Using Strobes for Underwater Photography
Just about any underwater photograph you view has made use of a submersible flash unit (strobe). Whether used for a touch of fill lighting, or to provide all of the light to illuminate a scene, understanding the proper use of strobes is essential to successful underwater photography. Flash units can certainly play an important roll above water too. However, ambient light for above water photography (such as in a tidepool or from a kayak) generally is the dominant source of light on the subject of the photograph. Underwater this is less likely to be true, particularly in the often-dim waters of California and New England. Lack of a strobe or the understanding for proper use places severe limits on the types of underwater photography possible.
Several factors conspire to lead to the reliance on strobe lighting. Water, being far denser than air, acts like a light sponge. Even clear tropical water steals about half the intensity in the first 10 feet of water. At typical dive depths of 50 or 60 feet, ambient light levels can be quite dim, particularly in plankton-rich waters. Fast film, a slow shutter speed, and wide-open lens would still be inadequate for forming a decent image. Strobes come to the rescue by providing a blast of high intensity light that easily illuminates the intended subject. The other reason for using strobe lighting is to bring out the true colors of objects at depth. With the red end of the spectrum extinguished by 30 feet, blue wavelengths predominate with increasing depth. Everything acquires a blue monotone that is readily apparent on film. Using a strobe, with its full light spectrum, brings out the complete palette of colors within its angle of coverage.
Electronic flashes are far more convenient to use compared to their predecessor, the flash bulb. When a strobe unit recycles, electric charge is transferred from the batteries to a capacitor, which stores the charge at a relatively high voltage (around 300 volts). When triggered, the capacitor dumps the charge into the flash tube. Strobe duration is on the order of less than 1/1000 of a second. This effectively freezes just about any motion from the camera or subject. Depending on the camera, a shutter speed from 1/60 to 1/250 second must be used in order that the strobe flash is in synchrony with the shutter being completely open. At fast shutter speeds, only a fraction of the film plane is exposed at any time since the two shutter curtains travel together to form a slit. Only at the sync speed (or any slower speeds) of the camera does the first shutter curtain complete its travel before the second (closing) curtain begin its action. If the shutter speed is set too fast, a portion of the image will lack any strobe lighting and will likely be severely underexposed.
Strobe and Ambient Light Comparison
Properly using light from a strobe requires understanding some key differences compared with the ambient light illuminating the subject area underwater. One is the rapid interval of strobe light, which effectively eliminates subject or camera motion as a problem. When using ambient light alone, any rapidly moving objects will re blurred if the shutter speed is too slow. Strobe lighting also restores color to your intended subject (at least over relatively short distances). Unless you are in shallow clear water, ambient light will not provide the brilliant frontal illumination that strobes are capable of. You thus can use smaller apertures for greater depth of field with close-up images illuminated by strobe. Forget about long distance lighting with your strobe though. Beyond five or six feet, usable strobe illumination is nearly completely absorbed by the water. Another property of strobe lighting is that it is a bit more complicated to use. This can be to your advantage though since it permits greater control over lighting. Strobes also add to the bulk of your camera system.
Strobe Distance Relationships - Manual Exposures
For any given strobe, the closer you are to the subject, the more light will be available. This means that at short distances (a foot or less), you probably can close the lens aperture all the way and still be capable of correct exposure. Try this at 3 or more feet and your strobe can strain all it wants yet never be capable of coming up with the proper exposure. Since strobe intensity per unit area falls off by 4X with each doubling of distance, you will have less light available for something 4 feet away compared to 2 feet.
You can compare strobe light output with guide numbers, a value that each strobe will be rated for. The guide number (GN) is equal to the aperture (A) divided by the camera to subject distance in feet (D) when using a film speed of ISO 100: GN = A/D. As an example, an underwater strobe with a guide number of 24 at a camera to subject distance of 3 feet should be set at an aperture of 8 (Aperture = GN / Distance). Guide numbers should be used merely as guides and not absolute values to be followed all the time. Use them to get a general idea of what aperture to set the camera lens at for a given distance. Bracketing around this value should typically be done to assure a proper exposure. You should do test exposures at various distance and aperture combinations to arrive at exposure combinations you prefer. Make sure the guide number you use is adjusted for underwater use – above water guide numbers are about double the value for underwater.
