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.