There was a question regarding composition. The common organizing principle is to use something called the 'rule of thirds', where you divide the viewfinder into an imaginary tic-tac-toe grid. (or you can do it in reality, if your camera has interchangeable view screens. All of my cameras have grid screens in them)
You start placing objects of interest along the grid lines, or at the intersection points. For example, a city skyline shot. Instead of parking the horizon line in the middle of the frame, put it along the lower 1/3 grid line, and let the sky take the remaining 2/3, particularly if the sky is pretty. Waterfront? maybe reverse it, put the horizon at the top and let the water and the reflections take up the lower 2/3.
In a portrait, the eyes are generally the center of interest, and as such can be placed at one of the upper intersection points.
Start thinking about lines. Diagonals are interesting. They lead the viewers eye into the scene.
A guy named Alain Briot, has a lesson on this, posted at the luminous-landscape.com website.
DOF in practical terms is a function of magnification, distance of lens to subject and f-stop. The larger the sensor (or film) the more magnification required, so less DOF. That's why when we used to shoot 8x10 cameras in the studio, we were often running an f-stop of 64, which required massive amounts of flash power.
