The author, ready to shoot with a pair of Sea and Sea YS-350 strobes and a Sea and Sea housing for the Nikon N90S. Photo by Mike Purcell/Waterhouse

Answer: Yes, maybe, sometimes.

• Hydrodynamic resistance. A second strobe provides more water resistance and can be awkward when swimming against the current. Likewise, a second strobe makes the system bigger and more ungainly. In restricted spaces like shipwrecks or tight coral crevices this can be a problem.
• Turbid water. Using two strobes increases the potential of backscatter on both sides of the frame rather than just one.
• Lighting wide angle. Here there are two schools of thought. One says that wide-angle photography should be done with twin powerful strobes mounted on long articulated arms so that reef scenics can be evenly lit across a single plane, or perhaps one strobe might be directed to a colorful foreground while the second would illuminate a nearby model.

I prefer to use a single powerful strobe for wide angle. For me, this system is quicker, more ergonomic, and better suited to my style. I use a customized diffuser that spreads the beam evenly across 180 degrees so that I can cover even the 16mm full-frame fisheye lenses. I also use a long articulated arm, but I handhold the strobe so that I can paint whatever portion of the scene best contributes to color or texture. In wide angle the sunlight is my secondary source of light, and I am always aware of the need to blend strobe and ambient light sources.

What Are the Best Arms?
While you can handhold a single strobe, a second strobe requires some form of arm system. Merely the need to have a hand free to trigger the shutter presupposes that the second strobe will be mounted to a tray or housing. Ideally, both strobe arms are articulated (ball joints are the most common form of articulation) so that the strobe head can be positioned quickly and stay where it's placed. The arms should also feature a means to quick-disconnect from the tray to facilitate handheld lighting or to fit the system into a confined space.

Ikelite, Sea and Sea, Aquatica and Ultralight offer high-quality articulated ball joint arms. Those who find their housings or Nikonos RS camera to be too heavy in the water may also appreciate Ultralight's buoyancy arm. It is a simple concept: a hollow arm designed to provide lift, but a good solution to an affliction I have been plagued with, "RS Elbow."

Do I Need Wet Connectors?
Photographers can't change film under water, so to shoot more than one roll of film per dive, you must carry more than one camera. For those using dual strobes, dedicating a set of strobes to each camera can get expensive and quite cumbersome. The solution is wet connectors so that the same strobes can be used for each camera by means of an electrical contact that is able to connect and disconnect under water.

This is not necessarily a new concept; in fact I have been using E-O connectors (named for their manufacturer, Electro-Oceanic) on my Nikonos and housed cameras for more than 20 years. E-O connectors are very difficult to find these days and because they are only suitable for manual exposures, they are impractical for those who prefer TTL automation. A new solution is available from Aquatica, which offers a five-pin TTL wet connector available in a variety of configurations to accommodate most any TTL strobe, camera or housing. TTL strobes can finally be used on multiple camera systems on a single dive.

Now if only all the camera and housing trays had a common quick-disconnect levered receptacle (like we have on tripods) rather than awkward 1/4x20 screws, it would be a wonderful world. Are you manufacturers listening?

Why Use Dual Strobes?
A second strobe is typically used for one or more of these three reasons:

• To extend the light coverage for wide-angle photography;
• To increase light output so that a smaller aperture can be used for better depth of field;
• To fill the harsh shadows often left by a single strobe.

Dual strobes are used to soften what might be distracting shadows cast by a single strobe.	Here the primary strobe is used to light the soft coral in the foreground while a slave strobe on an articulated arm is directed to the clownfish and anemone in the background. Photo by Stephen Frink

How Do You Trigger Dual Strobes?
The second strobe can be triggered by means of a dual sync cord or by a slave sensor, which reads the burst of light from the primary strobe and fires immediately. Since strobes discharge in the range of 1/7,000th second, and cameras with focal plane shutters typically synchronize at no greater than 1/250th second, slaved strobes make sense. But when it comes to reliability, dual sync cords are clearly more reliable. When using a slave strobe:

• High levels of ambient light may make it hard for the slave to read the primary strobe's burst, and it may fail to fire.
• Diffusers on the slave strobe may obscure the sensor, and it may fail to fire.

In choosing a strobe specifically for slave use, look for features like multiple power settings for manual output, or even TTL slave. A very useful accessory for Ikelite strobes is the remote TTL slave sensor. It replaces the sync cord on the secondary flash, and allows a separate TTL slave sensor to be mounted on the strobe arm positioned to face the primary strobe.

While this crocodilefish is resting on a single plane, and it would be possible to light it with a single strobe, twin flashes were used to assure minimal distracting shadows. Photo by Stephen Frink

How Do Two Strobes Affect Exposure?
When using a manual strobe, its guide numbers can be used to calculate exposures. For example, the Nikonos SB-105 suggests an underwater guide number of 33 for ISO 100 film. Divide the guide number by the distance in feet to determine an aperture. So for a strobe-to-subject distance of 3 feet use f/11 (33 divided by 3 = 11). Keep in mind manufacturers often overstate their strobe's guide numbers. Still, it's a starting point.

With two strobes aimed at exactly the same point, a new guide number must be used. A simple rule of thumb is to multiply the original guide number by 1.4. Taking the SB-105 example again, the guide number for dual 105s is 46.2 (33 multiplied by 1.4), and the aperture setting for a strobe-to-subject distance of 3 feet is f/16 (46.2 divided by 3 = 15.4).

However, guide numbers are difficult to use in the real world, and it's unlikely both strobes will aim at exactly the same spot, or be exactly equal in power. The alternative: dual sync cords that allow TTL exposure automation, or new slave strobes that also integrate TTL (like the Sea and Sea YS-30).

Another solution is to calculate the exposure based on the primary strobe, either TTL or manual, and then add a smaller amount of light to fill shadows with a variable powered slave. In this case, the second strobe is not adding intensity of light, but rather contributing to the quality of light. In this configuration, a TTL primary could be used and the second strobe operated at a lower power on manual without affecting exposure. The ultimate evolution of this concept is seen in the new Sea and Sea YS-350 system, which incorporates a type of rheostat to continuously vary the manual light output on either the primary or secondary strobes.