Light is all that we capture as photographers. The more we understand how it behaves the more control we have over out images. Underwater light acts differently and is affected by: 1/ Distance from subject 2/ Floating materials in the water – plankton, sand and so on. 3/ Depth 4/ Weather 5/ Time of day 6/ Surface conditions
Additionally water is 800 times denser than air. So what!? Well this increased density means water absorbs light, decreases contrast and sharpness resulting in dull monotone images. So being close your subject and/or having ideal conditions with minimum particles in the water is essential for clarity in images. There are many times when this is not the case and to get the best images you will have to repeatedly go back until the conditions and light are correct – much the same as other forms of nature photography.
Depth has the effect of filtering colour out of the water and past 5 meters red has already totally disappeared with all other colours other than blue rapidly disappearing by 30 meters. You can see this in the image above! Some of this light can be restored using strobes. The direct effect of water filtering light is, that no matter how powerful your strobes are, if you are more than 2.5 meters away from your subject (5 meters travelled for light to go from strobe to subject to sensor) you will not have red in your photo – no matter how big or expensive your strobe are!
The weather affects the amount of ambient light available with clouds additionally filtering out much of the light, this is exasperated in underwater photography compared to land based photography as the water surface reflects light too. The amount of surface light reflected is directly related to the angle of the sun and wave condition. The lower the angle and the smoother the sea (less waves!) the more light is reflected and does not make it into the water.
Now I come to how I set up my camera, it is not the only way, but hopefully this will give you some tips and get you thinking about your camera setup. To capture the photos I use only RAW. As mentioned the underwater world is not an easy environment for photography and cameras have not really been designed to work in this environment, so the ability to fine tune the images in post-process is critical and JPEGs just don’t cut it when it comes to the required post-processing!
For the setup of my camera with strobes I favour full manual control over ISO, shutter and aperture settings with autofocus on and in servo mode. I do not have TTL connectors (through the lens light metering) for my housing so the strobes are set to manual as well. In fact I am not sure how effective TTL is underwater anyway.
If I am not using strobes I use aperture priority (Av) mode and an ISO of 800+ trying to achieve as high a shutter speed as possible for a sharp image. (Note: It is nearly always necessary to underexpose by a full stop or two with the fisheye).
For focus setup I use single spot focus on the centre point as that is the most sensitive focus point and with the low light environment the autofocus can be fooled at times and end up hunting. You can get around this issue for close subjects, with a focus light – conveniently built into my strobes. This is much better than a torch as it automatically turns off before the strobes fire and back on again after.
A tip for focus lights use - if you use a red filter you are much less likely to disturb the animal before the strobes fire as most fish do not see red. A white light exposes them to predators and so they tend to move off.
For 10 years I used only natural light for my underwater photographs but recently have started using strobes which works especially well for macro. However the addition of strobes has also added complexity. It seems you do not get anything for free!