It’s the light that makes a photograph. The principle also applies to underwater photography.
Sunlight penetrates water. However, only twenty percent makes it to a depth of ten metres. Less than one percent of sunlight reaches the ocean at 100 metres depth. To make things more difficult, water filters the warmer side of the light spectrum earlier than blueish light waves. That’s why the deep blue is so blue. Red disappears at five metres depth, yellow light at ten metres, green light waves make it to 25 metres, then below it’s all blue.
Yet, the world in the deep blue of the ocean is all but monochrome.
Hence, we need a light source to restore colours. A flash or a continuous light, the brighter the better. And of course it should give a nice light quality, as it’s the light that makes a photograph. The warmer our light, the deeper the blue in the background.
Apart from colour, it’s also the shape of the light source that counts. In the case of underwater macro photography, I was looking for a focused beam of light to make the subject of the photo stand out. So I constructed a light shaping snoot for my flash.
The snoot helps to isolate the photo’s subject from the background to draw the eye on the delicate details of a crab, for example.
It can be tricky to focus the light beam onto the region of interest – which is often very small in case of supermacro photography. It takes some time and training – and stable buoyancy.