Having the right tools for the job is essential for fluo photography. In addition to your normal camera equipment, you will require a light source that stimulates fluorescence, so a powerful ultra violet (UV) or special 'blue-light' for continuous-light shooting, or my preference is 'excitation' filters mounted over the strobes that convert the conventional white-light to blue.
If using filtered strobes or blue-light you will also need a yellow ‘barrier’ filter mounted over the cameras lens that acts in exactly the same way as the mask visor, stripping away the excess reflected blue light to leave only emitted fluorescent light visible. This it is not required if using a UV light.
If using strobes, ideally you will also have a fluo-light for spotting, rather than simply taking random guesses as to what subjects may fluoresce. This will also greatly assists the cameras AF-system to lock focus. To make your life a little easier mount the light on the camera so your hands are free to access its controls.
One word of warning when you set up your camera – ensure no white light escapes from the internal flash (you will need to use tape on the inside of clear plastic housings), as this will be visible in the photos and ruins the shots!
In order to capture fluorescence on camera we have to take into account some key differences compared to conventional imaging. White light is reflected off whatever is within range of the strobe, whereas for fluo-imaging only fluorescent subjects will appear in the photograph.
The process of fluorescence is actually fairly inefficient making the light produced quite weak. Our eyes see the colours more brightly than the camera, as they are much more efficient at detecting light than the cameras sensor. However, newer generations of digital cameras have advanced considerably in this department and some models can now capture usable images in very low-light conditions.
What this means to the majority of divers that do not possess cutting-edge technology, is that you have to maximise the emitted light hitting the camera sensor in order to obtain good exposure. This can be achieved in a number of ways. First of all, you can set a wide aperture (f-stop) to maximise the light passing through the lens that reaches the sensor. However, this will have the effect of reducing depth-of-field an,d therefore, needs to be consciously considered at the time of shooting. Or you can increase the ISO, effectively making the camera sensor more ‘sensitive’ to light. But again this has a secondary effect – creating digital noise. Again this needs to be considered when depressing the shutter release if your camera does not perform well in low-light conditions.
One point to remember when dialling in your settings; we can reduce digital noise in post-processing, but we cannot create a greater depth of field.
Considering the above, increasing the amount of light the subject emits is beneficial, letting you set a lower ISO and use a smaller f-stop. Basically more light in, equals more light out! So cranking the power up on your light source will mean the subject fluoresces brighter. Also, the use of multiple strobes or lights will have the same effect. Multiple strobes set to high-power is how I now shoot, especially when lighting wide-angle scenes. Direct lighting will maximise the level of light reaching the subject, in turn creating more emitted light. There is no need to angle your strobes out to edge-light the subject, as you do not get backscatter in fluo-photos.
We all know that water absorbs light, so the golden rule of underwater photography ‘get close to your subject’ is even more applicable for the reduced light levels of fluorescence imaging. Accordingly I would suggest using a lens that has a short minimum focusing distance, enabling you to get the camera as close to the subject as possible. It also has to suitable for the size of the subject you intend to shoot, meaning you are not backing off to fit your subject into the frame.
To assist you in post processing, I would highly recommend shooting in RAW, if your camera is able to. You can then set Auto White Balance and adjust as necessary on the computer, without incurring image degradation. Adjusting the WB can have quite a big effect on how the image looks, taking the photo from very green to a warmer, more pleasing and natural yellow-hue. I have also found that it is quite common to need to boost the exposure and contrast a little, while decreasing the shadows, to make the image pop from a darker background.
One last point I feel I should reiterate. As an underwater photographer you should have excellent buoyancy control and be able to multi-task. This enables you to take good photographs. But, more importantly, you avoid damaging marine organisms. This is even more applicable when shooting in the reduced light levels that fluo-diving imposes.