 Take a look at the two photographs on this page. They share some important similarities. Both images are of similarly sized sharks. Both images were created with the same camera, lens and strobes. I used ISO (sensitivity to light of my digital camera’s sensor) 200 on each shot. For both photographs I was about 4 feet (1.2 m) away from the tip of the shark’s nose with my camera.
But one important feature is significantly different. In the photograph of the sixgill shark that I acquired on a night dive off Seattle, the water appears black, while the water surrounding the Galapagos shark I photographed during a daytime dive in Hawaii is a beautiful blue. The difference in the color of the water in the two images is significant not just in the way the photographs look, but with respect to what I had to do to get the proper exposure for each image.
In the case of the sixgill shark my exposure depended on the light emitted by my strobes. The key elements to consider were my ISO, the power of my strobe(s) and, ultimately, the distance from my key strobe to the shark’s head. Light from a strobe dissipates as distance increases, and you can see that intensity of the light falls off as you look from the shark’s head toward its tail.
To get a good exposure I needed to have a good idea what F-stop to use according to my strobe-to-shark distance for the strobe power and ISO I was using. Although it can be calculated, as is the case with many strobes, a chart on the back of my strobe that is provided by the strobe’s manufacturer provides me with that information.
For the Galapagos shark photograph, getting a proper exposure involved a slightly different process. The first thing I needed to do was determine the proper F-stop for the water in the background. I wanted the water to look blue (not black and underexposed or whitish blue and overexposed) just like it did to my eyes when I was underwater. So, I took a light meter reading on the water right next to the shark to establish my F-stop and properly expose the water. If your camera doesn’t have a built-in light meter, you can simply take a picture of the water and look at the camera’s LCD display, or the camera’s histogram (a graph that can tell you a lot about exposure), and make any needed adjustments.
Once I established my F-stop, then according to my ISO and the power setting of my key strobe, I was able to use the same chart on the back of my strobe to determine the right strobe-to-shark distance (using the shark’s head as my target) to properly expose the shark.
The subjects are similar, but getting a proper exposure at night and during the day when you want natural-looking blue or green water in your shot involves different techniques.
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