Photography / Buying Guide

Underwater Flash


There are many accessories that are helpful or necessary for taking photos underwater. Buying an underwater housing is just the beginning. An external flash is your most important accessory. Photography is the art of creating images with light, and while the control of available and artificial light is important for all photography, it is especially crucial when shooting underwater. Flashes come in a range of prices with many different features, and the choices can be confusing for the beginning photographer. You might wonder: why should I spend $1,000 on a flash when I could get one for $250? We'll explain why flash is necessary in the first place and then detail the important features that determine one price point from another.

Why is flash critical for underwater work? The short flash duration freezes motion. But even more importantly, the deeper you descend underwater, the more you lose color. The warm colors of the spectrum are lost first, and the problem is exacerbated with deeper descents. Warm salt water acts like a giant blue filter over your lens, while cold salt water or fresh water acts as a green filter. This is due to the algae growth in the water column. Digital sensors and film are both more sensitive to this effect than is the human eye. With the addition of artificial light, however, warm colors are returned to an underwater scene. When white light is used close to your subject, it cuts through the blue or green to restore true color. Flash is the best kind of light to use, because its color temperature (the “tone” of the light it emits) is the same as daylight. Even so, if the subject is beyond 4.5' (1.5m) from the lens, the water will act as a blue or green filter over your light source, destroying much of the flash's advantage. So no matter how powerful and expensive your flash, the golden rule of underwater photography is WHEN YOU THINK YOU ARE CLOSE ENOUGH, MOVE CLOSER!! This cannot be overstated: the closer you are to your subject, the better your photos will look.

Available light photos look like they were captured with a blue filter.

Using a flash brings back the bright colors.

Beyond blue/green color variance, another problem unique to underwater photography is backscatter. The world's oceans and other bodies of water have particles floating in them, even though the water appears clear to us. Because a camera's built-in flash is so close to the lens, the light travels straight through the water column and illuminates these particles, which reflect the light back to the lens. This causes an unsightly white speckled effect that looks like snow and is called backscatter. By having an external strobe aimed correctly from the side, those unsightly reflections are deflected from the lens, avoiding backscatter. Divers with bad neutral buoyancy technique can make the backscatter problem worse by kicking up silt. Because of this, neutral buoyancy is a very important skill for the underwater photographer to master. Between avoiding both color loss and backscatter, it becomes apparent that an external flash is not an underwater photographic luxury, but a real necessity. Which one to choose? Let's examine the primary features and their underwater advantages.

Backscatter can ruin your photo.

Proper lighting and diving techniques will help avoid backscatter.

What sort of flash sync do I need for my underwater photo rig?

In order for the flash to fire when the camera shutter opens, it needs to be connected to the camera. Be sure that your flash is compatible with your camera and housing. For DSLRs and some advanced point-and-shoot cameras, you would use a sync cord. In these applications, the inside of the housing features a connection which attaches to the camera's hot shoe. This connection leads to a bulkhead that attaches to an O-ring sealed sync cord on the exterior of the housing. The sync is then attached to the flash. When using a camera/housing combination that uses a sync cord, make sure the flash you buy has the bulkhead for a sync cord.

Most of today's point-and-shoot cameras, however, do not have a hot-shoe connection. For these cameras, the flash must have an optical slave trigger. If your camera has no hot-shoe connection, the pop of its onboard flash will trigger the slaved external flash. Many digital cameras use a pre-flash to determine proper exposure before the flash that lights the exposure; this pre-flash is so fast you don't notice it. If your slave trigger is not designed to ignore this pre-flash, it will fire your external flash when the camera's shutter is not open. So, for most point-and-shoot digital cameras, it is vital to have a pre-flash-compatible slave trigger. Some of these flashes use a fiber optic cable to efficiently move the light from the on-camera flash up to the external flash. Remember, you don't want your on-camera flash only to fire on your subject, or you will get the dreaded backscatter.

