The aperture controls the amount of light that passes through the camera lens. When you press the shutter button on your camera, you are opening the shutter to allow light to pass through the lens and create an image of what the lens sees. It’s just like how your eyes work to create pictures in your brain of what you are looking at. If you shoot with the aperture adjusted to the smallest opening, the smallest amount of light is allowed to enter. For example, if you are taking a photo in an environment that’s too bright — how should you adjust your aperture? Simple — adjust to smaller aperture to let in less light. The aperture’s sizes are measured by f-stops. A higher f-stop means a smaller aperture hole while a lower f-stop means a bigger aperture opening. Aperture refers to how wide or narrow the lens opens. Lenses these days open in a rounded motion, so your aperture is a circle (or a polygon, like an octagon, that is close to a circular shape). How wide or narrow it opens determines how much light can get through the lens at one time. Then your shutter speed determines how long the shutter stays open. Again, think about your eyes. Your pupil works like the aperture of your camera. When lighting is dim, you iris (colored part of your eye) will dilate your pupil (the black part) to a large aperture to allow more light to the lens of your eye. When you are in bright light, your pupils get really small, like a narrow aperture, to allow in less light.
Note: Remember that larger apertures are the smaller numbers on the camera settings, like f/1.2. Smaller apertures are the larger numbers, like f/22. This is important understand when you are setting your aperture.
Depth of Field (DOF)
The aperture also controls the depth of field, a larger opening will give you a narrow plane of focus.
Consider the “field” to be your scene, whether it is an actual field or something as small as your tabletop. Depth of Field refers to the distance within that scene you can see in focus. A shallow or narrow depth of field will focus on some plane within the full depth of the scene, and the rest of the scene will be out of focus or blurry. A deep or long depth of field will show all or most of the scene in focus, including the foreground and background. This is important for choosing an aperture value.
Shooting in Aperture Priority mode means that you choose your aperture, and then the camera will use its built in light meter to adjust the shutter speed accordingly. The larger apertures (smaller numbers) will deliver a shallow depth of field. This means that part of your image will be in focus, and other parts will be blurry. This is nice for portraits or flowers, or any subject that you want to stand out from the background. You may also want to stick to the larger apertures if you are shooting in low light.
How low/large you can go depends on your lens. Higher end lenses will have larger aperture options. Look at your lens. You might know that it’s, say, a 28-70 zoom lens, but do you see that other number next to your zoom range? That’s your largest/widest (lowest number) aperture possible with that lens at its shortest zoom setting. On some zoom lenses, the widest aperture possible will decrease as you zoom in on your subject.