What are AC and DC Packs and Heads?

Divide and conquerMonolights combine power supply and flash head into a single unit. An alternative approach is to separate the power supply and flash into two separate units–the AC power pack and flash head. Because there is no internal power supply, flash heads can be smaller; some are downright tiny, allowing you to place them in locations where larger monolights might not fit. Since the heads are smaller, there’s room for adding cooling fans without the head size getting too large or the fan too big and noisy.A separate, single-unit power supply can control more than one head. Usually the output for each flash is controlled separately in either symmetric (all flashes operate at the same power output) or asymmetric (each flash output is controlled independently) configurations. The power supply itself can be larger because the design needn’t be concerned as much about heat buildup affecting the flash head (as with a monolight) allowing more flash heads to be connected along with overall higher Watt Second output. A ratio approachIf some of these buzzwords are new to you, be sure to read part I of this series, “What are Monolights?” which contains a glossary of terms for those new to the world of studio lighting. One of the terms you won’t find there is lighting ratio and that’s especially important when using multiple flash heads. Power packs with asymmetric controls can be set so each flash connected has a different output intensity. Lighting ratio is the difference in the brightness of light falling on your subject from the main (sometimes called key) light and the other fill light, but there can also be tertiary lights that serve other purposes such as adding highlights to the subject’s hair (hair light) or illuminating the background (background light.) A ratio of 3:1 is considered “standard” or normal for color photography but photographers can be flexible in applying this rule.Dyna-lite M500XL is the company’s smallest power pack. It produces 500-Watt Seconds of flash power with a five f-stop range and a Variator to provide 1/3 stop fine-tuning to three flash heads. It weighs only 4.2 pounds, has a footprint of just 5 5/8×5 7/8 inches, and stands only 5 1/4 inches high including the handle.Like Monolights, power packs have input for a PC cable allowing it to be directly triggered when connected to your SLR’s corresponding PC outlet. Many have an optical “slave” that can be set to trip the flash when it sees another flash go off or may be triggered by an infrared source. Radio-controlled slaves are a popular option that allows a power supply to be wirelessly triggered without requiring a camera-mounted flash or cable connection. Some power packs, such as the Dyna-lite M-1000WI, have a built-in radio receiver.

Smith Victor’s PGIRT-20 Infrared Flash Trigger is an economical alternative to radio wireless triggering devices. It slides onto a camera’s hot shoe or can be connected via the supplied PC cord. An infrared signal can be sent to the infrared slave built-into the power supply and trigger the flash heads from up to 50 feet away.DC, tooMost power packs are AC powered but just as DC powered monolights are gaining favor with location photographers, DC power packs, such as Comet’s CBm-1200, are popular too. Many studio light systems give you a choice of using AC or DC power packs so you can have both options in your studio lighting kit and be able to use the one that fits the assignment or project you’re working on. If you’re thinking about the requirement for DC power, ask yourself the big question: How far away is the nearest AC outlet? If it’s longer than your longest extension cord, it’s time to think DC.Comet’s CBm-1200 is a small, lightweight, 1200 Watt Second battery–operated power pack for the photographer who wants a fast DC unit to work on location with Comet flash heads. The NHB-24 Nickel Metal Hydride battery takes only two hours to charge and offers quick battery replacement. A power pack checklistHere are a few things to keep in mind when considering a power pack and flash head system for your studio lighting needs. As in all other kinds of photography, this involves a series of tradeoffs between functionality, ease-of-use, and cost. You’ll want to carefully juggle your budget with your want list.Continuously variable output Many power packs (aka power supplies) such as the Norman P404 allow you to adjust power settings to suit the lighting setup you have. Output is symmetrical and the two outlets that this 400-Watt Seconds power pack offers are divided evenly so the power from one head is the same as the other. Lighting ratios can be controlled using a light modifier such as a lightbank (see Part IV of this series, coming soon!) or by varying the distance from the subject to from the lamp head.It is here we run into one downside of the power pack/head system. In a two light system, if a power pack fails, you can’t shoot. If you have two monolights and one’s power supply fails, you still have one light to finish the shoot. It won’t be perfect but fulfils the old newspaper adage of “f/8 and be there.”The Hensel Pro Mini 1200 provides full modeling light control and power adjustment. The power supply can also produce symmetrical or asymmetrical output through its four outlets.Power ControlWhen doing volume portraiture, such as store or school photography, it’s important to properly expose each subject without altering background exposure and still be able to make adjustments depending on the subject. Norman’s Tone Light Control uses a small controller that’s attached to the camera and allows just the subject lighting to be adjusted to any one of three levels on a per-subject basis.

The Norman 1200 Watt Second AC Power Supply with Tone Light Control can be ordered with a factory set TLC mode to control the fill light, or the main and fill in either two or three output levels. A switch on the power supply allows all 1200 Watt Seconds to be used on a single light head. PortabilityThe option of having a DC power pack allows you to take your flash system out into a cornfield or a racetrack to create real studio lighting in the middle of nowhere. This is also where you will run into the second downside of power pack/heads: cables. You’ll need to run a cable from the power pack to each head to make it all work. This isn’t a big deal in the studio, where you can tape them down or place them safely under cable runs, but on location, watch where you walk. Knocking over a light stand and flash head almost always results in a disaster, and it’s expensive, too.The Lumedyne 400ws Deluxe Extra Fast TTL Pack offers settings from six to 400 Watt Seconds. It has a seven-light battery gauge and two head jacks. You’re still going to need a handheld light meter that reads flash output. Flash users are always going to need a flash meter (a light meter that reads flash output) but photographers shooting with continuous light sources (covered in the next part of this series) can get by with just their in-camera meter.

The Gossen Luna-Star F2 is a digital incident, reflected, and flash meter. Both flash and available light are measured at the same time, so it’s easy to determine the balance between the two. (That’s another lighting ratio you need to consider when working under high ambient light.) This is the meter that I personally use and other than the fact that it uses the dreaded 9-volt batteries so beloved of German engineers, it’s been a real workhorse for many years.

Joe Farace is the author of a new book called “Getting Started in Digital Imaging” published by Focal Press (ISBN 024080838X.) It’s available in all the best bookstores as well as Amazon.com.