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Assembly and Operating Instructions for HiViz.com Kits

 

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Selecting a Flash Unit for High-Speed Photography

 

Assembly instructions for other kits

 

Criteria for selecting a flash unit

 

One of the most frequent questions I'm asked is what flash unit to use for high-speed photography. My answer is whatever flash unit you currently have or can afford if it meets the two criteria below. Here are the criteria:

  1. You must be able to adjust the flash duration to values low enough to "stop" most action. A duration of 1/20,000 s will suffice for most situations.
  2. You must be able to trigger the flash off camera through a connection to an external trigger circuit. That means that the flash meets one of these requirements: a) it has a PC jack to accept a standard PC cord, b) it can be actuated through a hot shoe adapter, or c) it can be triggered remotely by optical or wireless means.

You may have noticed that a criterion missing from the list above is the maximum brightness of light that the flash can provide. This is standardized through a specification known as the guide number, GN. The f/# for correct exposure is directly related to the GN at a particular flash-to-subject distance. That is, a greater GN means that you can use a smaller diameter aperture (larger f/#) at a given distance. I haven't been particularly concerned about this factor, because in most situations, I can just place the flash unit closer to the subject to increase intensity. This works if the size of the subject is small enough that the flash unit can provide full coverage of the region of interest. In situations where I've needed additional intensity or coverage, I've used two or more flash units synched to discharge simultaneously. With modern digital cameras, another approach is simply to increase the ISO, since noise at high ISO has become less of an issue with these cameras.

 

One thing not mentioned above but something that you need to be aware of is that some older flash models have high-voltage synch circuits up to as much as 400 V. Some trigger circuits aren't designed to withstand this voltage. If you plug a high-voltage flash unit into a such a trigger circuit, you'll burn it out. Note that HiViz.com trigger circuits are all designed to be used with flash units having high-voltage synch circuits up to 400 V. If you're using some other trigger circuit, then check first whether your flash unit is one that has a high-voltage synch circuit. You can find articles and videos on the web that show you how to measure the synch voltage. If your flash unit has a high-voltage synch circuit, then check the specifications of your trigger unit to see what synch voltage it can withstand.

 

The Vivitar 283 flash unit

 

For years I've used Vivitar 283 flash units. I still recommend them, and they can be purchased cheaply on eBay and other places. If you want a reliable assessment of the flash condition and a 365-day warranty, you can get that at KEH Camera. A Vivitar 283 in excellent condition is running less than $20 at KEH. When buying a used unit, keep in mind that the flash unit will not likely come with the synch cable that you'll need to connect it to a trigger circuit. The 283 uses its own proprietary cable--called a PC-1 cord--rather than a standard PC cable. You can buy a new PC-1 cord here. At $10, this may cost you more than your used 283 unit.

 

Unlike many modern flash units, the power output of the 283 can't be directly adjusted by turning a dial or punching a button to obtain fractional amounts of full power, such as 1/2, 1/4, 1/8, etc. Vivitar used to sell an accessory module that could be popped into place on the front of the flash unit in order to dial in power settings as low as 1/32. You may be able to find one of these on eBay. Search for Vivitar VP-1 Varipower Module, but keep in mind that they may cost more than the flash unit. Here's the thing, though: You don't need that module. There's a technique to trick the light-sensing circuit of the 283 into giving you the low flash power, hence, short flash duration, that you need for high-speed photography. See this article for that technique. If you find that you don't want to mess around with the calculations described in that article, just do one of these two things to get minimum duration: 1) turn the auto thyristor setting to yellow, or 2) remove the auto thyristor module and short between holes 2 and 5 with a paper clip. See the linked article for a diagram to locate holes 2 and 5. And don't worry, you won't hurt the flash or yourself. I've used this trick with dozens of 283s for 30 years.

 

The Powerextra DF-400 Speedlite

 

If you want to purchase a new flash unit inexpensively that fits the bill for high-speed photography, the Powerextra DF-400 Speedlite is one that I can recommend. I purchased one on Amazon for $26.99. There are units for a comparable price from other manufacturers; however, the DF-400 is the only one that I've tested. It is also the lowest priced one that I found in my quick search on Amazon. The DF-400 is shown in Figure 1. It includes the plastic stand shown. The unit is powered with 4 AA batteries. Figure 2 shows the opposite side of the unit. Note that there's a standard jack for a PC cord; however, the cord doesn't come with the unit. Therefore, you would need to purchase your own PC cord if you don't already have one. There's also a jack for external power. Like the PC cord, an external power cord isn't included.

 

Figure 1 Figure 2 Figure 3 Figure 4

 

The guide number of the DF-400 is about half that of the Vivitar 283; however, as discussed previously, I don't consider that an obstacle for most situations. If, however, you want more light output, there are more powerful models that will also cost you more. For example, the Powerextra DF-801 has a guide number of 60 and a price of $65. Figure 3 shows the back of the DF-400 unit, and Figure 4 provides an enlarged version of the controls. You turn the unit on with the slide switch at the bottom. After it charges, push the Test button to verify operation. Change the mode to M (for manual) if necessary. This is the only mode I use. Modes S1 and S2 are for wireless operation, but I haven't tested these. I usually avoid the use of wireless controllers as they can introduce a delay of a few milliseconds. If you're photographing a balloon burst, the burst is complete in a few milliseconds, so there would be nothing to see if you connected the flash unit to a trigger wirelessly.

 

In order to adjust the flash power, hence duration, simply press the +/- buttons. The specifications of the DF-400 state that the minimum duration is about 1/20,000 s. I haven't tested this directly, but I did take several photographs of balloon burst to evaluate the magnitude of the blurring of the rip for different power settings. I used a simple piezoelectric sound trigger for synchronization. I present the results below.

 

From top-to-bottom, I increased the flash power from the minimum setting in single steps. Each press of the + button provides a 2x increase in output, as judged from the photos. The aperture for Figure 5 was f/5.6, and I increased the f/# in 1-stop increments for successive photos. The exposure is consistent for the 4 photos. Clicking on any photo will open a larger version for closer inspection. My judgement is that the blur is acceptable for the photos taken at the two lowest power settings. For all photos, I used a Nikon D7100 with 105mm micro lens. The ISO was 200 and the shutter set for 2 s. Since I took the photos in a dark room with flash lighting only, the shutter setting was irrelevant to the exposure. The shutter only needed to stay open long enough for my assistant to respond to my signal to pop the balloon.

 

Figure 5

f/5.6, lowest flash power setting

Figure 6

f/8, one step higher than minimum flash power

Figure 7

f/11, two steps higher than minimum flash power

Figure 8

f/16, three steps higher than minimum flash power

 

Here are a few more notes about the DF-400. The flash head rotates up and down through 90 degrees, but it doesn't rotate side-to-side. There are pullout bounce and diffuser screens to cover the flash tube. The unit recharged very quickly. When set to minimum flash power, you can discharge the flash as quickly as you can push the test button. Reliability? I haven't used the unit enough to say, but for the price, it's worth the risk, at least for me.

 

Assembly instructions for other kits

 

 

 


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