Double exposure, Repeating flash and a ballerina

November 28, 2014  •  Leave a Comment

Stop MotionStop Motion. Double exposure, single strobe and Repeating flash
I recently completed a couple of shoots for the National Ballet Studio, who are based here in Dubai. When I was offered the shoots, I jumped at it, as it was a chance to get really artistic and creative again, and it was something totally different to the headshots and corporate portrait work I do normally.
I had a few ideas I wanted to shoot which involved movement, and put forth my ideas to the Directors, Helen Ainsworth and Elizabeth Johnson, during our pre-shoot meetings earlier in June this year. They bit.

One of the shots which I had envisaged would incorporate a double exposure, Repeating mode flash, together with single strobe, in order to portray a static and dynamic element within the same image. Great idea, but something I had never attempted before (I did not let on to this at the time however...). Cue the deep end, and the manufacturers manuals (Yes, I keep mine...somewhere)

Now to shoot Repeating mode (stroboscopic) flash, you need to do it in total or as near to total darkness as possible. You DON'T want ambient light.......
Helen and Elizabeth had originally booked a proper theater for this shoot, which guaranteed the required lack of ambient, but as luck would have it, they were dumped due to a double booking. Plan B -The media hall at a local golf club. The only problem was that it was a light and airy hall. Definitely not great for a shot which required almost total darkness....... however, Helen's husband was included in plan B. He works for a company who manufactures black drapes and hanging systems for theater productions, and it wasn't long before he had put up a 8 meter length/width by 3 meter high black fabric box for me, with black fabric for flooring (it wasn't perfect, as there was still a LOT of ambient light coming in, but it was what we had to work with). We all get thrown curveballs, we just have to learn to deal with them.

The equipment I needed to create the image was the following - for the Repeating mode element I would have to use my Nikon SB910's, triggered by a Nikon SU800 commander. For the static element I would use my Profoto 7B pack, one head with a Profoto 3x4 softbox, triggered by Pocketwizard Plus 2's.
The SU800 is a great trigger for the SB910's, as it has a Repeating mode Commander function - dial in all the settings from the camera position, whilst the PW's were the best for triggering the 7B pack - nice, simple, extremely reliable.
I didn't want to sit in Photoshop for hours editing and blending separate images, I try as much as I can to do it all in camera, so I knew the way to get the shot was to do a double exposure in-camera. Thankfully my Nikon D800 has a double exposure setting....and it was back to the manual I went. And doing a double exposure explains the why I needed to use two different lighting setups with two different triggers - I needed two different lighting setups whose triggering systems wouldn't interfere with each other whilst making the double exposure. Yes, I would need to swop triggers mid-exposure, but the one wouldn't set the other off.
I had fashioned a rail, about 6 meters long, elevated on Avenger C-stands, on which I clamped 4 SB910s's about a meter or so apart from each other. They all had their diffusion domes attached, and the rear-facing portions of the domes were gaffered with black tape. I wanted all the light going forward, and as little as possible coming back towards the camera. The rail was raised so the heads of the speedlights were positioned about three feet or so from the top of Jenna's head (our ballerina in these images), and she had to basically remain just under and parallel to the speedlights at all times.The sensor eyes were all pointing back towards the camera (the SU800 uses IR to trigger. Have those windows out of line-of-sight of the trigger and the only thing you'll capture is a look of surprise on your face).

Technically, there were quite a few things to consider:
ISO - I had to keep it as low as possible to help minimize the ambient, yet balance that with the need for the power setting and hence recycling time of the speedlights. The higher you put the ISO on the camera, the less power you need from the speedlights, and recycling times are quicker. Quite important when using Repeating mode. Also, I didn't have external battery packs for the speedlights, so the drain on the batteries in the speedlights would be quite significant.
Aperture - I settled on f/8, as I was anticipating some movement from the Jenna, not just parallel with the rail, but also some front to back movement as she completed her routine, and f/8 would give me some leeway as far as DOF went. f/5.6 would possibly have been too shallow with all the movement, and f/11 would have forced me to up the power on the speedlights - more power drain, greater recycle times.
Shutter speed - The static strobe portion was easy - 1/1250th, and that killed any ambient. The other part, the Repeating mode flash, was a little more involved. I eventually settled on 3 seconds, and here's why:

