon 580EXII fired via Skyports.  Lighting set-up here The action in this shot is entirely stopped by the strobes (roughly 1/2000th of a second flash duration).

Phew...so, what does all of this mean in the real world and why is it important?  If you are using strobes while shooting ANYTHING moving, then just like knowing what shutter speed to pick to create a sharp, crisp image, you should also know what flash duration you'll be getting from your strobe:

-hummingbird feeding
-gymnast during a floor exercise
-outdoor portrait of model on a windy day (blowing hair)
-pouring a bottle of Coca-cola into a glass for a product shot
-cyclist descending Cormet de Roselend during the Tour de France


IN SUMMARY:
1.  Flash duration stops the action and becomes your shutter speed (in low-light and dark settings)
2.  Flash duration changes (typically is fastest at full power)- check mfg specs
3.  If your strobe manufacturer uses the t.5 number, divide by 3 for the true stopping power of your flash.

So, what's the answer to the question?  For the most flexibility stopping motion, get the "A" head.  This is one of the main reasons I chose the Elinchrom system because most of their lights have very fast flash durations at a reasonable price.  However, just like buying any piece of equipment, consider why you need the faster flash duration head and how it will fit into your work as a photographer and buy what fits your needs.

And one final CURVE BALL to end on:  If you shoot location work during daylight (much of what I do) then your true stopping power becomes a combination of your cameras sync speed and your strobes flash duration OR worse yet, entirely dependent on your cameras sync speed.  In the next blog post we'll discuss using strobes with ambient light, on-location.

-Kevin Winzeler
Advertising and Commercial Photographer based in Salt Lake City, Utah
Tags:  Elinchrom, Ranger RX Speed AS, Commercial Photographers