Guide to Underwater Photography
The rules for exposure under water are much the same as photography on land, you need to adjust the amount of light that enters the camera, by making the appropriate adjustment to the aperture and shutter speed, and then temper as needed by tweaking the ISO.
Your aperture will be defined by the focal range you would like to achieve in your image. Remember that a larger aperture (smaller number e.g. f1.4, f2, f2.8) will provide a tighter depth of field, where the subject will be in focus but the foreground and back ground will be blurred. A smaller aperture (larger number e.g. f11, f22, f32) will provide a wider depth of field, where the detail in front of and behind the subject will also be in focus.
With your aperture chosen, you then need to select your shutter speed to balance. As a rule of thumb, aim to keep the shutter speed equal to or faster then 1/60th of second. Given that you're not going to have two feet planted firmly on the ground, any slower then that, and you may start to see camera shake in your results. Light is at a premium under water, so expect to have the ISO set to 200 or 400 to compensate.
By Specialist underwater cameraman Stuart Keasley
Colour balance does present us with a bigger challenge. The colours we see are the visible portion of the spectrum of light; different colours are represented by different wave lengths. Blue has the shortest wave length with the most energy, red is at the other end of the spectrum with the longest wave length and the least energy. Light travels quite effectively through air, so the difference in energy between the colours has negligible effect on how far they will travel and the colour balance will tend to stay the same from the light source until it reaches the subject.
However, the energy is lost very quickly when it tries to penetrate water. We see this as a gradual loss of colours, starting with the reds, yellows, greens and then finally blues. At just 6 metres, a normally vivid red object will appear a rusty and dull brown. Inadvertently stray too close to a coral and cut yourself at 20 metres depth, your blood will run a rather disconcerting green. Down at 40 metres, everything will be swathed in a blue hue.
There are a number of options open to us to address this problem;
- Choose your dive time and site to minimise the effect. A shallow dive into flat calm crystal clear sea under the midday sun will have much better light penetration, and therefore less colour loss, then a later afternoon dive into a choppy sea where the swell has kicked up the bottom and reduced the visibility to a few metres. However more often then not, the dive times and locations are dictated to us by the dive operator and the tides, the opportunity to pick and choose is rarely ours to take.
- Place a red filter in front of the camera lens to help restore some of the natural colour balance. The filter blocks the yellows, greens and blues to bring them more in line with the reduced level of reds. Unfortunately filters will only give accurate results for a certain depth in certain conditions, anything else will still leave an imbalanced image, the further away from the required conditions, the less appealing the image will look. In addition, we are already struggling in a low light environment, putting a filter in place to further reduce light is not the ideal.
- Use the custom or manual white balance setting. A control introduced with digital cameras that lends itself particularly well to this problem. A white object, in digital terms, is an object that has equal amounts of red, green and blue to a high exposure. If you change the balance of those three colours, you will see the object take on different hues and tints. With custom white balance, you show the camera what you believe is a white object, a white slate, the sand, or even your hand if nothing else is available. The camera will then look at the colour that has recorded, calculate how much red, green or blue needs to be added to return that colour to white. The calculation is then stored and applied to any subsequent photographs. This does have limitations, the act of boosting the required channels will introduce noise or imperfections, to the picture. The more you boost, the worse the noise gets. The camera does also need some of the colour to work with. If you are at a depth, or in conditions, were all of the red has been wiped out, then the camera will not be able to bring anything back.
- Use an external strobe light (flash gun) as a local light source. The final technique, and arguably the optimum one for bringing out the vibrant colours for which the marine environment is renown. The high burst of light replaces ambient light and will restore your subject to its natural glory. It does add an extra level of complexity to your exposure calculations, and will place a noticeable dent in your finances, however if you really want to wow your friends and family, then it is the only way to go.
Using a strobe light
There are some does and don'ts to using a strobe. While the strobe will produce a powerful burst of light, its energy will still be absorbed by water. So get as close as you can to your subject, when you think you can't possibly get any closer, get closer still. This will reduce the amount of water between you and the subject, and so allow as much light as possible to reach it from your strobe. You should also place the strobe light on an arm, so the light comes in from a different angle, this will reduce the amount of back scatter, caused by light hitting particulate in the water between you and your subject.
Also, have a thought for the sensitivity of the creature you are blinding with flashes. The majority of marine animals don't have eyelids, or any way of protecting their eyes from bright light. A half hour photo session in the midst of a group of over zealous divers probably won't do their nerves any favours.