Light Underwater – Considerations with Photography
Underwater photography is much the same as land photography; that is, it is the art of capturing the light onto the sensor. The issue with underwater photography is that light behaves very differently underwater than on land. The major factors effecting the light are refraction and absorption.
Refraction of Light
Another property of water to be aware of, albeit not as important as the absorption issue, is refraction. Technically, refraction is the change in direction of a wave due to a change in its speed, which occurs when passing from one medium to another - such as between the ocean and the air in your mask or dome port.
Water has a refractive index of about 1.33, while air has a refractive index of about 1.0003. Practically, this difference in refractive index makes objects appear bigger and closer underwater, approximately 25% bigger. The example commonly used to demonstrate this property is placing a pencil halfway in a glass of water. The pencil will appear to bend at the water's surface due to the bending of light rays as they move from the water to the air. Simple, right?
Remember that practically, what you see in your viewfinder or LCD screen is what you get. Refraction is an issue that you need not worry yourself over. There are ramifications for how the ports on your housing are designed, and where your lenses focus underwater. However, the manufacturers design ports to work within the parameters of refraction, and in certain situations, like when shooting a DSLR and a mid-range zoom lens, you may need to use a dioptre in order to obtain the best focus underwater, but this is a totally different topic from lighting.
Absorption of Light
Ever been frustrated with blue/grey images with dull and monotone colours. Don’t worry this is totally normal. This is because light waves are absorbed in water due to the fact that water is around 800 times denser than air.
The effect this has is that the light wave is absorbed into the water and simply put the light disappears. However if you remember your school days light is not just one colour it is made up of a spectrum of colours with different wave lengths. Simply put you can consider that light is made up of three colours. If you had three torches producing exactly pure red, green and blue and shone them on a perfectly white wall this would be the result:
We can see here that then even combination of red, green and blue creates white. Combinations of just two colours create the secondary colours of yellow, cyan and magenta. If we then take this one step further a combination of 100% red and 50% green creates orange. Violet is between magenta and blue. This then creates the rainbow... We can see this in a rainbow on land ROYGBIV (Red, Orange, Yellow, Green, Blue, Indigo & Violet). Red carries the lowest amount of energy and is the fastest to disappear underwater with blue being the last (See diagram). The depths of this absorption is different due to many factors such as weather, surface conditions and visibility. The diagram below demonstrates the typical depths for wave length light loss:
There are three general ways of getting round this problem with underwater photography. You either reintroduce light with torches or strobes (Flash) or adjust the camera’s meter to adjust the white spectrum (White balance) to a new standard. White balance works by adjusting the sensors sensitivity to the different wave lengths to create a ‘new’ white. Or you use filters or magic filters that attach to the end of the lens to adjust the colour the sensor picks up directly.
You also need to consider that when using either methods to rebalance the spectrum the subject distance also needs to be considered. If your subject is only one meter from the camera you will have all of the colours of the spectrum again. However the background from the subject will start to lose colour the further away again giving you a dull backdrop (Which is not always bad).