For underwater, I use a pretty static setup without much variability. To me, the underwater choice is all about the most flexible camera setup; No lens changing happens underwater. But in air, I use a range of gear.
My photo opportunities typically range from closeups to people to city scenes to broad landscapes (or seascapes — you get the picture). Cities and monuments are often most picturesque and/or easiest to shoot without large crowds in low light, either at dawn or dust. Many scenes are at their most spectacular at the extremes — shooting into the sun or after dark when the sun or lights are the main photo attraction. Distances in cities are shot from either short-range at street level or very expansive for skyscraper-height city views. Landscapes outside the city could be shot with anything, but the focal length should be carefully chosen to frame the scene to best advantage. Small subjects need macro, and people are best at portrait focal lengths around 100mm.
My “core” lens usually includes just four main lenses. I carry an extreme wide-angle lens to shoot big things (buildings and marquees) from ground level, for expansive land- and city-scapes, and to capture room-filling scenes indoors. Also, nothing beats a good wide-angle for great depth of field to permit unconventional quick shots without much focus adjustment. For greatest flexibility near to far and framing for interest on the fly without lens changes, my “main” lens is an extreme (wide to telephoto) zoom. But aside from that one lens, I tend to prefer fast lenses (f/2.8 or better) to permit hand-held shooting even in low-light situations. To cut down on weight and reduce number of lens changes, I favor zoom lenses.
So what is my usual? Four main lenses:
I augment these basic lenses with a very few basic filters. Only a couple of filters are routinely needed in this digital world. Shooting in camera raw format, most stuff for which we once needed filters when shooting on film (color balance and focus effects) are easily adjusted later, on the computer. I tend to use only a couple of filters all the time: A circular polarizer to cut reflections and bring out more colors in bright light; and a range of neutral-density filters to control light when shooting into the sun or when light is not well balanced across the frame. Those filters are still needed with digital because they do not change colors but do allow us to manage light and therefore contrast. Too much contrast (very high dynamic lighting range) may exceed what the camera sensor can capture, resulting in blown-out highlights or lost details in shadow. What the camera can’t capture, cannot be saved. Recently I have begun to experiment with high dynamic range (HDR) processing, but it is still worth my while to carry neutral density filters.