Flash Duration

Yesterday I had the pleasure to assist at the Mark Wallace Pocket Wizard tour, which made a stop at SPA studio in Seattle. We showed all the folks that showed up how to shoot the new PocketWizard Flex and Mini in TTL mode.

Mark also brought a Profoto Accute 600B with him to demonstrate the difference between studio strobe and small flash, and we spent a good two hours going over some fundamental of lighting. Nice set of information Mark shared for those that are still new to the topic, and there are always a few nuggets even for the old hands.

One of the issues he brought up, and that made me do a bit more research is flash duration, and why it matters. All in the quest of always producing the best image possible, and knowing all the twists and quirks of your gear to get the max performance out of it.

Incidentally, of the images I shot earlier this week of Terra, two had movement / action in them, and I noticed some amount of motion blur when magnifying the shot during retouching. Not enough to be a problem. But it shows that I didn’t have full control of that part of the shot, something that shouldn’t have happened.

They were shot at 1/200 @ f/13, 82mm. That makes it save from a handshake perspective. Since we were in studio, all the motion stopping is really a function of the flash duration, and it wasn’t fast enough to fully freeze Terra who was making a fast move. So that brings me to flash duration and controlling it, and how the different equipment produces different results. After researching this, I found that I was not in the optimal range of my gear, and that I could have given myself more margin.

Flash duration is a function of the design of the flash. And most flashes will have different flash durations at different power settings. In general more expensive gear gets shorter durations, which is good, which was the point Mark made during the presentation yesterday. However, in looking at different specs, it turns out there is a lot of improvement that can be gained by just understanding your gear.

So let’s look at various gear choices and their respective flash durations:

First lets look at the Canon 580EXII since that was the topic of discussion yesterday: Based on the Canon manual, flash duration is 1.2ms for a normal flash – or shorter. That comes out to 1/833s presumably at full power. They don’t publish the same specs as studio flashes, so some of this is a guess. Most people guess that a 580EXII comes in at 50Ws or thereabouts.

Now lets look at the Profoto Accute which I was shooting with earlier this week. It has a power output of up to 2,400Ws and the flash duration varies between 1/320s to 1/3200s. It turns out the Accute 2400 has worse flash duration specs, even at the same energy setting than it’s smaller borther the Accute 1200. It is worst at the full 2400Ws (meaning channel A+B) with 1/320s. At the lowest setting of 75Ws it comes up to 1/1800s. At that same power the smaller brother Accute 1200 comes up to 1/3200s.

I don’t recall exactly how I had my light configured, but I do know that the larger fill light was on the 2400, and I believe at least at half power. So that would have put me into the 1/560s range and definitely not optimal. Also turns out, reading the manual that the bracket control affects flash duration negatively, and should be set to –1. Another option is to use the Accute Twin head, which can achieve the same flash duration at higher power because it uses two tubes, which provides it more efficiency. But it doesn’t get faster than 1/3200s. So lesson learned – put the motion freezing light on the 1200, dial the power down if possible, by being less aggressive on the aperture and maybe by compromising on the ISO setting if needed.

To round out our look at flash durations by gear, let’s look at other Profoto models: The D4 1200 has flash durations of 1/1000s to 1/7500s but at a steep price premium. Going one step higher to the Pro 8 that becomes 1/2200s to 1/12000s. Awesome motion stopping, but very expensive. Interestingly enough the Accute 600B, the battery version, actually has better performance than the AC version. It comes in at 1/1000s at full power of 600Ws, and goes up to 1/6800s at it’s lowest power setting.

Looking at the more popular AlienBees, the AB400 with 160Ws at full power is rated at 1/6000s and 1/3000s at 1/32 power, which is a mere 5Ws. The bigger AB1600 is rated at 1/1800s at full power and 1/900s at 1/32 power which is about 20Ws. It’s a bit surprising to see the higher power be a shorter flash, when all other gear is reverse. I always do take the ABs specs with a grain of salt, for all the magic math around ‘effective Ws’ they do, and these flash duration ratings seem too good be true without some caveats as well.

And for completeness, the Elinchrom Ranger, coming in at 400Ws, of between 1/3000s to 1/6000s with the Ranger heads, going down to 1/1300s to 1/3200s with the Quadra heads.

OK, that were a lot of numbers. Where did we end up after all this exercise:

Assuming that 50Ws is at the low end of your light needs, you can get 1/800s with the Canon speedlite. The smaller Profotos get you to 1/3200s on AC and 1/6800s on battery. The big Protofos all the way to 1/12000s. The Elinchome to 1/6000s. And for the ABs – well if you take the numbers at face value somewhere around 1/4000s probably. As you need more power, let’s say 600Ws you are out of luck on the Speedlite, as it doesn’t play in that league. The small Profotos get you still 1/1000s on battery or AC. The Elinchrom model I looked at only went to 400Ws, so it would be maxed out. And the AB1600 would now be at full power and at face value deliver 1/1800s.

What’s the conclusion of all of this? Actually simple: know you gear, use it correctly. Every once in a while it’s actually good to pick up that user manual and re-read it, you might discover some details you didn’t know. And that can make the difference between a clean shot, and something that could have been better.