What Is Aperture?

Whether you shoot film or digital photography, your camera acquires images through the interplay of three primary exposure settings — shutter speed, ISO (International Organization for Standardization) setting, and, you guessed it, aperture.

The exposure settings you choose dictate both the duration and the amount of light entering the camera. Shutter speed controls duration, while aperture allows you to adjust the volume. The larger the aperture diameter is, the greater the amount of light flowing through the camera's lens, which lets you control aperture size.

Different lenses have their own range of aperture settings. If you've ever examined camera specifications, you've undoubtedly seen alphanumeric symbols like f/1.4, f/5.6 and f/22. Each of these numbers correlates to a specific lens aperture size or f-stop. F-stop numbers are fractions that tell you the exact diameter of the aperture at a given f-stop setting. The system is a bit counterintuitive, though, because smaller numbers indicate a larger aperture and larger numbers correspond to smaller openings.

Manufacturers calculate f-stop numbers for a particular lens by dividing its focal length by the aperture's diameter. Focal length determines field of view, which varies from lens to lens. A wide angle lens — for instance, a 24mm lens — has a very broad field of view and is often used for landscape photography. A telephoto lens, for example, a 500mm lens, has a much longer focal length and a narrower field of view. And zoom lenses (such as a 70-200mm lens) let you twist the lens to move through a range of focal lengths.

Focal length is measured in millimeters and often printed on the lens or camera body. By doing a bit of math, you can calculate the aperture size for a particular focal length. If you have a 100mm lens with the f-stop set to f/4, the size of the aperture would be 25mm.

Regardless of the focal length of the lens you're using, you now know that by changing the f-stop, you're altering the volume of light that passes through the lens. If fractions and f-stops aren't your thing, don't worry. You can still learn to change aperture settings to amazing effect using a few settings on your camera.

There are thousands of camera lenses on the consumer market. Every lens offers its own range of potential f-stop settings, which is critical to remember if you have an SLR (single-lens reflex) camera that lets you attach dozens or hundreds of different lenses. Each lens has its own maximum and minimum aperture setting.

Lenses are often marketed by their maximum aperture because the bigger the aperture is, the more useful the lens will be, especially in low-light situations. For example, a lens that can open up to f/1.4 is considered fast because it lets in a lot of light, allowing photographers to use faster shutter speeds to take sharp photos, even in dark environments.

Fast lenses are often complicated to manufacture and, as a result, they are usually more expensive than slower lenses. Lenses as fast as f/1.4 aren't uncommon, but you'll rarely see them with f-stops as large as f/1.0. However, these kinds of ultra-fast lenses do exist, such as Leica's 50mm f/0.95, which costs around $11,000.

Regardless of the type of lens, modern cameras make changing the aperture size very easy. Most models are equipped with both a manual (M) or aperture priority (Av, for aperture value) mode. Manual mode lets you control shutter speed and aperture independently. In Av mode, you pick a desired f-stop, and the camera automatically and continuously changes the shutter speed to maintain an even exposure.

Light levels will affect your choice of f-stop. In very bright light, you can use just about any f-stop setting, because you're able to control exposure with shutter speed. But if you want to make crisp photos in low light, you'll have to use the widest aperture to let in as much light as possible. Otherwise, your pictures could be underexposed.

Aperture size affects more than exposure; it also impacts depth of field. In short, depth of field refers to how much of an image is in focus. When only one part of an image is sharp — for instance, a single petal on a full flower blossom — photographers say that the picture has a shallow depth of field. Alternately, deep depth of field is obvious in a sweeping, broad landscape, in which the flowery foreground and mountainous background are all relatively crisp.

Lenses with wide maximum apertures, such as f/1.4, are great for shallow depth of field. You can isolate one part of a subject, keeping it in focus while blurring the rest of the image. Many photographers use this technique to great artistic effect.

Telescopes rely on the principles of aperture to help people see far-off objects in space. Image quality and magnification all start with the aperture, which is very different from the kind found in cameras. Telescopes have two basic jobs — to collect light and to magnify images. That second job depends entirely on the first, so in order to build a quality telescope, a manufacturer ensures that the device can gather as much light as possible. A larger aperture means the scope has more light-capturing potential.

With telescopes, the term aperture doesn't refer to an opening as it does in cameras. Aperture indicates the diameter of the main optical element, which is either a lens or a mirror. As the aperture size increases, more light enters the scope, making your target objects appear brighter. This results in better visual information for your eyes and means that you can see fine details and faint objects in space that a telescope with a smaller aperture would miss. Thus, provided it's made with high-quality construction methods and materials, a large-aperture telescope creates superior magnification and extremely sharp details.

You can change a telescope's magnification power by using different eyepieces. However, unlike cameras, telescopes don't have adjustable aperture settings. Telescopes come with just a single aperture size, which will be listed on the specifications. Telescope aperture sizes are generally listed in inches or millimeters. Three-inch (76.2mm) telescopes are much less expensive than 8-inch (203.2mm) versions simply because larger apertures are harder to manufacture.

Whether you're gazing far into space with telescopes or taking snapshots of the anthills at your feet with a point-and-shoot camera, aperture plays a major role in how those images appear to your eyes. Now that you have an understanding of how aperture affects visual characteristics, you can see your world (and others) in a whole new way.

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