Digital users can choose from a rapidly expanding range of accessories. These include flashunits, strobe arms, filters, macro, wide angle and even SuperWide lenses.
Before plunging ahead and investing heavily in accessories it ís important to think through whether they actually offer significant benefits for the kind of photography you want to do.

The reason for taking the minimalist approach is that entry level digital systems are extremely versatile as is. So they are much less dependent on add on equipment to get good pictures. That said, for certain types of photography and in some conditions, additional equipment can make your picture.

Flashgun?
Built in guns have their advantages. The light is always aimed at your subject, exposure is automatic and there is no additional bulk and drag.
Seperate flashguns are needed in low visibility conditions. To avoid backscatter, light reflecting off debris between the light source and the subject, you have to move the light source off the camera. Typically you'll want to get the strobe 30 cm to 45 cm off the camera. "Built on" strobes provide a little additional power, but do nothing to reduce backscatter.
Separate flashguns may also be essential with some add on lenses that can obscure the built in strobe. For extreme close ups off camera flash may also be needed to light your subject evenly.
Built in flashguns provide only one kind of light - full frontal. If you want to light your subject creatively to enhance textures or create a mood, then an off camera flash will let you do this.

Choosing a flashunit

Digital stills cameras don't automatically operate well with all flashunits. If you already own an underwater film camera then it ís quite likely your existing strobe won't work well (or at all) with a digital camera.
Digital cameras present two problems for flashgun designers. Firstly, they don't measure the light in the same way as film cameras. That has meant going back to the drawing board for underwater flashgun manufacturers.
Digital cameras put out a series of pre flashes to determine correct exposure. Many digital cameras and housings cannot easily be wired to an underwater strobe. So the strobes have to be fired by a slave. The light from the cameras own strobe triggers the underwater gun. With film cameras it ís relatively simple to slave a strobe. The triggering strobe fires once and the slave fires with it.
With pre flashes it ís more complicated. The slave gun has to fire at the right time. So it has to fire in sync with the final flash from the built in gun. Complicating things further, the number of preflashes isn't standardized.
To get over this, some guns are designed with high speed recycling - they simply fire with all the preflashes and the main flash. Others offer a programmable feature that can be set to ignore preflashes.
Another issue is putting the right amount of light on to your subject. This is less of an issue with film cameras because print film is easily manipulated and corrected at the processing stage. So for entry level film cameras a simple flashunit with just one power setting is usually adequate.
Digital cameras require much more accurately controlled lighting. There are four principle methods of getting the right exposure with off camera strobes. Itís important to understand the differences and to thoroughly check the small print to confirm the strobe you are considering really does operate the way you think it does. At the moment we feel that some manufacturers and suppliers are creating confusion by claiming greater degrees of automation than their guns strictly provide. It also does not help that the industry has not standardized terms to describe how strobes work and what they can and cannot do.

Manual flash units do not provide automatic exposure. If you use a gun designed for a film camera, and can get it to sync with your camera, you may be able to get good exposures. However it takes experience and skill. Usually a strobe designed for a film camera has only got one to three power settings. This does not give you a lot of discretion for shooting at different distances or for adjusting for different shades and reflectivity.
A manual strobe for digital cameras will usually have six or more power settings. Like the dimmer on a light switch, having so many choices gives you lots of scope for lighting all sorts of subjects over a range of distances, from supermacro to distance shots.
To shoot with a manual gun, you take a picture, review it and then adjust the power of the gun until you have the exposure you like best. With a little bit of practice it becomes much less hit or miss than it sounds! Youíll probably get the exposure right first time more often than not. However it is a problem with moving subjects that keep varying their distance, because you have to change the power settings in time to their actions.

