When you press your camera’s shutter release button, a hole opens up to let the image sensor view the scene. This opening is the aperture, which you can adjust to let more or less light hit your camera’s sensor.
Aperture is measured in “f-stops”: f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22.
Note: the large apertures (larger openings) are given smaller numbers.
The size of the lens opening is doubled, or halved, when you move from one f-stop to the next. The smaller the f-stop number, the larger the opening.
When you change your camera’s aperture, the main thing that will change is the depth of field (DOF), which controls how much of your shot is in focus. With a large depth of field, most of your image will be in focus (a large DOF is typically used for landscape shots). When you use a small depth of field, part of the image will in focus and the rest will be blurry. A shallow depth of field is often used for portraits (it keeps the focus on the subject, rather than the background), macro photography, and creative shots. Smaller f-stop numbers (large apertures) decrease the depth of field, while large f-stop numbers (small apertures) increase the amount of the scene that’s in focus.
If you’re working in your camera’s manual mode, keep in mind that when you change the aperture, you’ll also have to adjust the ISO and shutter speed to maintain exposure. (More on that here.)
You can also work in Aperture Priority Mode — usually indicated by “A” or “Av” on your camera. In this semi-manual mode, you select the f-stop number and the camera will automatically select the proper shutter speed.