The aperture of the lens refers to the size of the opening inside the lens where the light passes through to reach the film or or the digital sensor.

The aperture is measured in f-stops as indicated by the series below:

f/1  f/1.4  f/2  f/2.8  f/4  f/5.6  f/8  f/11  f/16  f/22  f/32  f/45  f/64

Each progression in f stop to the right in the above table lets half as much light through the lens as the preceding number.

The f-stop number represent the ratio of the focal length of the lens to the diameter of the lens diaphragm opening of the lens and is calculated as follows:

Lens f stop, f = D/F

Where: D = Diameter of the lens iris

and F = Focal length of lens

A higher aperture (between f/16 to f/64) will give greater depth of field but will also let less light to the film or sensor thus requiring lower shutter speeds. A lower aperture (f/1.4) will give a smaller depth of field but will also enable you to separate a subject from the background by selective focus. Generally a lens is referrred to as a fast lens when it has a large aperture opening, equal to or greater than f/2.8.

Aperture f/1.8

Aperture of f/1.8 in an 85mm lens gives shallow depth of field.

Aperture f/5.6

Aperture of f/5.6 in 85mm lens will give
reasonable depth of field and shutter speed.

Aperture f/16

Aperture of f/16 in a 85mm lens will give great
depth of field but requires a higher
shutter speed.