To guarantee a sharp image when using either the flat or dome ports, it is best to calibrate the lenses underwater by placing a focus chart in a swimming pool or tank. Even on location, the hotel pool will offer a lot more control than an ocean or lake. Set the camera housing on a tripod and hang a focus chart on a C-stand. If possible, calibrate your lenses at night or in an indoor pool or tank. Crosslight the focus chart with two 1200 watt HMI’S, 2’ in front at a 45-degree angle to the chart. Starting at 2’, tape measure the distance from the underwater housing’s film plane to the focus chart. Eye focus the lens and mark the housing’s white focus knob data ring with a pencil. Slide the camera back to continue the same process at 3’, 4’, 5’, 6’, 8’, 10’, 12’, and 14’. This should be done for all lenses. Once a lens has been calibrated, you must establish reference marks between the lens and data ring so that you can accurately sync up for underwater focus when lenses are changed during the shoot. After marking the data rings underwater with pencil, go over the calibration marks with a fine point permanent marker on the surface.
Dome Port – To calibrate a lens with a dome port, use this basic formula to determine the starting point for underwater focus: Simply multiply the inside radius of the dome by four. That number will be the approximate distance in inches from the film plane that the lens should be set on as a starting point for underwater eye focus calibration. The most commonly used dome radius is 4”. Multiply the 4-inch dome radius times 4. That gives you a measurement of 16” at which to set your lens in the housing to begin calibration for underwater photography. If a lens cannot focus close enough to take advantage of the dome port, use a plus diopter to shift the lens focus back. Ultimately, your lens should be able to focus down to at least 10” to be able to follow focus from 1’ to underwater infinity. When using most anamorphic lenses with the dome port, you will have to add a +3 diopter to the lens to shift close focus back in order to focus on the aerial image.
Flat Port – The refractive effect of the 25 percent magnification produces an apparent shift of the subject towards the camera, to ¾ its true distance. As a general rule, for flat port underwater photography, if you measured your camera to subject distance at 4’, you would set your lens at ¾ the distance, or around 3’. For critical focus, especially on longer lenses and when shooting in low light, underwater eye focus calibration is recommended. Shooting through a window or port of a tank or pool is the same as using a flat port. If you do shoot through a tank window and want to minimize distortion, the camera lens axis must be kept at 90* to the window’s surface. Camera moves will be limited to dollying and booming to keep the lens perpendicular to the window’s surface. Panning and tilting should not be done unless a distortive effect is desired.