Learn from Sport Diver Images Columnist Jason Heller about underwater photography terms. From aperture to zoom lens, it's the terms that will help you gain a clear idea of how to get your best shot. Aperture The aperture moderates the amount of light entering your camera lens. The wider the aperture, the more light hits the sensor. Backscatter Particles in the water column that appear as grains of dust on your images. By placing strobes on articulated arms extended away from the camera, you achieve even and soft lighting with limited shadows — and reduces backscatter. DSLR Digital single-lens reflex camera. Focal Length The focal length of a lens is defined as the distance in millimeters from the lens' optical center to the focal point (once always the film plane, and now more commonly the digital sensor). Wide-angle lenses have short focal lengths, provide a wide angle of view and a greater depth of field, while telephoto and macro lenses have longer focal lengths and compress perspective. Macro Lens Macro lenses have long focal lengths and are differentiated from others by their ability to focus on tiny subjects close to the lens itself. The most commonly used macro lenses are 60 mm and 105 mm, and the close focus distance allows you to fill a frame with life-size, or larger than life-size images of very small subjects. However, the closer the subject is to the lens, the shallower the depth of field. Compensate by shooting at the smallest possible apertures (highest f/stops). Megapixels Megapixels are derived by multiplying the number of horizontal pixels by vertical pixels (for us tech geeks, this is the square of the linear resolution). Therefore, doubling the megapixels doesn't double the pixels, rather it equates to less than a 50 percent increase. Prime Lens A prime lens has a fixed focal length. Super Macro Want to shoot the smallest of the small? You'll need extra focal length and magnification to accomplish this specialized form of imagery. It helps to also have good eyesight, stable hands, perfect buoyancy and patience. A teleconverter is a secondary lens mounted between the camera body and the main lens to extend the focal length and provide the necessary magnification. A 2x teleconverter doubles the magnification and is commonly used for super-macro underwater shooting. Wide Angle Lens These lenses can be fixed, zoom "rectilinear" (no image distortion) or fisheye, which creates a unique super-wide perspective with purposeful and creative distortion. Wide-angle lenses require a dome port, and some lenses will additionally require an extension ring, which sits between the housing body and port. Because you can shoot very close with a wide-angle lens and yet still make your subject appear to be several feet away, wide-angle lenses allow you to "take out the water" (and its associated backscatter), and make even slightly turbid water appear extremely clear. Zoom Lens A zoom lens allows you to vary focal length within a defined range (zoom in or out). Zoom lenses provide flexibility for creative composition and multiple perspectives. However versatile topside, long zoom ranges do not work well underwater and should be avoided.