Aperture and a Rock

This is a rock under a train car. In one image, the rock is the only thing in focus, and in the other the train is in focus. This is because a larger aperture, the shallower the depth of field. Depth of field refers to not how far away the focus is, but how deep the the field of focus is. Smaller apertures allow for a greater depth of field. Also, smaller sensors have greater depth of field, that's why cell phones easily have many people in focus and can focus easily in good light.