May 27, 2010

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

The following question came recently:

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

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)., 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

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 ASCommercial Photographers


    Jase said...

    Great article and very thought provoking.
    Something I'm struggling to work out right now with my SB-800 but failing miserably...

    Glyn Dewis said...

    Very useful article Kevin.
    Very useful to know a little more about what the strobes are capable of rather than just diving in and buying the latest model without fully knowing what it's offering.


    Kevin Winzeler said...

    @Jase - I didn't talk about speed lights, but should have mentioned one thing: The flash duration of speed lights gets SLOWER as you INCREASE power. So an SB-800 or 580EXII at Full power has a longer flash duration (around 1/800th of a second). However, due to how speedlights cut off the light at lower power settings (shorter curve in the graph above), they have an incredibly fast flash duration as you decrease the power. This is one advantage of speed lights and why you see a ton of speed lights at lower power settings used for stuff like hummingbird photography, a water balloon bursting, or trying to stop a speeding bullet.

    580EX II - flash to sensor 1-3 feet roughly:

    Manual at 1/16 power = 1/15,000 second +/- 25%
    Manual at 1/32 power = 1/19,000 second +/- 25%
    Manual at 1/64 power = 1/31,000 second +/- 25%
    Manual at 1/128 power = 1/35,000 second +/- 25%


    Frozen Forever Photography said...

    Hey Kevin,
    I understand why most flashes have a faster duration at lower power settings. However, I dont understand why the Rangers speed up as you increase power. Does this apply to most power pack set ups?


    Kevin Winzeler said...

    A lot of pack systems and monolights actually work this way (shorter flash duration at greater power). Not sure exactly why. The message is to check and know what your system does and/or before you buy to think about what you need and how the system you buy fits that need.

    Frozen Forever Photography said...

    Thanks for brining this up. I checked all my gear agian and it looks like I have a mix of strobs and durations. My Q-flash & 120j's work like a sb-800. However, my white lighting and alien bees work like your rangers.

    I now am going to make a cheat sheet for each strobe.

    thanks again,

    Sebastien said...

    Hi Kevin,

    at the end of your post you mention: "In the next blog post we'll discuss using strobes with ambient light, on-location".
    Have you actually wrote something yet ? I would like to know more about using my Ranger RX speed A over the sync speed of my 5DMKII, is there any tips to work faster than the 1/200th under the bright sun of New Caledonia island (where I leave) ? Thanks for sharing, I like your work man !


    Anonymous said...

    Very Interesting!
    Thank You!

    Kevin Winzeler Photography said...

    Hi Sebastian:

    I have the content, but have yet to put together the "ambient stobe" post. Thanks for the reminder however!

    1/200th is about the max with the 5D. I've tested other cameras (1D series) with the Elinchrom Quadra (not Ranger) hooked up via Pocket Wizard FlexTT5 to achieve higher sync speeds (in the 1/800th range), but can't seem to achieve much higher than the standard with the 5D due to the larger sensor size.

    You can always use ND's on the lens if you're trying to achieve a wider aperture and shallow depth of field (I recommend the Tiffen IR ND series for least amount of contamination).

    I'll be back in NYC for PhotoPlus late October and will meet with the Pocket Wizard/Elinchrom folks to see if they've had any success on the larger sensor cameras. Stay tuned.


    Anonymous said...

    Great post, I am almost 100% in agreement with you