Put most simply – 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 can get in – the smaller the hole the less light.
Aperture is measured in ‘f-stops’. You’ll often see them referred to it 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 as well – this means if you increase one and decrease the other you let the same amount of light in.
One thing that causes a lot of confusion among new photographers 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.
Depth of Field and Aperture
There are a number of results of changing the aperture of your shots that you’ll want to keep in mind as you consider your setting but the most noticeable one will be the depth of field that your shot will have.
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.
Small (or shallow) depth of field means that only part of the image will be in focus and the rest will be fuzzy.
Aperture has a big impact upon depth of field. Large aperture (remember it’s a smaller number) will decrease depth of field while small aperture (larger numbers) will give you larger depth of field.
It can be a little confusing at first but the way we remember it is that small numbers mean small DOF and large numbers mean large DOF.