Manual focus, auto-focus and focus-lock are the basic choices facing the photographer underwater. Each has its uses. Each has some disadvantages.
Manual focus comes in two basic forms, SLR manual focus and range finder focusing. Both types rely on the photographer to set the focus properly. Most people are familiar with the SLR camera’s manual focus. Look through the viewfinder, twist the lens barrel, make the image sharper or fuzzier, line up the edges of the image in the focus target ring/area of the viewfinder, and take the picture! It’s a pretty simple way to get things done.
Range finder focusing requires a bit more practice to get right. With a range finder camera system, such as the Nikonos camera systems, you estimate the distance to the subject, set that focus distance on the barrel of the lens, compose and shoot. The difficulty with the range finder system is, to get the focus right, you must correctly estimate the distance. You can end up with a lot of fuzzy pictures if you have difficulty judging distance underwater. Both manual systems can be a lot slower to get focused than auto-focus for the inexperienced.
Lots of the newer cameras have auto-focus. Look in the viewfinder or at the LCD screen, partially depress the shutter release, the camera automatically focuses on the subject, then fully depress the shutter release to take the picture. At least that’s how it works on land. The underwater world is an environment that is not very auto-focus friendly. Auto-focus works by looking for contrast, such as the edge of an object. The higher the contrast, the easier it is to focus on. Several things conspire to evade auto-focus underwater. Most of the critters are trying to camouflage themselves into the background, thereby reducing their contrast. Poor visibility can contribute to the auto-focus having a difficult time. The particles and other “stuff” floating close to you in the water can have more contrast and edge-like characteristics than the bottom or the critters you’re trying to get a picture of a few feet away. Low light conditions contribute to an auto-focus system having problems, depending on the low-light operating capabilities of the camera system. When the auto-focus can’t find anything to focus on, it starts to “hunt,” focusing in and out, trying to find something with some contrast. The time it takes for the auto-focus system to hunt for something to focus on if it misses the subject the first time, combined with the noise that might scare away shy critters, makes non-working auto-focus a frustrating experience. Auto-focus works best in good light, clear water and high contrast subjects. To help auto-focus do its job, you can find an edge to focus on, shine a spotting light on the subject if it’s dim or very low contrast, and get as close as possible.
Focus-lock tries to eliminate some of the problems that occur with auto-focus by locking on a specific focal length or distance. The photographer focuses on a subject that is the same distance from the camera as the real subject, locks the focus at that distance, then aims the camera at the real subject, moving the camera closer or father away to achieve the correct focus. By doing this, the photographer avoids the “hunting” issue and is able to achieve focus in an area or plane that doesn’t have an edge or notable feature to focus on. Obviously, focus-lock takes a little while to set up.
Some housing controls give you the option of changing the auto-focus/manual-focus switch underwater, some don’t. Some systems are only set up for one option. More options frequently translate into more opportunities to get the picture that you wanted.