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3.3 THE APERTURE CONTROLS LIGHT AND DEPTH OF FIELD | ||
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The aperture diaphragm, a ring of overlapping leaves within the camera lens, adjusts the size of the opening in the lens through which light passes to the image sensor. As it changes size, it affects both the exposure of the image and the depth of field in which everything is sharp.
The aperture can be opened up to let in more light or closed (stopped down) to let in less. Like the shutter speed, the aperture is used to control exposure. The larger the aperture opening, the more light reaches the image sensor in a given period of time. The more light, the lighter the image.
| The Way It Was: Early Apertures
A variety of designs in the past century and a half have enabled photographers to change the size of the lens opening. A form of the iris diaphragm, used in today’s cameras, was used as early as the 1820s by Joseph Nicephore Niepce, one of the inventors of photography. Waterhouse stops, used in the 1850s were a series of blackened metal plates with holes of different sizes cut in them. To change apertures the photographer chose the appropriate one and slid it into a slot in the lens barrel. With wheel stops, different size apertures were cut into a revolving plate. The photographer changed the size of the aperture by rotating the plate to align the desired opening with the lens. |
Like shutter speed, aperture also affects the sharpness of your picture, but in a different way. Changing the aperture changes the depth of field, the depth in a scene from foreground to background that will be sharp in a photograph. The smaller the aperture you use, the greater the area of a scene that will be sharp. For some pictures—for example, a landscape—you may want a smaller aperture for maximum depth of field so that everything from near foreground to distant background is sharp. But perhaps in a portrait you will want a larger aperture to decrease the depth of field so that your subject’s face is sharp but the background is soft and out of focus.
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A shallow depth of field can make part of an image stand out sharply against a softer background. This emphasizes the sharpest part of the image. |
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Great depth of field keeps everything sharp from the foreground to the background. |
Aperture settings are called f-stops and indicate the size of the aperture opening inside the lens. Each f-stop lets in half as much light as the next larger opening and twice as much light as the next smaller opening. From the largest possible opening to increasingly smaller ones, the f-stops have traditionally been f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45. No lens has the full range of settings; for example, the standard lens on a digital camera will range from about f/2 to about f/16. Notice that as the f-stop number gets larger (f/8 to f/11, for example), the aperture size gets smaller. This may be easier to remember if you think of the f-number as a fraction: 1/11 is less than 1/8, just as the size of the f/11 lens opening is smaller that the size of the f/8 opening.
How wide you can open the aperture, referred to as its "speed," depends on the len’s maximum aperture (its widest opening). The term "fast lens" usually applies to lenses that can be opened to a wide maximum aperture for the focal length. For example, a lens with a maximum aperture of f/2.6 opens wider, and is faster, than a lens with a maximum aperture of f/4. Faster lenses are better when photographing in dim light or photographing fast moving subjects. With zoom lenses the maximum aperture changes as you zoom the lens. It will be larger when zoomed out to a wide angle, and smaller when zoomed in to enlarge a subject.
| How To: Selecting an Aperture
Look in your camera manual for a section on aperture preferred or aperture priority, or apertures: _______________________________________________________________________ |