Which of the following methods is often used to enhance depth of field in photography?

Enhancing depth of field in photography is primarily achieved by using a smaller aperture. When the aperture is reduced, allowing less light to enter the lens, it increases the range of distances that can appear sharp in the image. This effect is due to the way optical physics works; a smaller aperture means more of the scene is captured in focus, both in front of and behind the subject. This is particularly useful in landscape photography, where the goal is often to keep everything from the foreground to the background in sharp focus.

The other methods mentioned have different effects. Increasing ISO enhances sensitivity to light but does not directly influence depth of field. Using a larger aperture actually decreases depth of field, making the background blurrier, which is often desired in portrait photography to isolate the subject. Reducing shutter speed affects motion blur and exposure time, but it does not change depth of field. Thus, employing a smaller aperture is the technique that most effectively enhances depth of field in photography.