The main factor was the dance routine timing. Jenna had to do a routine that would cover the portion of the rail below where the speedlights were mounted. I wouldn't trigger immediately as she started her routine, but only as she was in range of the first light. She then had to continue parallel to the rail, and complete her routine to end below the last speedlight, on a spot that I had marked out where the metered Profoto 7B with softbox would be waiting for her. That was close to 3 seconds as you can get. And then she had to remain perfectly still while I changed triggering systems to trigger the 7B pack. Not a lot to ask of poor Jenna, who was already closed to finished from all the previous shooting.
So knowing what time I needed for my shutter speed, I took a test shot at 3 seconds with my aperture set at f/8, and was not surprised at all to see that ISO400, which I had hoped would be OK, was not even close to being OK. Way too much ambient light in the exposure.
So I dialed the ISO down, test, dialed down, test, and eventually settled on ISO125. I didn't kill the ambient totally with the aperture and shutter speed combination, but it was as close as I was going to get considering the power setting and recycle times of the speedlights. I had originally set them to 1/64 power, but with the drop in ISO, I had to increase their output by a stop to 1/32 power. I knew I would be about a 1/3 stop under in exposure off the speedlights due to the ISO setting, but I was not willing to increase their output again to the detriment of recycle times. I was shooting in RAW, so I had that flexibility to push a bit in post-production if I had to.


When it comes to Repeating Flash mode, there is a formula to use to determine your shutter speed:
Number of times the flash fires in total before it switches off / Hz (number of times the flash fires per second) = Shutter speed
I had the shutter speed determined already, and I figured that I wanted each flash to fire 12 times (plain thumbsuck), so the Hz worked out to 4. Each flash would fire 4 times per second for a total of 12 times over 3 seconds. But I decided to push it a bit and dialed in 5Hz/12times/3 seconds. And believe it or not, the SU800 let me do it, and it worked!! When I tried 6Hz, the Commander unit just blinked at me. It'll only let you push your luck to a certain point....
I did try experimenting with the settings a bit regarding the Hz/times, but eventually decided that the weird combination was just where I needed to be.

Here are a few test shots we did trying to get the balance right, everything the same except for the Times/Hz combination

2Hz/6 times test2Hz/6 times test
This was image was taken at 2Hz/6times, Repeating mode, single frame only

3 Hz/9 times3 Hz/9 times This image was taken at 3 Hz/9 times, Repeating mode, single frame only. You can see the marked difference in upping the Hz/time combination.
I hit the shutter button a bit early, thus Jenna didn't make it through the frame totally. But you can see how much closer her body is spaced in the image as compared to the one above, even though the routine was slightly different.
5 HZ/12 times. Why do it by the book? And here is the combination that I originally decided on. 5 Hz/ 12 times for 3 seconds. Technically, according to the book, it should be 4Hz/12 Times.....

So then it was down to the final shot, double exposure at 3 seconds, and then at 1/250sec. We had the start and end points marked out with tape on the floor, and just after Jenna started her routine, I triggered the Repeating mode using the SU800. Jenna ended up a little off the end point, but now had to remain perfectly still in her end pose while I switched to the Pocketwizard, changed the shutter speed to 1/250 and took the second part of the exposure firing off the 7B pack.
You can see in the final image at the top of the post that there was perhaps a slight timing issue, as her tutu on the left side of the Repeating flash portion is not as correctly exposed as I would have liked it to be, but I love the how the double exposure integrates the two "frames" together. Overall, I am extremely happy with the image, and my client even happier. And that makes for a good day.

BTW: I always shoot tethered (Capture One, Tether Tools) into my laptop when I'm on location. Here's an interesting fact - the D800 will not let you use the double exposure function when tethered. I tried to select it, and it was greyed out. Some head-scratching ensued, I disconnected the tethering cable, and there it was, ready to go. I'm not sure what it didn't like, perhaps it'll work with wireless tethering....but if you ever encounter the same scenario, there's your solution.



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