An aperture is a hole or an opening inside a camera lens, through which light passes to enter the camera body. The size of the aperture is expressed as an f-number or f-stop (such as ƒ/2.8), with a lower number indicating a larger opening. A wider aperture allows more light to pass through the lens and reach the image sensor, resulting in a brighter image. In addition, a wider aperture results in shallower depth of field, which means that objects in the foreground and background will appear blurry while objects in the middle remain in focus. A narrower aperture has the opposite effect: less light enters the camera, resulting in a darker image with greater depth of field (foreground and background objects will be more clearly visible).
The term aperture can also refer to the actual diameter of thehole or opening inside the lens. For example, ƒ/2.8 indicates anaperture diameter of 2.8 millimeters (mm). The relationship betweenf-stop numbers and aperture diameters is not linear; each increasein f-stop corresponds to a halving of the diameter (and thus halfof the area) ofthe aperture opening. So ƒ/4 corresponds to anaperture diameterof approximately 2 mm, while ƒ/5.6 corresponds to anaperturediameterof approximately 1 mm