Forget about the size of your camera's sensor or the ergonomics of your housing. Quality of light is the single most important factor in how your underwater photo turns out. No one specification tells the full story of a strobe. The strobes you choose will be in your gear kit long after the camera you originally paired them with... So get to know the numbers and make a fully informed decision!
This number is used to express the power of a flash. When researching underwater strobes, you typically see a number that represents the strobe’s power at ISO 100 with a distance of 1 meter from the flash to the subject.
Without getting too technical, here are some basics about guide numbers:
Another way of quoting the power of a strobe is the amount of energy stored in a strobe's flash capacitors. For example: at 160 watt-seconds, the Ikelite DS161 has nearly twice the flash capacitance of the SEA&SEA YS-D1, but is only quoted at half the power in guide number. How can this be? It’s because a strobe’s power is translated through a flashtube, bounced off a reflector, passes through a lens, and so forth. Power is just one piece of the puzzle. There are several other factors to consider when measuring "quality of light."
By definition, the strobe’s angle of coverage tells you the area your flash covers within a one-stop fall off. At Ikelite we measure this across a flat surface perpendicular to the direction of the strobe at a distance of 1 meter. Alternatively some manufacturers choose to measure this on an spherical surface centered on the strobe. The difference? Depending on the reflector, this could turn a 65-70° angle of coverage into 90-100°.
The reality? Reflectors for straight flashtubes are designed to provide a narrower beam so that flash output is concentrated and the effective guide number is higher. The average underwater strobe with a straight flashtube and no diffuser measures around 70° across a flat surface. Underwater strobes with round flashtubes generally have a wider angle of coverage around 90-100°
It's probably no surprise that a straight flashtube produces a rectangular beam pattern and a circular flashtube produces a circular beam pattern. The angle of coverage of a strobe with straight flashtube is narrower top-to-bottom than its quoted angle of coverage side-to-side.
This seems like a good thing, because we shoot mostly rectangular images. However, our strobes are typically extended out at some angle from the housing. The rectangular flashtube is usually positioned at some angle in relation to our frame. This affects our effective angle of coverage. A circular flashtube covers more area overall, and maintains its full angle of coverage regardless of its orientation to the camera.
Wide angle lenses fit a lot more in a frame than macro lenses. So in practice, the difference is more obvious when attempting to evenly light a wide angle scene than a close-up scene.
The one big secret of the great underwater photographers: they take A LOT of pictures. Your odds of getting "the shot" will increase with the number of shots you take of any given subject. The number of photos you can take depends on the shutter lag of your camera—practically a non-issue these days—and the recycle time of your strobe. Recycle time is the time it takes for a flash to charge up its capacitors and be ready to fire again for the next photo. The recycle time is most affected by the type batteries the strobe is powered by.
We use AA batteries everywhere: in our computer mouse, our TV remote, our flashlights, and more. In fact, they’re probably the most popular battery in the world for low-draw electronic devices. A strobe is a different animal though. Unlike electronic devices, strobes draw a huge amount of pulse current. You can only get so much pulse current (read: energy, power, output) out of even the best AA batteries.
The recycle time of a strobe powered by AA batteries will be in the 3-4 second range at best. That’s OK in systems where your strobe triggers off of the camera's flash, which also takes a relatively long time to recycle. For close-up work a slow recycle time is less of a problem. Many small critters will keep you waiting a while in between shots until they look just the right way. A smaller strobe powered by AA batteries is also hugely advantageous as you can get it close to your lens in tight spots. But in fast moving shots, you’ll only get one or two shots off when that turtle or shark passes. Make them count!
Wide angle work is a different beast. Unless you’re a true savant at pulling the trigger at precisely the perfect moment, rapid firing is critical when shooting larger, fast moving subjects. When you get serious about wide angle, you’ll crave and truly appreciate the power and lightning fast recycle time provided by a high-capacity dedicated battery pack. Dedicated battery packs provide more flashes per charge, have longer lifetimes, and are less prone to floods due to assembly error. Meaning that you can travel with less of them than with sets of AA batteries.
Recycle time for any strobe improves by using an electrical sync cord, using TTL, and/or by setting your strobe to less than full power.
Direct sun is perceived as “white light” at about 5500K.
"Warm" strobes are in the 4600-4800K range. This is great for wide angle because your tropical photos will have a very rich, blue water background. It gives a bigger pop of color to soft corals, and makes your underwater models' skin look more naturally warm and alive (less corpse-y).
"Cool" strobes are in the 5500-6000K range. This looks totally natural in close-up shots where the strobe is the primary source of light, rather than the sun. It also makes green fresh water settings look more green and fresh. In tropical wide angle scenes, cool strobes will give you less oranges and reds in the coral and a slightly washed out blue water background.
If you’re shooting RAW, color temperature can be tweaked in Lightroom or Photoshop, but it never hurts to shoot the right quality light for the right situation!
The concept of Thru-The-Lens (TTL) flash metering has evolved beyond its original, very literal definition. There are now a variety of methods by which the camera collects information about ambient light and adjusts flash output for correct, TTL exposure. Most commonly, digital cameras bounce one or more small "pre-flashes" off the subject to help decide how long to fire the main flash.
This is a pretty sophisticated process which starts and finishes within milliseconds. For a strobe to provide TTL exposure, it must either watch and mimic the camera’s flash (via fiber optic, optical slave, or LED trigger) or receive timing data directly from the camera (via sync cord attached to the camera’s electrical TTL hot shoe).
TTL strobe exposure which relies on mimicry is "universalized" to provide the near-best-case-scenario for any of hundreds of different camera models. Which make these systems very adaptable, and don’t require any direct attachment to the camera. Again, recycle time suffers, and the camera has no idea that the strobe is attached so it bases its calculation on its knowledge of the built-in flash.
TTL strobe exposure which relies on a direct electrical connection has the added advantage of true data transmission between the camera and strobe. There is a waterproof connection involved, but also faster recycle time, rapid firing capabilities, and more accurate exposure.
Choosing the right combination of equipment for your shooting scenario, you can increase your chances and get a higher percentage of strong images.