There are two basic types of studio flash: powerpack-and-heads systems and monolights. The former provides a lot of power and control, but is a bit unwieldy, and the cables connecting the lamp heads to the powerpack are just waiting to trip someone. Monolights are self-contained; the lamp head and the powerpack are built into a single unit. Like the pack-and-heads systems, they can run off AC power, but many monolights also can be operated on battery power, making them great location light sources.
Most monolights are much more powerful than shoe-mount flash units. More power means you can shoot at smaller apertures to increase depth of field, especially important when using umbrella reflectors and the like, which greatly reduce intensity. While shoe-mount flash units are rated in guide numbers, monolights and studio flash are rated in watt-seconds or joules (one joule = one watt-second). Guide numbers are measures of output, while watt-seconds and joules are measures of generator power, so they can’t really be compared (output depends on generator power, flash head and reflector), but, roughly, 600 watt-seconds or joules would give an ISO 100 GN of around 80 in meters or 262 in feet (much depends on the reflector).
Monolights are also bulkier than shoe-mount flash units, so if bulk is a concern, pay special attention to the size and weight of the monolight and battery pack (if a separate unit) in the specs when evaluating units. Many of today’s monolights are more compact than previous generations. Of course, you aren’t going to use a monolight on-camera, so bulk is mainly a concern when carrying the gear to and from locations.
Battery technology is better today than in previous generations, with battery packs that are more compact and provide more flashes per charge. When checking specs, make sure you’re comparing like with like: The number of full-power flashes will be much lower than the number of 1/32-power flashes. The same applies to recycling times. A unit will recycle much quicker after a 1/32-power pop than after a full-power one. Power setting also affects flash duration. Duration is longest at full power and briefest at minimum power. It’s not a bad idea to carry a spare battery or two if you plan to do a lot of shooting at a location that doesn’t have access to AC power; in such cases, battery packs that have removable batteries are a big plus.
One of the big advantages of monolights is the wide range of excellent light modifiers available—parabolic reflectors, umbrellas, light boxes, snoots, grids, beauty dishes, barndoors and more. Many units accept standard "S" modifiers. Check to see what’s offered for each unit you’re considering to make sure what you need is available. For a wedding shooter who wants more light-shaping at his or her disposal, the array of sophisticated modifiers is incredibly beneficial.
The Gemini 750Pro monolight from Bowens provides flash durations up to 1/2300, quick 1.5-second recycling and 750 watt-seconds of power (ISO 100 GN 100 in meters/328 in feet). It features a proportional modeling light, a color temperature of 5600K (+/-300K) and optional remote radio triggering via the PocketWizard BowensGEM receiver module or Pulsar remote trigger card. The 750Pro has a cooling fan, and it can be powered by AC or an optional Bowens Travelpak battery, which can handle two Gemini units. Gemini 750Pro dimensions are 6.7×16.1×5.2 inches; weight is 8.8 pounds. The Travelpak (Small) measures 6.7×5.5×8.1 inches and weighs 11 pounds; the Travelpak (Large) measures 6.7×5.5×9.4 inches and weighs 14 pounds. The Large pack provides about twice the number of flashes as the Small. www.bowensusa.com