Aperture is the term used to describe the amount of light that passes through a camera lens. DSLR lenses have an iris within them, which will open and close to allow certain amounts of light to reach the camera's sensor. The camera's aperture is measured in f-stops. Aperture has two functions on a DSLR -- as well as controlling the amount of light passing through the lens, it also controls depth of field.
The Range of F-Stops
F-stops pass through a huge range, particularly on DSLR lenses. Your minimum and maximum f-stop numbers will depend, however, on the quality of your lens. Image quality can drop when using a small aperture (we'll explain that term later in the article), and manufacturers limit the minimum aperture of some lenses, depending on their build quality and design. Most lenses will at least range from f3.5 to f22, but the f-stop range seen across different lenses can be:
f1.2, f1.4, f1.8, f2, f2.8, f3.5, f4, f4.5, f5.6, f6.3, f8, f9, f11, f13, f16, f22, f32, f45
DSLRs have more f-stops than many film cameras.
Aperture and Depth of Field
Let's start with aperture's simplest function first: Its control of your camera's depth of field.
Depth of field simply means how much of your image is in focus around your subject, meaning a small depth of field will make your main subject sharp, while everything else in the foreground and background will be blurry. A large depth of field will keep all of your image sharp throughout its depth.
You use a small depth of field for photographing things such as jewelry, and you use a large depth of field for things like landscapes. There is no hard or fast rule, though, and much of choosing the right depth of field come from your own personal instinct as to what will suit your subject matter.
As far as f-stops go, a small depth of field is represented by a small number. For example, f1.4 is a small number and will give you a small depth of field. A large depth of field is represented by a large number. For example, f22 is a large number and will give you a large depth of field.
Aperture and Exposure
Here's where it can be confusing.
When we refer to a "small" aperture, the relevant f-stop will be a bigger number. Therefore, f22 is a small aperture, whereas f1.4 is a large aperture. It's extremely confusing and illogical for most people as the whole system appears to be back to front! What you need to remember is that, at f1.4, the lenses iris will be wide open and allowing a lot of light through. Therefore, it is a large aperture.
Another way to help remember this is to recognize that aperture actually relates to an equation where focal length is divided by aperture diameter. For example, if you have a 50mm lens and the iris is wide open, you might have a hole that measures 25mm in diameter. Therefore, 50mm divided by 25mm equals 2. This translates to a f-stop of f2. If the aperture is smaller (for example 3mm) then dividing 50 by 3 gives us a f-stop of f16.
Changing apertures is referred to as "stopping down" (if you are making your aperture smaller), or "opening up" (if you are making your aperture larger).
Aperture's Relationship to Shutter Speed and ISO
As aperture controls the amount of light coming through the lens onto the camera's sensor, it has an effect on the exposure of an image. Shutter speed, in turn, also has an effect on exposure, as it is a measurement of the amount of time that the camera's shutter is open.
Therefore, as well as deciding on your depth of field via your aperture setting, you need to remember how much light is entering the lens. If you want a small depth of field and have chosen an aperture of f2.8, for example, then your shutter speed will need to be relatively fast so that the shutter is not open for long, which could cause the image to overexpose. A fast shutter speed (such as 1/1000) allows you to freeze action, while a long shutter speed (such as 30 seconds) allows for nighttime photography without artificial light. All exposure settings are determined by the amount of light available. If depth of field is your primary concern -- and it often will be -- then you can adjust the shutter speed accordingly.
In conjunction with this, we can also change the ISO of our image to help with lighting conditions. A higher ISO (represented by a higher number) will allow us to shoot in lower lighting conditions without having to alter our shutter speed and aperture settings. However, it should be noted that a higher ISO setting will cause there to be more grain (known as "noise" in digital photography) and image deterioration can become obvious. For this reason, I only ever alter ISO as a last resort.