Focus is only one of the factors affecting the apparent sharpness of your photographs, but it is a critical one because it determines which parts of the picture will be sharpest. To understand how, imagine the part of the studio setup on which you focus as a flat plane, much like a pane of glass, superimposed from one side of the setup to the other, so that the plane is parallel to the camera's film plane. The parts of objects cut by this imaginary plane will be in critical focus, the sharpest part of your picture. This plane of critical focus is a very shallow band and includes only those parts of the scene located at identical distances from the camera. If you manually or automatically focus on different parts of the setup, the plane of critical focus moves closer to or farther from the camera. As the plane moves, various objects at different distances from the camera come into or go out of critical focus. On an SLR camera, the plane of critical focus is what's normally shown sharpest on the viewfinder's ground glass. On point and shoot cameras you don't have this feature, but some cameras will outline the sharpest part of the scene on the monitor or in the electronic viewfinder. There are three ways to focus many cameras, autofocus, manual focus, and focus lock. Let's look at each.
When using autofocus, the plane of critical focus in your image will usually be the area that falls within the focus area in the center of the viewfinder when you press the shutter button halfway down. The biggest problem with autofocus is getting it to focus on the right part of the setup. If the main subject is too small or off center, the camera may focus on the background. If there is more than one item in the setup it may focus on the wrong one.
If you are having a problem with autofocus, check first that you aren't too close to the setup. Next check that you are in macro mode when you should be, or not in it when you shouldn't be (it's easy to forget). Autofocus also has problems with setups that have little contrast, when the object in the focus point is brighter than the rest of the scene, when the subject is poorly illuminated,
when both near and distant objects fall within the focus point, or when the subject is moving. If the camera can't focus, some cameras beep or blink a lamp. If this happens, or if the camera focuses on the wrong thing, use manual focus or focus lock.
When photographing with a 35mm SLR, you focus by turning the ring on the lens. Very few digital cameras have this feature, or one that's anywhere near as easy to use. However, some digital cameras do have a form of manual focus control that can help a great deal in some situations.
If your camera doesn't have manual focus, you can position the plane of critical focus using focus lock. Most digital cameras have a two-stage shutter button. When you press it down halfway, it sets focus and exposure. Some cameras beep and illuminate a lamp when these settings are locked in. If you don't release the shutter button you can then point the camera anywhere else and the settings remain unchanged. This lets you set the focus on a specific part of the setup to control both focus and depth of field.