What effect does closing the aperture at a fixed focal length have?

Closing the aperture at a fixed focal length has a significant impact on the depth of field and the f-number of the lens. When you close the aperture (meaning you make it smaller), the f-number increases, which directly relates to how much light is allowed to enter the camera. The f-number is the ratio of the focal length of the lens to the diameter of the aperture, and when the diameter is reduced, this ratio increases, resulting in a higher f-number.

As the aperture is closed, the depth of field, which refers to the range of distance within a scene that appears acceptably sharp, also increases. This occurs because a smaller aperture allows for a greater depth of field, bringing more of the scene into focus in both the foreground and background. This is particularly beneficial in landscape photography or situations where you want a broad range of the scene in sharp focus.

In contrast, opening the aperture results in lower f-numbers and shallower depth of field, which is often used to isolate subjects by blurring the background. Therefore, closing the aperture indeed increases the depth of field and correlates with a higher f-number.