Saturday, October 4, 2008
The vibrant colors we see in photographs of the underwater world are the result of artful application of artificial light. Here are some recent musings on the state-of-the-art of strobe photography today:
Underwater photographers tend to obsess over which camera to house, and then which housing to use it in. But, it seems less attention is paid to an equally critical part of the imaging equation … which strobe(s) should be used.
Why are strobes so important? Well, obviously because color in underwater photography wouldn’t happen without them. Water is a medium 800 times denser than air, and it is cyan in color. Unless, of course, you are in British Columbia where it is green in color or a mangrove lagoon where it might be brown in color. Whatever the specific hue, it is like a massively strong filter over the lens of an underwater camera, and the deeper we go, the stronger the filtration. Plus, the filtration effect is spectrally selective. In clear tropical waters reds are the first colors to drop out, logically since red is the exact complement of cyan. I suppose magenta would be the first color to drop out in green water since they are opposites on the color wheel (and on the Photoshop color balance sliders), although I don’t have enough experience in green water to say that with certainty. It hardly matters though, because after the first 15 feet in depth the whole warm end of the spectrum fails to record on film or digital. Yellow goes away next, and by about 30 feet blue and green and black are all that records. Digital offers some white balance tricks to make the colors stay faithful at greater depth, and special filters can be used to help shallow water available light photography, but none of this changes the reality of our very significant dependence on artificial light (strobes).
But, what strobes are best? Well, to a great extent it depends on your budget, naturally, but independent of price other variables include the kind of camera and housing you are using and the kinds of things you shoot.
Strobes for Compact Cameras - This is very significant because the kind of camera determines the kind of synch system that might be used. The strobe obviously has to fire when the shutter is open to shine onto the recording medium (analog or electronic), but that is not a slam-dunk in the digital era, especially with compact cameras. Digital cameras use a preflash system to determine the proper amount of light striking the subject, and therefore reflected back to the sensor. This is a brief burst of light, large enough to be detectable to the sensor but small enough so the primary flash, emitted immediately subsequent, still has enough juice in the capacitor to dump a full power flash if required.
Many submersible strobes of the analog era have a “slave” mode, whereby they can tell when a primary strobe fires, and will trigger cordlessly in synchronization. This is a notoriously unpredictable way to trigger a flash, because in high ambient light (shallow water) it may not “see” the difference between the primary strobe and the available light on the scene. But, even in a best-case scenario, where the slave strobe actually does fire, it will likely trigger from the preflash and have no chance of being recycled adequately to synchronize with the primary “real” flash. People who housed digital compact cameras (point-and-shoot), and thought to use conventional submersible slave strobes were disappointed to find out how poorly they worked, if at all.
In phase two of the strobe-in-the-digital-world story, certain manufacturers made strobes that could detect preflash and delay the primary synch until the main strobe went off. Sealife created the SL960D, able to work with cameras that have up to 4 preflashes and has five different settings on a synchronization mode, one of which is almost certain to work with the preflash of almost any compact camera. It has a deflector on the tray that bounces light to a slave sensor on the strobe head, or can be triggered by means of a fiber optic (more about that later). Ikelite had several solutions, including a slave-sensor equipped DS50 (now known as DS51), and their later solution, the AF35 Autoflash kit. This is an integrated unit including a camera tray, articulating arm, and an integrated slave sensor positioned perfectly to pick up the on-camera flash from a compact camera either encased in their housing or any of the many clear OEM housings offered by Olympus, Canon, Nikon, or the rest. While not TTL, it does offer exposure automation by mimicking the duration of the camera’s built-in flash.
The next step up the technology chain for those cameras that operate exclusively by the preflash-main flash paradigm (and far more sophisticated in terms of reliability) is the fiber optic options. Here some level of mounting bracket is employed to hold a fiber optic perpendicular to the camera’s built-in flash. The perpendicular aspect is critical because fiber optic does not transmit light well when it comes in through the side, but shot directly down the pipe it runs very efficiently through a small coiled cord, terminating directly at the submersible strobe’s slave sensor. Gone is the ambiguity of whether there is too much ambient light on the scene, and predictability ramps up significantly. Two cameras that do this extraordinarily well are the Sealife DC800 and Sea and Sea DX1G. Both have dedicated digital cameras that fit precisely into their custom housings, and each offers a robust and reliable means of mounting the fiber optic into just the right spot for the internal flash.