Most strobes come with a table of suggested apertures for each camera to subject distance, based on its characteristic guide number. A strobe might have a recommended aperture of f/5.6 at 4 feet and f/11 at 2 feet. Note that in going from 2 feet to 4 feet (doubling the camera to subject distance) the lens aperture should be opened 2 stops (from f/11 to f/5.6). A similar relationship occurs when doubling the distance from 1 to 2 feet. In this case the aperture setting would shift from f/22 at 1 foot to f/11 at 2 feet. Strobes often are provided with convenient charts that can be used for determining aperture for a given distance and film speed. Of course, recommended apertures should only be used as a general guide. You should do test exposures to get a feeling for the settings that work best for you.
Exposure bracketing at any distance can be done in several ways with manual strobes. You can of course open or close the lens aperture to regulate the amount of light reaching the film. Another way is to move the strobe either closer to or farther away from the subject. This is easy to do if you handhold the strobe. Move it closer to provide more light; back it off to decrease the amount of light. You can also usually vary the strobe power with a switch on the strobe. Most of the time you will probably use full power, but if desired, you can typically shoot at ½ power, or even ¼ or 1/16 power. Lower power settings come in handy if you need to open the lens to a wider aperture. For example, if you need to photograph something at f/8 from 2 feet away, you can use the ½ power setting to make a proper exposure (2 feet would normally require a full power setting at f/11 for a strobe with GN = 24).
For many photographers, determining apertures based on distance is a pain they would rather avoid. Through-the-lens (TTL) exposure control largely eliminates this problem. A sensor in the camera (separate from the light meter) determines when the appropriate amount of light has been provided by the strobe. The sensor picks up light reflected by the subject area and extinguishes strobe output when it calculates a correct exposure. This means that you don’t need to worry about how close you are – the camera takes care of the calculation. Of course, you need a camera and strobe that are capable of TTL exposure control. You also must make sure that the aperture is open sufficiently wide to cover the maximum output of the strobe. Even a TTL strobe will underexpose the subject area if you insist on setting the aperture to f/22 for something that is 4 feet away. Most TTL cameras and strobes indicate by a blinking flash-ready light if the strobe is firing at maximum output, a sign to open the aperture a bit.
Under many circumstances, TTL exposure control works well. Since the sensor is reading a small area in the center of the image, however, a small highly reflective area in the center may cause the strobe to cut exposure sooner than you would like for exposing the intended subject. Likewise, highly reflective areas in other parts of the image may be overexposed since the sensor does not include them in determining strobe output. In such cases you can adjust exposure by manipulating the ISO dial (increase from 100 to 200 for example to reduce exposure when there are reflective areas outside the sensor area). You may also decide to go to manual exposure where you can control output from the strobe. Wide-angle photography, in which you are combining ambient light from the background with strobe illumination of the subject in the foreground, is another situation where manual strobe exposure may be the way to go. Since the ambient light exposure is based on the light meter reading and the strobe exposure is based on the TTL sensor, it’s possible to overexpose a brightly-lit foreground.
When using a single strobe, the typical ideal position is above and to the left of the camera, angled down about 45 degrees to the subject. This position reduces the effects of lighting particles in the water (backscatter) and forms a pleasing shadow behind the subject. Angles greater than 45 degrees form shadows that are excessively harsh and unnatural; angles less than 45 degrees increase the chances of backscatter and can create a less desirable flat lighting.
