There are two basic types of studio flash systems: two-piece (consisting of a power pack and one or more flash heads) and monolight (in which the flash head and power pack are combined in one unit). The traditional separate power pack system generally offers more power and quicker recycling, and you can plug several flash heads in a single power pack. Monolights, being self-contained, are easier to position just about anywhere and to transport, and cost less. There are power-pack systems that can run on battery power, and many monolights offer this capability, so both types of systems can be taken into the field when necessary, but a monolight is more compact.
A monolight is lighter and takes up less space than a separate flash head and power pack. This monolight advantage lessens as the number of heads increases, however: Since each monolight contains both a flash head and power pack, the monolight weighs more than a pack-and-head system flash head alone. This is mainly a consideration when hanging the head on a boom.
With a pack-and-head system, the power pack plugs into the wall plug via a cord, and each head plugs into the power pack via a cord. This puts a lot of cords on the floor, stretching around the studio from the power pack to the wall outlet and from the power pack to each flash head. With a monolight, the unit plugs into the wall plug, and that’s it. Each monolight can be plugged into the nearest wall plug, thus minimizing the cords-everywhere problem. But you need a socket for each monolight you want to use.
Of course, if your system—pack-and-heads or monolights—offers wireless control, then you just need one wall plug for either system. And if your system provides battery operation, you don’t even need that.
If you do only headshots, you can work with a fairly low-powered flash head or system because the head(s) will be quite close to the subject. If you light large subjects or areas, you’ll want a more powerful system. Flash heads are often rated in watt-seconds (w/s), or in joules in Europe, but watt-seconds aren’t a measure of output; they’re a measure of power. Current monolights range from under 200 w/s to over 1300 w/s. All other things being equal, two systems of equal power should produce equal output. But all other things aren’t always equal—flash tube, cable, the nature of the capacitor, reflector and more enter the equation. A better indicator of flash power is estimated output (usually given as the aperture required at a specific distance, often 10 feet), when available. If you can try the system (in the store, or by renting it for a day, or via a friend who has it), you can determine this yourself with a flash meter.
Besides maximum power, you should consider the ability to adjust the power of the flash burst. It’s easier, in a multi-light setup, to adjust the power of each flash head appropriately than it is to physically move each unit closer to or farther from the subject to establish the desired lighting ratio. Lower-cost units might operate only at full power, or at full and half power. Better units might let you adjust power down to 1/32 power or less, in full-stop or fractional-stop increments, or even continuously.
With some flash systems, the color of the light changes with the power. Shooting RAW with digital cameras, you can deal with this in postproduction, but it’s best to get a system that minimizes color shift with power setting. Some manufacturers provide data on this, others don’t.
Better (generally, higher-priced) monolights have better repeatabi-lity—each flash at a given power setting is just like the last one. Lesser units might provide slightly stronger or weaker bursts from flash to flash.
Camera-mount auto-flash units have very short durations—generally, from around 1/1000 second at full power, down to 1/20,000 or so when used at very close range or set to minimum power in manual mode. Studio flash units have slower durations— often starting much longer than 1/1000 and rarely going above 1/2000. This isn’t a major consideration, unless you want to do those "frozen-milk-drop" kind of studies, in which case a camera-mount unit is a better choice than a studio unit.
Because the flash burst is very brief, you can’t study your lighting as you can with continuous "hot" lights. So, most studio flash systems provide modeling lamps, which are continuous-running bulbs adjacent to the flash tubes that let you see what the lighting looks like. Unless you work in a very dim environment, you’ll want a system with powerful modeling lamps. In multiple-light setups, it’s also helpful if the modeling lamps are proportional; that is, if they automatically brighten or dim to match the power setting you use for each flash head. With some systems, you can manually adjust the brightness of the modeling lamps.
It takes time for a flash unit to recharge between bursts, and this is known as the recycling time. With some types of photography, it doesn’t matter if you have to wait several seconds between shots; with other types, quick recycling is essential. Flash manufacturers publish recycling times for their units at various power settings (units recycle more quickly after brief low-power bursts than after longer full-power bursts). Bear in mind that these published times may be somewhat optimistic, especially for battery-powered units as the battery wears down.
While dedicated camera-mount flash units are triggered via the camera’s hot-shoe, studio flash systems are triggered via sync cords. So you need a camera body that has a PC socket to connect the studio flash system. All pro DSLRs and many mid-range models have this.
Most of today’s DSLRs offer wireless off-camera flash with TTL exposure control when used with appropriate dedicated flash units. Studio flash units don’t offer automatic exposure control, and exposure is generally determined via a flash meter. But many of today’s studio systems do offer either built-in or accessory means of triggering wirelessly via infrared or radio signal. This eliminates cords running from camera to power supply or monolight units, and is the preferred way of working with studio flash today.
Paul C. Buff
Profoto (MAC Group)