Digital Underwater Photography

Strobes

As mentioned on the Equipment page, Strobes section, strobes help bring back the colours underwater.

Automatic Flash

Many cameras will have a setting option of automatic flash. In this scenario, the camera decides on the correct output power as well as whether to fire the flash under the current light condition. When automatic flash settings are set to on, the flash will not fire when aiming into blue open water because enough ambient light exists. On the other hand, when aiming at subjects that are close to the coral reef, or under low-lit conditions, the flash will be activated.

You should remember that all strobes have a maximum of distance they can reach. You can choose to have your strobe on all the time but if your subject is too far away, it will have no effect on the subject.

Many camera housings come with built in diffusers for internal flash. A diffuser helps disperse the flash light more evenly.

Backscatter and Artefacts

Backscatter is the reflection of particles in the water back to the direction they came from. It is an important concern in cameras where the strobe is fixed near the lens, thus even the smallest amount of sediments in the water will show up as backscatter. Only use your internal flash in very clear water and turn it off in water that is not clear to avoid backscatter effect. You should also try to get as close to your subject as possible to limit the amount of water between the camera and the subject.

A good solution to avoid backscatter is by using an external strobe mounted on arms and position it away from the lens.

External Strobes

External strobe is the best flash option in underwater photography. When using external strobes, you may choose between single strobes to a two strobes setup. When selecting an external strobe, choose one that is specific for digital cameras with automatic and manual settings and that is compatible with your camera and housing.

One problem with a single strobe is that it can create a harsh shadow effect behind an object. To avoid this effect, try and shoot in an upward position so your subject is lit by natural sunlight as the main light source and using your strobe as a fill light. Another solution can be to use multiple external strobes from two different angles.

Strobes Positioning

When using a single strobe, position it away from the camera either above the camera or at 45 degrees angle and aim it at your subject.

Waz taking a photo of a cruising shark

When using multiple strobes of unequal power, the powerful main strobe simulates sunlight and the weaker fill strobe balances the main one. You should position your main strobe off the camera and above the subject about 45 degrees. The smaller strobe should then be positioned on the opposite side at a lower angle.

When using multiple strobes of equal power, set both strobes on either side of the camera at camera level, aimed straight ahead. This setup is very useful for very wide lens. In silty conditions, aim the strobes slightly away from the camera to avoid backscatter.

Strobes Temperature

Another factor to consider about strobes, though not as important, is colour temperature. Strobe output is measured in degrees Kelvin and the important thing to remember is that the lower number indicates warmer light. Most strobes are generally rated at about 5200K, approximating the colour of direct sunlight; a "warm" strobe is rated around 4800K. Warmer strobes have a tendency to shift light output very slightly towards orange. If your subject is a diver, this has the effect of making flesh tones a bit more lifelike and pleasing. The light emitted from the strobe is usually white balanced to sunlight in order to produce normal colours.

TTL

Determining apertures based on distance can be hard for many. Through-the-lens (TTL) metering 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, calculates a correct exposure and output correct flash power. This means that you don’t need to worry about how close you are—the camera takes care of the calculation.

In theory, TTL automatically adjusts for any lighting situation, adjusting the strobe’s output for perfectly-exposed pictures, regardless of the aperture chosen or prevailing light conditions. In essence, light pours out of the strobe, reflects back from the subject, and then a sensor measuring the amount of light striking the image sensor stops the output when an ideal exposure is detected.

If the subject and foreground are approximately equally distant and the TTL sensor is able to get sufficient reflective light to calculate the proper exposure, the strobe can more easily provide ample power. The problem with TTL metering is that in real life several situations will confuse the sensor to give an incorrect exposure. For example, a situation that might mislead the sensor is when there is an open expense of water. The strobe will fire at full power because there is not enough reflective return. A second condition that can confuse the strobe is when the subject is back lit by natural sunlight. The strobe might consider the sunlight as overpowering source and will fire at minimal power. A third situation that will fool TTL metering is a very small reflective area or highly reflective area. The TTL metering will interpret this situation as one that requires the least amount of light and thereby under exposing the image. Under the right conditions, TTL metering works well. As long as you avoid those situations, which mislead TTL metering (like sun bursts), you can generally rely on it to give pleasing exposures. To counter the limitations of TTL metering, you can calculate the proper exposure by the guide number method, which is easy and is never fooled by other factors.

Know Your Strobe Guide Numbers

Guide numbers are a way of rating the strength of a strobe’s output. Basically, the higher the number, the stronger the strobe. The number can usually be found in the specifications literature provided by the manufacturer. Be sure to note whether the guide number provided is for use when shooting on land or underwater. If you are given a topside guide number, divide by three to obtain an approximate underwater guide number. Some guide numbers are provided for a specific ISO setting because the guide number will change with the ISO setting. Essentially it is a ratio between aperture and subject distance, using ISO settings as a reference point.

A guide number is used in two versions of what is known as the exposure formula in order to help you determine the correct f/stop or strobe-to-subject distance. Our formulas are:
Guide number divided by distance equals f-stop
or
Guide number divided by f-stop equals the correct strobe-to-subject-distance

For example, if you are shooting macro, you will want to select the lowest f-stop number your lens can work with. Let’s say for the example we set the aperture at 5.6 f-stop and our strobe guide number is 24. 24 divided by 5.6 equals 4.28 feet, meaning our strobe should be about 1.5 meters away. 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 to for a given distance. Bracketing around this value should typically be done to assure a proper exposure.

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.

Please consult manufacturer’s operation manual for proper strobe operation.

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