Many economical flashes have built-in pre-flash compatible slave triggers, but do not allow use of sync cords. Some offer a built-in, pre-flash-compatible slave trigger as well as a sync-cord bulkhead. For people who are planning to start with a point-and-shoot camera and move up to an SLR once they get more experience, these dual-function flashes are a better option. Also, some SLR housings accept fiber optic cables or a sync cord. Fiber optic cables are reliable and a very efficient way to fire your flash. Since they are a wet connect, they remove a possible failure point on both the housing and flash. Some flashes do not have a built-in slave, but they do have an optional slave that can be connected to the flash’s sync-cord bulkhead. An advantage to this system is that you can use a 15' or 3' extension and have a second flash behind a diver, or hidden behind a coral formation to light up an area. This allows you to light up the background in a cave, or a shadowy area on the side of your scene. 

How much power does my flash need to have?

How much light a flash produces is indicated by its guide number or watt-second rating. Guide numbers really aren't a measurement of power but are instead the maximum flash-to-subject distance when a lens is set to f/1.0 and the ISO speed is 100. Thus, if a flash were rated at a guide number of 100, it would project 100 feet at ISO 100 with a lens aperture of f/1.0, but only 25 feet with a more realistic lens aperture of f/4.0. Some manufacturers give us only this specification, so it is all we have to go on. Watt-seconds are also not a unit of light output but rather a unit of electrical usage. Nevertheless, they do give a good idea of how bright a flash will be. Essentially, the higher the watt-second rating, the more light the flash will produce. Because water is denser than air, you'll need a flash with some punch—but never forget that the goal is always to be as close to the subject as possible. Some flashes have a dial that allows seamless regulation of increases and decreases in the power. This is a very handy feature: when shooting in manual flash mode you can change your power setting instead of changing your f-stop or strobe-to-subject-distance. 

Can I use TTL and auto exposure control?

TTL stands for Through the Lens. In TTL systems, the exposure information that the camera sends to the flash was measured at the digital sensor or film plane instead of some external location upon the camera. For underwater work, this becomes problematic. Generally speaking, TTL works best when shooting in macro mode, or any other time that the subject completely fills the frame. When shooting wide angle, with abundant negative space in your image, a TTL system will tend to overexpose the subject. Because of these limitations, many photographers believe that in today's digital world TTL is not worth the trouble when shooting underwater. When photographing digitally, many photographers set to the camera's manual exposure mode and use the "shoot, review and adjust" method. If your flash has a power dial, this is very easy and gives you lots of control. Other photographers feel, however, that leaving the camera and flash to do the thinking allows the photographer to concentrate on composing that once-in-a-lifetime image. If TTL is important to you, make sure your flash and housing are compatible and they allow TTL control.

TTL works best with macro work, where the subject fills the frame.

Negative space can fool TTL and overexpose the subject. Here, manual exposure control was used.

How much angle of coverage should my flash have?

Because you want to work very close to your subjects, the angle of light coverage is very important. When photographing underwater, aim to shoot with a macro lens for very small subjects or wide-angle lenses for large objects. In this way you will maintain as close a working distance to your subject as possible. So if you're using a wide-angle lens which covers an 80-degree horizontal area, you'll need a flash with a horizontal illumination angle of at least 80 degrees. Flashes that have a coverage angle of 100 degrees are best suited for wide-angle work. Using a diffuser over the flash will help to spread the light out over a wider area and give you nice, soft light. It will also help to blend the flash light with any available light.

Many experienced underwater photographers use two flashes. With two, it becomes easier to cover the area seen by your wide-angle lens. When using two flashes you can also use longer flash arms to keep the lights further off to the side—thereby keeping the flash heads away from the center of the scene and reducing backscatter. To achieve a more captivating lighting effect, it is best to use one flash as the main light with the second flash used as a fill light. The fill light is for softening the shadows and shouldn't overpower the main light. The best way to achieve this effect is to have two identical flashes with power dials. With the flashes an equal distance from the subject, you can easily dial in different power settings for variable light ratio effects. Some housings have dual bulkheads, allowing you to connect two sync cords. Some housings have one bulkhead, but they have dual sync cords. These cords have three connectors—one for the camera and one for the two flashes. It is also possible to have one flash connected with a sync cord and have the second flash fired by a slave sensor. 

