Aperture and shutter are, to an extent, interchangeable as controls for exposing an image. Where the lighting is dim or extremely bright (these are relative values, and depend very much on the sensitivity of the film) there may not be much choice in the settings, but over a wide range of lighting conditions, there are likely to be several possible combinations that will give the same exposure, from a small aperture and slow shutter speed to a wide aperture in combination with a fast shutter speed.
This choice is valuable, allowing either good depth of field with the possibility of some motion blur, or selective focus together with frozen movement. There may be good reasons for favoring either of these effects, for instance, to focus attention on one object in a scene by means of a shallow depth of field or allowing a controlled amount of blur to convey the speed of a galloping horse. It is also important to have an appreciation of priorities. If stopping movement is critical, then some depth of field has to be sacrificed; if depth of field is more important, some blurring may be inevitable. In many automated cameras this choice is catered to in a selection of different programs.
