Aperture is “The size of the opening in the lens when a picture is taken.”
When you hit the shutter release button of your camera a hole opens up that allows your cameras image sensor to catch a glimpse of the scene you’re wanting to capture. The aperture that you set impacts the size of that hole. The larger the hole the more light that gets in – the smaller the hole the less light.
Aperture is measured in ‘f-stops’. You’ll often see them referred to here at Digital Photography School as f/number – for example f/2.8, f/4, f/5.6,f/8,f/22 etc. Moving from one f-stop to the next doubles or halves the size of the amount of opening in your lens (and the amount of light getting through). Keep in mind that a change in shutter speed from one stop to the next doubles or halves the amount of light that gets in also – this means if you increase one and decrease the other you let the same amount of light in – very handy to keep in mind).
One thing that causes a lot of new photographers confusion is that large apertures (where lots of light gets through) are given f/stop smaller numbers and smaller apertures (where less light gets through) have larger f-stop numbers. So f/2.8 is in fact a much larger aperture than f/22. It seems the wrong way around when you first hear it but you’ll get the hang of it.
The main reason photographers change the aperture is to control Depth of Field.
Depth of Field (DOF) is that amount of your shot that will be in focus.
Large depth of field means that most of your image will be in focus whether it’s close to your camera or far away (use small aperture).
Small (or shallow) depth of field means that only part of the image will be in focus and the rest will be fuzzy (use large aperture).
Choosing the correct aperture (f-stop)
Aperture f/5.6 and lower
This will isolate the subject from the background. The main subject will be in focus while the background is blurred. This is best used for portrates or close up shots of objects such as flowers and birds.
Aperture f/8 to f/16
Used when depth of field does not matter because the distance between the main subject and the background is small. This is generally used for photo’s taken where all objects are in the immediate surroundings and you want the entire scene in focus. Examples would a dog playing in the garden or a model sitting the the bonnet of a car .
Aperture f/16 or greater
Used when a large depth of field is important. Generally used for landscape photography where objects can be miles apart but you want the entire scene to be in focus. An example would be flowers in the foreground and a mountain range in the background. Many photographers don’t recommend going above f/22, if you do a tripod is recommended as a slower shutter speed is required to allow enough light to reach the sensor.