What about batteries and recycling times?

When a flash fires, it takes a certain amount of time for the power to build up so the flash can be fired again. This is known as recycling time. A faster recycle time is always better, for it is very frustrating to have taken a photo and then miss a second, superior photo because the flash won't fire.

Recycle times are given by manufacturers to reflect the time you must wait once the flash is fired at full power. When photographing at a close distance in TTL mode or when turning down the power, the flash will recycle much faster than the specified recycle time. One of the factors affecting recycle time is the kind of battery that powers your flash. One brand, for example, uses a special rechargeable NiMH battery pack. This battery allows the flash to recycle in only one second after a full-power discharge, and it will fire approximately 250 times at full power on one charge. The disadvantage of this kind of battery is that you need to carry along an expensive backup battery in case you deplete the power in the one you are using. In comparison, there are others that use AA batteries. Alkaline AA batteries are readily available almost anywhere, but the flash needs 2.5 seconds to recycle and you get only 230 pops at full power. If you switch to rechargeable NiMH AAs, the flash will recycle in only 1.9 seconds and you will get 330 pops at full power. If you're on an expedition where it is impossible to recharge batteries, you are better off with a flash that uses AA batteries.

What are aiming lights, and do I need them?

Some underwater flashes have a built-in aiming light, which is a constant light source in the middle of the flash tube that helps you visualize the area the flash light will illuminate. Since everything underwater looks 25% closer, many new photographers incorrectly aim the flash in front of their subjects. This only exacerbates the backscatter problem, with very little light ending up upon the subject. Built-in aiming lights help avoid this problem.

Some flashes offer an aiming light that uses a halogen bulb. Since the bulb has a warm color temperature it is easy to see, even in bright sunlight. However, if you're very close to your subject and shooting at a slower shutter speed with a wide aperture, this light might produce a warm color spot in the middle of your image. When using such a flash in a similar situation, you might need to turn off the aiming light before shooting. There are other flashes that use LED aiming lights. LEDs have the same color temperature as daylight and flash tubes, but it really doesn't matter, since the aiming light goes off when the flash fires.  Another benefit of aiming lights is that they will also help your auto-focus camera work in low-light environments. 

What is the purpose of arms and camera trays and how do they work?

Once you've decided on the best flash, based on the features you need, you must correctly position your flash. To achieve this, you must attach the flash to an arm and attach the arm to your housing.

There are a few different kinds of light arms. Ball-joint and flexi arms are the most popular. Ball-joint arm sets have two or more segments, with each segment featuring a ball on each end. They are then joined together with a ball clamp. Each segment can be moved independently and can be extended or folded against itself. This allows for maximum flexibility in shortening or extending the arm. There's one brand of arm that allows the flash to be mounted with a flash-head adapter directly to the top segment. You can also use a ball-head adapter attached to the top ball segment with a ball clamp. This allows the flash to be adjusted without moving the whole arm. 

Flexi arms consist of a group of interlocking pieces that can be moved into different positions. They do not offer the flexibility of the ball-joint arm but they are economical and easier to use. For macro work you must use shorter arms. Flexi arms are ideally suited to macro work, and for wide-angle work, a ball joint arm system works best.

There is a third kind of arm. It consists of a 12" tubular flash arm and a mounting block. The design provides rotation in all directions and allows the tube to slide up and down for length adjustment. The arm provides versatility at a very reasonable cost.

After you've chosen an arm set, you must mount the flash arm to the housing. Most SLR housings allow the arm to mount onto the housing's handle.

For small point-and-shoot cameras, a tray is attached to the bottom of the housing, like a tripod, and arms can then be attached to the tray. Whatever style of arm you choose, remember that the purpose is to position the light. The strobe arm should be an extension of your own arm, rigid yet flexible and responsive.

Click here to compare features on the many flashes available. Once you've decided upon a flash-and-arm configuration, it is time to get wet and start capturing images. With your new housing and lighting, you'll bring back amazingly colorful photographs that will astound your non-bubble-blowing friends!!

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