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 scene on which you focus as a flat plane, much like a pane of glass, superimposed from one side of the scene to the other, so that the plane is parallel to the front surface of the image sensor—traditionally called the 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. When using a twin lens camera, such as a Fuji Real 3D camera, when you focus on a part of a scene you not only place the plane of critical focus, you also place the position of the stereo window. In playback mode or using a photo-editing program you can adjust the position of the stereo window but not the plane of critical focus.

As you manually or automatically focus on objects nearer or farther away in the scene, the plane of critical focus and the stereo window both move closer to or farther from the camera. As the plane moves, 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 or covered by the active focus area. On point and shoot cameras, it will be the area covered by the active focus area, often the center of the viewfinder or monitor.

You can lock focus, exposure and the stereo window, on any part of the scene by putting it in the center of the viewfinder and pressing the shutter button halfway down. Without releasing the shutter button, you can then recompose the scene and press it all the way down to take the picture.