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An aperture is an opening. In the world of photography, people use the term to describe how much light is admitted into the camera. The width can be controlled manually by the user, or automatically by the camera. Aperture width has a profound impact on the appearance of the final photograph, and the concept is often introduced at a very early stage in the study of photography as a result.
You usually hear aperture designated with a number, such as “1.8” or “16.” The smaller the number, the wider the opening. These numbers follow a set sequence of increments known as stops or f-stops, and in photography notation, aperture is usually indicated with an “f” followed by a slash and the number, as in “f/8.” A common range is f/8 to f/1.4, with f/5.6, f/4, f/2.8, and f/2 between. Stops are divided by a factor of roughly two, so f/4 allows twice as much light in as f/5.6, and f/2 allows in four times as much light as f/4.
When cameras are sold, the aperture range of the camera is included in the camera's technical specifications, so that photographers have an idea of the capabilities of the camera. A wide range can be very advantageous, as it allows the photographer more flexibility. Some basic point and shoot cameras have only one setting, which can be very frustrating.
Now that you have an idea of what aperture is, you probably want to know why it matters. In the basic sense, the wider the opening, the more light is entering the camera, shortening the necessary exposure time. Width also has an impact on the depth of field, meaning the area in which things are in focus. When a camera is set to a small aperture, it has a greater depth of field, bringing a range of objects into focus. The wider the opening, the shallower the depth of field.
To get an idea of how aperture changes the outcome of a photograph, think about shooting a scene at a sports event. If you shoot with a wide aperture, allowing yourself only a brief exposure, you will be able to freeze an athlete in motion. However, the crowd and the rest of the field will be blurred out, because of the shallow depth of field. On the other hand, if you shoot with a narrow opening, lengthening the exposure, you can get a shot of the whole field, and you will see the athletes as blurs, because they moved while the film was being exposed.
Many cameras come up with f-stop and exposure recommendations on their own, but some allow users to force a specific aperture or exposure life. As a general rule, if you decide you want a narrower aperture, you need a longer exposure, to compensate for the loss of light. If you want a wider one, you will need to shorten the exposure, or the picture will be over-exposed. Some cameras have a variety of shooting modes which pick the best aperture and exposure for the task, allowing users to select things like “sports mode” or “portrait mode.”
Let's say you want to take a long-exposure picture, like a photograph of stars as they move across the sky. In order to do this, you would need a very large f-stop, allowing a small amount of light into the camera so that you could leave the shutter open for a period of hours. On the other hand, if you are taking a photograph of a footrace on a cloudy day, the camera might recommend a long exposure to ensure that the picture comes out. If you want a picture of your friend crossing the finish line, you can widen the aperture, picking a lower f-stop number, thereby decreasing the exposure time so that you can get a crisp picture of your friend.