Camera Controlled Flash
Our definition of a camera controlled flash is a system where the cameras own internal flash is used to control the exposure of the slaved gun. To do this a fiber optic cable is placed in front of the cameras own flashunit. When you take a picture the internal gun fires and triggers the external gun like an ordinary slave. The camera sees a burst of light and assumes it came from itís own gun. When the camera determines that the subject has been properly lit, it turns off the built in gun. This in turn shuts off the external gun, ensuring a properly exposed flash picture that is fuss free and consistent.
Because the sensor that controls the flashgun is built into the camera you cannot miss aim it. It ís also less prone to being affected by backscatter as it is not normally in line with the strobe. A further benefit is that if your camera has a flash compensation feature, this will also control the external strobe as well. This can be useful as it allows you to customize the exposure to your taste. For example if you photograph divers it ís quite common for the auto exposure to be fooled by dark equipment on their bodies. This causes the automatic exposure to boost the flash power to compensate. The result is that the divers face is often greatly overexposed. By setting the flash compensation exposure to underexpose a little, this problem is solved. If your camera does not have this feature you'll find it provided on some underwater guns. These are normally just selected on a dial and can be faster to use than a menu based selector.
Also because the external gun is controlled directly by the cameras own built in gun, adjustments made to the camera are automatically relayed to the strobe. If you alter the aperture or film speed there ís no need to make any additional adjustments to the strobe. So there ís almost no scope for making an error by being forgetful or narced.
Camera controlled flash is our recommended solution for working with currently available consumer camera and housing combinations represented by Olympus, Sony, Canon and similar own label suppliers.

Lenses

Wide Boys

Wide angle lenses serve three main purposes. They let you shoot large subjects, operate in low visibility and manipulate perspective.
Water has two main effects on underwater photography. It isn't really clear. Even crystal clear water only equates to a foggy day topside. The debris in the water column physically obscures your subject, making it look soft or out of focus. The more water you have between your camera and the subject, the worse the picture looks.
To keep the picture sharp, you need to get as close as possible to your subject. By reducing the water column between your camera and your subject, you also reduce the amount of detritus you have to shoot through. With large subjects like whale sharks and wrecks, a wide angle lens is essential for good photography.
The second effect of shooting through water is that it quickly absorbs colour. Youíll only get bright reds, for example, in shallow water over short camera to subject distances. To counter this flashguns are often used. However flash has a limited range through water. So getting close, even with flash or video lights is also vital to capturing vibrant images. Wide angle lenses let you get close enough to large subjects, like other divers, to get good colours.
In low visibility, wide angles become vital for anything other than macro and close up photography. In the typical conditions found in the UK, you might have to photograph large subjects like your buddy from under a metre away. Wide angles let you do this.
Wide angle lenses also let you play with perspective. Close focus, wide angle, is an example of this. By placing a smallish subject in the foreground a few inches in front of the lens you can make it tower over a diver just a little further away. Half and half shots or split levels are also possible with some set ups.
For consumer cameras, wet lenses are readily available. These either screw directly into the housing or mount via an adapter. They can be fitted and removed underwater in a matter of seconds, providing great flexibility. Holders let you store your lenses safely, usually on your strobe arm.
Wet lenses can sometimes be used in air. Typically they cover 85 to 100 degrees - roughly equivalent to a Nikonos 20mm or 15 mm lens. The exact coverage will depend upon your cameras own lens.
Prosumer cameras like the Nikon D100, Fuji S2, Canon D30, 60 and D10 use normal land lenses placed behind special ports attached to the housing. You need to choose your lens before the dive.

Macro Lenses

Most consumer digital cameras have a macro or extreme close up facility built in. This lets you photograph tiny subjects like Nudibranchs or Porcelain crabs for instance. However there may be limitations on how close you can really shoot underwater because of flashgun limitations. Some cameras donít permit the use of flash at very near distances because the flash cannot light the whole frame evenly. In this case an off flash will not work because there ís no flash from the camera to trigger it.
Macro lenses usually attach to your housing via a screw thread or an adapter. They allow you to shoot from further back while still filling the frame. This lets you use your built in flash or an off camera strobe as you prefer. You can also use much of your zoom range for more control over the final composition. Keeping your distance also helps to avoid damaging coral and can make shy creatures easier to photograph.
Prosumer cameras use macro lenses that are designed to provide a continous focusing range from a few centimeters away to infinity. For even higher magnification Prosumer camera owners may add teleconverters or close up lenses.