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A primer on lens aperture, and how you can use it to take better pictures.
In photography, aperture refers to the size of the opening in the lens. As the opening gets wider, the camera's sensor (or film) absorbs more light. Along with shutter speed and ISO, aperture affects exposure—whether a photo is too dark, too bright, or just right. The aperture setting also influences the depth of field in an image—that is, how much of the photo is in focus.
This is meant to be a quick, functional guide to aperture for casual photographers who want to venture away from auto mode and exercise a bit of control over their cameras. There are countless resources that explain the intricacies and higher-level concepts of aperture. (Also note that this is not a guide to Apple's photo-editing software.)
Why Does Aperture Matter?
Adjusting aperture is a fundamental principle of photography. Until the late 1970s, even casual photographers needed a working knowledge of how to control it. Today, any camera can adjust aperture automatically—most point-and-shoots and smartphones don't even have an option for manual control—and you can capture serviceable snapshots without understanding it.
But if you want to unlock the real potential of your DSLR, mirrorless system camera, or high-end compact camera, aperture control can help get you there.
First and foremost, aperture is one of the two primary influences on a photo's depth of field. A wide aperture creates a shallow depth of field, where your subject is in sharp focus, while the rest of the image is pleasantly blurry, almost dream-like. A narrow aperture flattens out the image—the separation between the foreground and background isn't as dramatic, and more of the photo is in focus.
Wide aperture settings also make it easier to shoot in dim lighting. A small aperture limits the amount of light that gets through the lens to strike the sensor. You can offset this problem by using a higher ISO setting or a slower shutter speed, but you'll increase problematic image noise in the first case, and motion blur in the second.
The aperture setting also affects sharpness, at least to a certain extent. If you take two shots with the same lens, focused on the same subject, the details on the subject will generally be sharper at f/8 than at f/2, for example. There are a ton of variables here—lens quality and sensor size, being the biggest—and it's a difference you'll probably only notice if you look closely. So for the purposes of this introductory article, it isn't a major consideration.
How Do I Use Aperture in the Real World?
For the sake of simplicity, we're going to assume that you're using an interchangeable-lens camera, like a DSLR or mirrorless compact system model. The easiest way to start experimenting with aperture control is to set your camera to aperture priority mode. It's usually marked as "A" or "Av" on the mode dial (or sometimes in the menu). This setting lets tinker with f-stops, without throwing off the exposure—your camera will automatically adjust its shutter speed (or sometimes the ISO setting) to compensate for the f-stop you've chosen.
Let's take a look at a few real-world situations where adjusting the aperture can transform good snapshots into great pictures.
Say you want to take a really striking portrait of a friend, where his face completely dominates the frame. In auto mode, your camera will probably pick a relatively narrow aperture (if you're in bright light, that is). The resulting photo will have a bit of separation between your buddy's face and the background, but it won't look very dramatic.
Try switching to aperture priority, opening up the aperture as far as it will go (remember, lower f-stops mean a wider aperture), and taking another shot. The depth of field will be much shallower, making his face really pop out from the blurry background.
Aperture range depends on the lens, and not all lenses can create such a strikingly shallow depth of field as you see in the examples above. We shot the sample photos above with a mid-range mirrorless system camera and a high-quality lens. Its aperture range starts at f/1.4—significantly brighter than a typical kit lens, which starts at f/3.5. So if you're shooting with a starter kit, you won't be able to achieve the same dreamy look in your shots. (If you're looking to upgrade your lenses, we have a handy guide on where to start.)
It's also worth noting that sensor size has a lot to do with depth of field, too. If you have a compact camera with an f/2 lens, it'll still have a relatively deep focal plane compared to an SLR with an f/2 lens—shots will look flatter.
As we mentioned, you'll also want to open up the aperture if you're shooting in low-light situations. Light is a precious resource in dim environments, and a wider aperture captures more of it than a narrower aperture. Simple.
Are there any situations where you'd purposely want to use a smaller aperture? Of course. Landscapes are a prime example—a situation where you want everything to be in sharp focus. And even with portraits and low-light shots, opening up the aperture too much can have undesirable effects. The focal plane might be too shallow for your tastes—it isn't necessarily a flaw, just a characteristic that you need to bend to your will.
Aperture is a major piece of the puzzle of photography. Once you've put in some practice and grasped the concept, you'll notice the quality of your photos improve, and the rest of the manual controls on your camera won't seem so intimidating. It's an important, but approachable step toward becoming a more capable photographer.
Photos: Aperture chart via Wikimedia Commons; all others by Reviewed.com