Defining Aperature

Aperture is a hole or an opening through which light travels. The larger the diameter of the aperture, the more light reaches the film/image sensor, and vice versa. The smaller the aperture, the larger the depth of field, and vice versa. The greater the distance between the lens and the subject, the larger the depth of field. The shorter the lens focal length, the larger the depth of field.

Short Depth of Field 

Middle Depth of Field

Far Depth of Field