Optics take a dive
Underwater photography is about more than just a waterproof housing. But with the right equipment and appropriate know-how you can create fascinating images.
Under the water’s surface a new world begins, a world with its very own conditions. Water is some 800 times denser than air and therefore has a higher refraction index relative to the incoming light. Absorption, dispersion and diffraction change. And a photographer has to be prepared for this optical shift in circumstances.
But first things first: you need to make your equipment watertight. Anyone who wants to get full use of his camera in a lake, pool or ocean needs a housing made of polycarbonate or metal. This not only protects the camera reliably in the deep, it also allows the camera to be operated through built-in mechanics as you descend. High-quality housings are designed for a specific camera and allow different lenses to be used via an exchangeable port.
Mirrorless system cameras are also an alternative to reflex camera equipment. For example, US underwater photographer Kevin Palmer likes using ZEISS Touit lenses on the NEX series from Sony for his deep-dive shoots. “Not every lens excels under water. When taking pictures through water, the imaging performance of the optics is weakened by the water’s density and particulates. Close focusing is critical to minimize this and not every lens performs well behind a dome port. That’s why I was so enthusiastic about the Touit 2.8/12. Despite these conditions, the lens delivers a very sharp, contrast-rich picture. And the color rendering is beautiful.
Viewed through optical flat glass, all objects appear a third larger and a quarter closer under water. Which is why two lens types are traditionally used in underwater photography: macro and wide-angle lenses. The changed light refraction in water extends the focal length of the optics by a factor of 1.33. In other words: a 35mm wide-angle lens (based on full-frame coverage) approximately corresponds under water to a standard focal length of 50 millimeters. Macro and wide-angle lenses require different types of glass for underwater housing. For focal lengths longer than 28 millimeters, or a vantage point smaller than 75°, the glass overlay is a flat port. For wide-angle lenses, however, a flat port leads to blur in the corners of the image, vignetting and “pincushion” distortions as a result of the larger field of view.
The solution is a dome port. Combined with the captured air, this so-called dome port acts like an additional lens under water to correct optical errors. The dome port creates a virtual image (as if you were projecting a slide in front of the curved glass of the port), which is located at a very close distance in front of the lens. So when the subject is 10 feet (three meters) away that means the virtual image is at about one foot (30 centimeters). It’s not the actual image that is photographed but the virtual image, on which the lens is in fact focused.
Under water you therefore need to use a lens with as short as possible minimum focusing distance and a focusing range that allows as many small steps as possible. Why? Because when the real object moves, so does the virtual image, but only in the millimeter range. According to Kevin Palmer, “For underwater photographers, it’s a great thing that modern optical design like we now have with the Touit 2.8/12 focuses a lot more on improving the performance in the area of minimum focusing distance.”
Another challenge under water is lighting and colors. A blue background dominates under the ocean’s surface and this creates havoc with the exposure meter, because it shows a picture that’s too dark. If you rely on the automatic setting of the camera, the subject in the foreground becomes overexposed. The solution is to use the manual setting in combination with selective exposure measurement. The optimal measurement point lies at a medium distance somewhere in the background, which is how you get balanced colors and a pleasing medium blue.
The deeper you go, the more the prismatic constituents of light – red, orange, yellow, green and blue – are absorbed. The red portion is affected the most: starting from a depth of just 17 feet (5 meters) natural light no longer has any red; and from 50 feet (15 meters) it has no orange. And that’s also true for the horizontal distance to the subject: even when you go beyond five meters, a subject in shallow water will lose its redness. So to retain the natural colors, the white balance of the camera must be corrected. Having automatic underwater programs helps, but a manual method is far more precise: hold a white or neutral-gray plastic card in front of the lens, manually conduct a white balancing and the colors are back to normal – provided the depth of the dive doesn’t change.
The filtering effect of the water is also called ’extinction’. This term covers not only color absorption, but also the light’s decreasing energy with increasing depth. Put simply: the deeper you dive, the darker — and colder — it gets. That’s why artificial light in the form of external amphibian flashes and underwater strobes, as well as constant light with photo lamps, all play an important role. With the right flashes, the full colorful majesty of the underwater world can be captured in pictures, even at greater depths. But be careful: when calculating the strobe, always pay attention to the real geometric distance. A subject 6 feet (two meters) away appears only to be only 4.5 feet (1.5 meters) away. This is because our eyes are lenses looking through a flat port (our mask). The light must also travel twice as far as the subject is distant – in the example above, the light must travel through 12 feet (4 meters) of water – 6 feet to the subject and 6 feet back to the lens. The physical consequences of light appear very dramatically underwater. With practice, settings become more intuitive.
Anyone daring to enter the deep blue with a camera should be proficient in the following two areas: scuba diving and photography. But if you’re up for the challenge and go downthere armed with the right high-quality equipment, you will uncover unique treasures — incomparable images from another world.