Harsh shadows can destroy the natural appearance of your images, particularly for close-up photography. With a single strobe, it can be difficult to produce a nice balanced lighting on subjects that have some dimension, such as a fish looking toward the viewer. One side of the face may be lit properly while the other side is cast in a deep, dark shadow. An effective means for remedying this situation is to use a pair of strobes. In this way light can strike the subject from two sides and provide balanced illumination. Generally for the most natural appearance, it’s best to have a second strobe with 1 to 2 stops less output than the other. This keeps a slight shadow on the subject, thus adding a sense of depth. Equal output from both strobes leads to flat, somewhat uninteresting lighting. If you do use a pair of strobes with equal output, then you will also need to make an exposure adjustment to compensate for the doubling of light on the subject. The typical remedy is to close the aperture 1 stop (such as from f/8 to f/11). If you do reduce the output of the second strobe, exposure compensation may not be necessary. Several methods are available for reducing the output of the second strobe:
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Use a smaller, less powerful strobe
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Attach a white diffuser to the front of the second strobe
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Set the second strobe to ½ or ¼ power
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Position the second strobe farther away from the subject
With manual exposure strobes, a slave unit is a convenient means for providing the second strobe. Slave strobes require no connection to the camera. A sensor in the front picks up the light from the main strobe, which triggers the slave to fire. Since light travels so fast, both strobes will fire in the time it takes the shutter to open and close. Slaves work best when close to a relatively large subject area. Otherwise the slave won’t reliably pick up the reflected light from the main strobe and will not be triggered. You can also connect both strobes to a T-connector. In this case the camera will trigger both and you don’t need to worry about the second strobe not being set off. Paired TTL strobes can also be used. Since both will have the same light output, you can position the second one farther away or cover it with a white diffuser to reduce its output for more natural lighting.
Hand holding two strobes would require three arms on your part, unless you have an unusually patient dive buddy to help. Strobe arms solve this problem quite well. Various types of articulated arms are available that connect the strobes to a tray that attaches to the camera or housing. Arms should be sturdy yet light, and enable the strobe to be placed in a variety of orientations. They permit precise, repeatable positioning of the strobes.
Most strobes now are equipped with detachable sync cords. The cord provides the electrical connection between the strobe and the camera that enables triggering of the strobe. Some, like those for the Nikonos system, are dry connectors with o-rings that must not come into contact with water. The Ikelite connector system, on the other hand, can be detached from the housing (but not from the strobe!) underwater. This can be handy if you need to switch strobes while submerged. You should minimize strain on the sync cords, particularly excessive pulling or bending. Broken wires inside the cord will lead to frustrating intermittent firing or no strobe triggering at all.
Although strobes have become much lighter and smaller than models of ten or more years ago, they’re still relatively hefty compared to above water flashes. Most of the bulk is made up of batteries that provide the electrical charge and the capacitors that store the charge. When you trip the camera shutter, the capacitors are triggered to dump the charge into the flash tube. Strobes typically use rechargeable nicad batteries. Their advantages include the repeated ability to recharge and a fast recycle time. Alkaline batteries can also be used in many models. They will typically provide more flashes before losing charge, but are heavier, take longer to recycle between flashes, and must be discarded when used up.
The biggest strobes are generally used for wide-angle photography. Lenses with a wide angle of coverage, such as the Nikonos 15 mm (94 degrees), will require a strobe with a correspondingly wide angle of coverage. Strobe spec sheets will indicate what the maximum angle of coverage is. The typical strobe suitable for wide-angle photography will cover an angle between 90 to 110 degrees. For strobes that don’t quite cover this much area, a white plastic diffuser can help to spread the beam. When doing this however, you lose 1 to 2 stops of strobe power and will need to compensate your exposure accordingly. Small strobes typically cover areas of about 60 to 70 degrees. They are not necessarily less powerful within the area they illuminate, however, and may require about the same exposure as for a large strobe. You can’t expect to cover a wide-angle area with a small strobe, although it’s possible to use a pair to blanket most of the subject area.
A wide variety of strobes are
available for Nikonos and housed camera systems. Most come with TTL exposure capability.
Here are
the major manufacturers of
underwater strobes. Guide numbers (GN) are rated for underwater but
must be taken with a grain of salt. Manufacturers may inflate these numbers to make the
strobe appear more powerful than it really is. Make sure to do your own exposure tests to
find out the true capabilities. A guide number of 33 means that at a film speed of 100,
you can use an aperture of f/11 for a subject 3 feet away. The higher the guide number
rating and degrees of coverage, the more suitable is the strobe for wide angle
photography. The downside is the far greater expense and size.
Nikon