What aspect of a photograph does aperture primarily control?

Aperture primarily controls depth of field in a photograph, which refers to the range of distance within a scene that appears acceptably sharp. By adjusting the aperture setting, which determines how wide the lens opening is, photographers can manipulate this range. A larger aperture (represented by a smaller f-number) results in a shallower depth of field, where the subject is in sharp focus while the background is more blurred. This effect is often used in portrait photography to highlight the subject against a soft backdrop.

Conversely, a smaller aperture (larger f-number) increases the depth of field, allowing more of the scene, from the foreground to the background, to be in focus. This is particularly useful in landscape photography, where getting as much detail as possible is usually desired.

The other options—color saturation, image resolution, and contrast levels—are influenced by various other elements of photography, such as the camera's sensor, the processing methods used, and the lighting conditions, rather than aperture control. Thus, the primary function of aperture revolves around how much of the scene is in focus, making depth of field the correct choice.