The aperture of the lens determines how wide the lens is open to let light in to expose the picture.
Aperture values are incremental and are always displayed in f-stops.
The table above shows aperture values in full exposure stop increments.
When the aperture value is increased by 1 full stop (f4 – f5.6) the amount of light hitting the camera sensor is halved. Similarly if the aperture value is decreased by one full stop (f2.8 – f4) the amount of light hitting the sensor is doubled. Much like the iris of the human eye which becomes larger to let in more light the iris or aperture of the lens is made larger to let more light in, and smaller to let less light in.
The aperture is typically used to control the depth of field in an image. The larger the aperture (lower f-number e.g. f2.8) the less of the image there is in focus. The smaller the f-number e.g f16, the more of the image there is in focus.
Larger apertures are typically used to isolate details in images and render the background out of focus. Smaller apertures are typically used to keep everything in focus. Landscape photos are typically taken at smaller apertures.
The wider the lens is open, the smaller the f number. For example f1.4 is a very wide setting that will allow a lot of light through the lens onto the sensor, however a very small aperture for example f32 let's only a small amount of light onto the sensor.
The following diagram illustrates a lens aperture.