>Here's how slow shutter speed came into play: At midday in the shallow water at the top of the statue (about 15 feet deep), the bright ambient light from the sun would have overpowered the flares. So I planned the shoot for very late in the afternoon. This meant more of the sun's light would bounce off the water's surface than would penetrate it — due to the oblique angle of the setting sun. My light-meter reading would be several stops darker at that time of day. I used a strobe to front-light the statue and my model, but the low ambient light allowed me to set my shutter to 1/30 of a second (the slowest shutter speed on my Nikonos camera), and that gave the light from the flares more time to register on film, making them appear brighter.
>Making hand-held light appear brighter is just one of several reasons to embrace slow shutter speeds in underwater photography. Two of the most important uses are bringing more light into backgrounds (specifically, the areas beyond that which can be illuminated by strobes) and creating intentional blur to enhance the perception of motion in a photograph.
>The physical characteristic that guides all underwater photography is the density of the medium. Water is 800 times denser than air, and light transmission is correspondingly reduced. That big fill-light in the sky, the sun, is very powerful, but transmission is variable according to where on the planet you are, the time of day, depth, water clarity, cloud cover and surface conditions (a choppy surface transmits less light than a slick calm one).
>Increasing Background Light
>Down to about 50 feet deep, light is absorbed at a degree equal to about one stop for every 10 feet of depth. Deeper than 50 feet, that relationship begins to fall apart. Light from strobes is not nearly as bright as the sun, of course, and illumination is reduced at a degree equal to about one stop for every foot of distance from the strobe head. Based on these approximations, I like to think of light in terms of "vertical" (that which comes from the sun and penetrates down into the water column) and "horizontal" (that which comes from the strobes and strikes the subject).
>Note the wide-angle shipwreck photograph; this image exemplifies the way both forms of light should be considered when calculating an exposure.
>The wreck photo is an anniversary shot, too. I was hired to document the decade of growth on the wreck of the Spiegel Grove off Key Largo, Fla., on the occasion of the 10th year since it was intentionally sunk as an artificial reef. It's now a riot of color, thanks to encrusting sponges and deep-water gorgonians, so I knew I would use the light from my strobes in the foreground to restore the color lost at the 90-foot depth. I used an 8-15mm zoom lens (at 13mm) so I could work very close to the gun emplacement and bring ample artificial light to the foreground. The diver in the photo is a compositional element, providing a bit of human interest as well as scale to the immense, 510-foot ship. Given the strobe-to-subject distance and the reflectance of the foreground I chose an aperture of f/8, an ISO of 320 and a strobe power setting of 50 percent. That was all pretty well revealed on the camera's LCD screen in my first test shots, but the real experimentation came with the shutter speed.
>If I had taken the shot at 1/125 (a pretty standard shutter speed for underwater photography), the background would have been quite dark, and there would have been no differentiation of the superstructure in the far distance. Not only that, the inevitable particles in suspension in the water column (backscatter) would have been very obviously illuminated as white spots against an indigo backdrop. Instead, I chose to shoot at 1/30. A slower shutter speed meant a lighter-colored background and the backscatter less obvious.
>This was still fast enough to allow me to hand-hold the camera. There is a dampening effect of the water that makes steadying the housing at slower shutter speed practical; more significantly, the speed of the strobe light in the foreground (approximately 1/4,000th of a second in duration) would preclude any evidence of minor camera shake. Slow shutter speeds like this, or even slower, are my go-to techniques for deep reefs and wrecks, anything with perceptible ambient light in the background and something colorful that may be effectively lit by the strobes for the foreground. I've found I can usually hand-hold the housing with shutter speeds as slow as 1/10. Be aware that at speeds that slow it is likely that motion from subjects such as fish or divers will blur somewhat.
>On a more recent project at the Georgia Aquarium I knew enough to kick the ISO up to 640, use f/8 and a half-powered strobe for the whale shark and porkfish in the foreground and to go with a relatively slow 1/40 shutter speed to reveal divers, nurse sharks and other background details. The "vertical" light in the background was challenging that day; the light streaming through the skylights was that of overcast skies, but, gratefully, the workflow enhancements of the digital realm made adjustments on the fly (swim) much easier.
>Let's use the photo of the competitive swimmer as an example. I wanted to portray her speed and athleticism, but it was midday in very shallow water. To get my shutter speed to 1/25, I had to reduce my ISO to 100, my F-stop to f/22 and use a neutral-density filter (a polarizer, in this case) over my lens to reduce the ambient light on the scene. Because I was at f/22, I had to kick my strobe up to full power for it to have any significant effect. This meant my recycle time would be slow enough that I might only get one shot per pass as the swimmer practiced her racing starts. Again, the goal was to blend the vertical light from the sun with the power of my strobe, the horizontal illumination.
>A strobe can be set to trigger at the beginning of the shutter travel or the end. In almost any underwater exposure there are two forms of light striking the subject: the available light from the sun and the strobe light. If the shutter speed is slow enough, there will be a difference between the distances traveled by the subject as frozen by the strobe and as blurred by the ambient light. The blur can be intentionally exacerbated by panning in the direction of the motion. This makes the background indistinct, but if properly timed it can minimize the motion of the fish. The flash will freeze the action, of course, but with front-curtain sync the flash goes off, and the fish continues to swim (and be recorded). The motion will be shown forward of the fish. With rear-curtain sync, the fish moves through the slow shutter speed and at the end, the strobe goes off and freezes action. That's aesthetically preferable and perceptually more realistic.
>© Alert Diver —Winter 2013