Taking pictures underwater can be frustrating. Sometimes you can spend a long time taking that just perfect shot, only to be disappointed later when you download the pictures from your card and they have a deep blueish hue, or a are filled with white spots. Then odds on you have a problem with the lighting in your photos. One of the keys to taking great photos underwater is to understand what is going on with light and particles in the water. Once you have, the results should be a quantum leap in the overall quality of your underwater pictures.
If you remember your open water course, you will have learned that objects appear 33% closer and 25% larger underwater due to the refraction of light as it passes from the water into the airspace inside your mask. This property of refraction produces many interesting effects as light passes from water to air, the most well-known is the beautiful rainbow. After rain with the air laden with moisture, white light is split into its base color components as it passes through the water particles in the air.
The reason this phenomenon happens is that each component color of white light has a different wave length, with blue being the shortest and red being the longest. As the light passes from one medium to another it is refracted, with the shorter wave lengths refracting more and thus the light being separated out. These different wave lengths cause light to behave very differently in water than in air.
When it comes to water, light has a problem travelling through it uniformly due to the increased density of water (compared to air). The lights with the longer wave lengths struggle to penetrate deeper and deeper into the water and are lost. This is why a red colored object will start to appear more greenish as you start to take it deeper, and as you progress it will become a brownish/blue color. Eventually the only color is a darkish blue, and this can seriously affect underwater photos.
One of the first methods you can use to compensate for light issues underwater is to adjust the white balance of the camera. This is completely electronic and does not use an external source of light. By re-calibrating the camera sensor to an object known to be white such as a white slate (or sand or bare hand), the camera electronics can then re-adjust the effect of the different wavelengths of light that hit its sensor to compensate for the depth. This is possible because the camera knows that white light is composed of a spectrum of wave lengths and what each wave length corresponds to color-wise. As such, the highest wave length will always be a red color, and the shortest wave length will always be a blue color.
While this technique is great for novice photographers and in shallower water, it is far from perfect, since so much of the subtlety of colors are lost, especially at depth.
To get great results from your underwater pictures you will eventually have to use strobes, which are simply an underwater flash. These are much more powerful than a normal land flash, since they have to put so much more light in the picture. As a result, unlike their land counterparts it is rare to point the strobe directly at the subject of the photograph, since this will often result in over-exposed pictures with large swathes of white across it. Another unfortunate result may be that the flash will light up every tiny particle in the water and the picture will be covered in white spots called backscatter. It takes time and practice with your own setup to get the best results with your strobe positions, since each system has its own quirks. The one underlying principle though is that you are generally aiming to reduce the amount of light hitting the object to a level that will produce great pictures without them being over-exposed.
A recent addition to the underwater photographer’s arsenal is the snoot. It is primarily used in macro photography, and can vary from a small dedicated light fitted to the top of the camera to an attachment that covers a regular strobe leaving a small hole with a nozzle to really control the amount of light produced by the strobe. They are more commonly used in quite artistic macro shots where the entire background is darkened and only the colorful subject in the foreground is lit.
The principles of lighting in macro and wide angle are fundamentally the same, the main difference being a technical one. The bigger the subject of the picture, the greater the arm span of the camera strobes that is needed. This is so that you can point the strobes “off target” and still get the desired lighting effect. Some dedicated wide angle systems will have arms that span nearly a meter each way from the camera body.
Finally, the key to mastering underwater lighting is practice, practice and more practice.