Underwater Strobes for Digital Cameras

When starting with a digital camera and underwater housing, you'll quickly encounter the limitations of the underwater world: little ambient light and color, except for blue. In addition, the placement of the camera's internal strobe causes the light to reflect on the particles thereby generating "back scatter", that make it look like there are snow flakes in the photo.

An external strobe helps to reduce these effects, but both the camera and strobe must meet some conditions for good results. This article explains a number of these aspects and how to choose a suitable strobe.

An overview and specification of various underwater strobes is available through the menu on the left.

Digital Strobes

Pre-flash

Most digital cameras use a special flash (or even multiple) to determine correct exposure and color settings. This so-called "pre-flash" is generated before the actual flash that is used to take the photo.

Conventional slave strobes are not aware of this phenomenon and will therefore be triggered by the pre-flash and are not capable of recycling quick enough to fire again on the actual flash.

For this reason, special "digital" strobes are required that can ignore the pre-flash. Some models are even models that are able to generate the pre-flash.

Digital flash types

Two types of flashes are used with digital cameras: full-flash and metered-flash.

A camera that uses full-flash will always fire it's strobe at full strength and will adjust the CCD's sensitivity to create the right exposure.

Metered-flash cameras will stop the internal strobe once the right exposure has been obtained, so-called "quenching".

TTL strobes

TTL (through-the-lens) is a method in which the camera and strobe communicate to obtain the right exposure. The camera evaluates the conditions and informs the external strobe about the required strength and duration. In theory, TTL should simplify underwater flash photography, but the influence of aperture size, shutter speed, composition etc. is still to big to completely rule out the need for manual adjustments.

Connection

In order to use TTL functionality, both the camera and strobe must be directly connected with a cable. The housings will therefore need special features to allow the cable to pass through.

TTL, E-TTL, i-TTL, ...

Digital SLRs do not use the conventional Nikonos TTL protocol, but use their own proprietary version like E-TTL (Canon), D-TTL and i-TTL (Nikon, Fuji). These cameras can generate multiple pre-flashes (sometimes up to 15) as well as a pre-flash with an extended duration. Standard TTL strobes are therefore not capable of working with these cameras. In order to work with these TTL protocols, conversion equipment is required or strobe manufacturers should support them in future models. The Sea&Sea YS-55TTL/E strobe has got Canon E-TTL support and Ikelite is also working on their DS-50 and DS-125 flash systems to support digital TTL protocols (read the Ikelite E-TTL article for more information).

Application

TTL can provide excellent results but generally requires more specialized (i.e. expensive) equipment. This will in most cases disqualify the use of TTL for the beginner or occasional digital underwater photographer.

Flash Housings

Besides the special underwater strobes, many manufacturers also provide housings that allow underwater use of populair surface flashes, such as the Nikon SB-800, Canon Speedlite 580EX and Olympus FL-20.

When combined with a suitable camera (usually a digital SLR), camera housing and sync cord, all restrictions on digital protocols are lifted and full TTL compatibility is achieved.

Slave strobes

In Slave Mode, a strobe will "watch" the environment with a light sensor. As the camera's internal strobe fires, the amount of light will trigger the sensor and cause the external strobe to fire too.

A disadvantage of this method is that the camera is not "aware" of the presence of the external strobe. This will cause the camera to not be able for the huge amount of light generated by the strobe. To prevent overexposure, the camera must be prepared for the additional light manually. The strength of the external strobe must in many cases also be decreased, but this is a feature that is available on even the cheapest models like the Epoque ES-150DS and Sunpak G-flash.

The Ikelite strobes can even handle metered-flash cameras and stop at the moment the internal strobe stops. This will further decrease the amount of light and improve exposure.

Auto TTL

Some strobes, like the Sea&Sea YS-90Auto for example, offer a special slave mode that claims to automatically adjust the power output to the conditions. This method is based on setting the camera's aperture on the strobe, and the strobe matching its output by measuring the ambient light with its own meter.

This mode may not always be usable given the fact that not all compact cameras show the aperture to the user (nor allow manually setting it). Furthermore, the position of the strobe may cause it to measure area with different brightness than the camera is looking at, and thus still producte over or underexposed photos.

S-TTL / DS-TTL

Strobes such as the Sea&Sea YS-110 TTL and the Inon D-2000 offer these optical slave-TTL types. With S-TTL (Optical Syncro TTL) and DS-TTL (Digital Slave TTL) the external strobe not only watches the start of the camera's internal flash, but also when it stops. When the camera has determined the proper amount of light for correct exposure has been received, it will stops its strobe and therby also the external strobe indirectly.

Connection

No direct connection (as with TTL) is required for these types of strobes; an underwater camera housing that permits the use of the camera's flash is sufficient. However, this also increases the chance of misfiring. Where there are for example no objects in range to reflect the light of the camera's internal strobe, the external strobe's sensor might not receive enough light to trigger.

To prevent this, special sync-cords or fiber-optic cables are available. These are attached to the housing (e.g. using velcro) and channel the light from the internal strobe directly to the sensor. Another benefit is that these usually shield of the internal strobe, thereby reducing the backscatter.

Application

Slave strobes are applied mostly with the average digital cameras and housings used by underwater photographers.

Because both strobe and housing do not require any special features, a digital underwater set with external strobe will be significantly cheaper than a TTL set. But the camera must still provide a reasonable amount of manual settings and a lot of experimentation may be required to get good results. Also read the usage tips for external strobes.

Choosing the right strobe

1. Check if the strobe and camera can work together: TTL-mode requires both a compatible housing and camera, otherwise your choice is limited to slave strobes. For a future proof strobe you may best select one that handles both.
2. In case of a slave strobe: verify that the camera's flash can be used in the housing and that the pre-flash can be disabled or that the strobe can synchronise with the pre-flash.
3. Choose a model with a power matching your needs: a higher guide number is useful for wide angle photography or when shooting macro with very small apertures. But it may also increase the backscatter by reflection on dust particles. A variable power setting gives you more flexibility.
4. The angle of coverage must also match your camera: an extreme wide angle of the camera must be matched by the angle of light produced by the flash. A diffuser may help to further increase the strobe's beam.
5. A short recycle time is important if you want to shoot series of photos in quick succession. A long recycle time may just miss you that one shot in an underwater stampede.
6. And finally the number of flashes obtained from a strobe must match the number of photos you think you'll be taking during a dive. Also take in account repetitive dives on sites where you cannot switch the batteries.

Related information

• Sea&Sea YS-90DX and Epoque ES-150DS review
• External strobes: usage tips

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