When using these point-and-shoot compact cameras topside very accurate exposures happen routinely in the strobe mode. The preflash kicks out to the subject, evaluates how much primary light needs to be dumped, and tells the main flash when to main quench. Unfortunately, getting that same information to an external submersible strobe, whether triggered by slave or fiber optic, is tough. Inon’s D2000 does a nice job with what they call “S-TTL”, and their brackets, trays, and strobe arms are custom fit for many of the most popular point-and-shoot digitals, and the Sea and Sea YS17 offers both TTL and manual modes. But, most strobes of this nature use a rotary power dial, like a rheostat, to determine how much manual strobe power is dumped. Examples of this application are found in Sea and Sea’s YS27DX and Sealife SLD61, both offering extremely reliable fiber optic synchs and multiple manual power settings. With the immediacy of exposure review through the camera’s LCD, often this is perfectly adequate. Unless, of course, the shot that is used to evaluate the exposure is the once-in-a-lifetime shot of mating great white sharks and the second shot, the one with just the right amount of strobe power dialed in, never happens.
Ikelite offers yet another option for certain compact cameras that have hot shoes, like Canon’s popular G9. Their housings offer not only the ultra-reliability of a hard-wired coiled cord, but for certain models of compact cameras from Olympus, Canon, and Nikon, they also offer TTL by means of conversion circuitry integrated into the Ikelite housing. Ikelite offers a nice explanation of TTL via submersible strobes in digital cameras and various terminologies at http://ikelite.com/web_pages/dig_definitions.html. Their DS51 or new DS160 are terrific choices for this application.
Strobes for Digital SLRs – The simple issue of getting a strobe to fire in synchronization with the shutter is a much easier matter with a D-SLR. Virtually all have hot shoes on top of the camera prism, and these connect directly to external bulkheads on the housing that adapt to the traditional Nikonos 5-pin connector, Ikelite connector, or the European favorite (now my favorite as well), the S6 connector. All the most popular strobes from Ikelite, Sea and Sea, and Inon have multiple power selector switches so that the amount of strobe light is controllable, to a point anyway.
The differentiators between all of these strobes, aside from price of course, are power supply (some use AA batteries; either NiMH, NiCAD, or alkaline) while others have a dedicated power pack and battery charger. Among the more elegant solutions for dedicated power packs in terms of ease of installation in the field is the new Seaflash 150 (battery pack unscrews from back of strobe from threaded compartment) and Ikelite DS160 (where the entire rear part of the strobe is a power pack that easily locks into place). Other features that distinguish one D-SLR strobe from another include peak power capacity, recycle time, handling ergonomics, weight and size (for travel convenience), and beam angle.
TTL – Getting Through-the-Lens exposure automation (TTL) to work with a digital camera has been challenging, to say the least. Dedicated strobes for Nikon (i-TTL) and Canon (e-TTL) are available from the manufacturer, but the protocols that make TTL work are closely guarded secrets that the camera companies have no intention of sharing with those who make underwater strobes. To figure out how to make these strobes work with accuracy, the UW guys had to reverse engineer the TTL technology and figure out how to make it work with their lights. For years I’ve said this was a bogus quest, but this year, for the first time in my photo seminars I teach in Key Largo, I’ve had multiple shooters show up in class with TTL systems that actually work.
On the very first day of class we take exposure slates underwater, and with the student kneeling in the sand at a fixed distance (3-feet away) we do exposure tests at a series of F-stops, from F-5.6 to F-22. This allows us to deduce what is the proper full and half power aperture for that distance and a subject of average reflectance, but it is also an empirical test for any form of exposure automation. Theoretically, if TTL works, the amount of strobe light striking the exposure slate should be approximately the same at F-5.6, F-8, and F-11. I would expect F-22 to be dark because the strobe will have already dumped full power. Even TTL can’t extract more than full power from the strobe, so whatever F-stop is right for full power is also the upper limit of TTL.
This year in a single class I had three shooters with 3 different Ikelite systems that had predictable and reliable TTL. Two were Ikelite strobes with Ikelite housings, and one was a Subal housing equipped with a conversion circuit, and a special Ikelite cord optimized for TTL. Another strobe that worked remarkably well in TTL was the Seacam Seaflash 250 (now replaced by the Seaflash 150). Now that the Seacam Seaflash 150 strobes are shipping I am eager to do a full field test on one very soon.
In terms of the universe of TTL options, they did not happen to show up in class and therefore I can’t say from personal experience how they worked, Sea and Sea also offers TTL-enabled strobes and external conversion circuits that mount onto the housing. Mathias Heinrichs has also developed a high-tech cable with integrated circuitry that enables a variety of popular submersible strobes to work with a variety of digital SLR cameras.
While manual strobe exposure is still fine for me, I am no longer the absolute skeptic regarding TTL in digital. I think we have crossed the threshold of predictability and it will continue to get better. While camera-makers continue to chase megapixels and sensors offer ever-higher resolution and reduced noise; the strobe manufacturers continue to attack their obstacles by making strobes more powerful, more reliable, faster to recycle, and more energy efficient.