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The aperture of the lens controls the amount of light that passes through on its way to the camera's sensor or film plane. A camera's aperture is also commonly called its f-stop, though 'f-stop' technically refers to the diameter of the opening created by internal adjustable blades, rather than the entire mechanism.
While a camera can function without the use of a shutter, all cameras must have an opening for light to enter and make an exposure. Early photographers used cameras with fixed apertures (Camera Obscura, Pinhole Cameras, early View Cameras), and timed the exposure via a watch or a clock. They controlled the length of the exposure by simply removing and covering the lens with a lens cap.
Where is a Camera's Aperture?
The aperture diaphragm sits between the front and rear glass elements of a lens. Manual-focus lenses use internal rings—or cams—that rotate in unison with the outer aperture ring (the one with the f-stop numbers on it), to dilate the aperture blades. Digital/Autofocus cameras use dials on the camera body—either near the shutter release button or on the back, close to the LCD screen—to change aperture settings.
What Do the F-stop Numbers Mean?
The numbers on a lens's aperture ring (or on the LCD screen of a digital camera) indicate the size of the diameter—or f-stop—at a particular setting. The numbers usually run from the widest opening to the narrowest, as follows:
2 - 2.8 - 4 - 5.6 - 8 - 11 - 16 - 22 - 32
Some may ask why the numbers increase if the diameter is shrinking. Well, they are actually in descending order. These numbers represent fractions that refer to the diameter of the aperture in relation to the length of the lens (i.e.: 1/2 - 1/2.8 - 1/4 - 1/5.6 and so on). Say, for example, you are using a 50mm lens and your camera or meter is telling you the correct exposure is at f/2.0. This means that the aperture diameter is 1/2 the length if the lens: 25mm/50mm = 1/2 or f/2.
How Does it Work?
The iris of the human eye (the colored part) regulates how much light enters by dilating, thus changing the diameter of the pupil. The iris will dilate the pupil as wide as possible at night or in low light. In bright light the iris shrinks the size of the pupil, thus restricting the amount of light entering. A camera's aperture works similarly. It is adjusted automatically by the camera, or manually by the photographer. When aperture blades expand towards the center the lens is being closed—or 'stopped down'—slowing down the light entering the camera.
Conversely, a photographer is 'opening up the lens' when retracting the aperture blades, allowing light to enter quickly. The aperture stays open at its widest f-stop during composing and focusing. When a camera is fired, the aperture rapidly stops down to the selected f-stop, then opens back up again.
Depth of Field and Apertures
Aperture also controls depth of field (DOF), the narrow or broad area in which things are in focus. Smaller apertures (remember, f/11 - f/22 means 1/11th - 1/22nd of the lens's length) produce a larger depth of field from front to back. Large, or wide, apertures (f/1.2 - f/4) give a lens the ability to create a narrow swath of focus, or 'selective focus'. Most lenses have a scale on them displaying—in feet or meters—the length of the effective DoF for each f-stop. As the lens is stopped down, the depth of field increases, creating a larger area of focus, and vice versa.
Those lenses with large apertures are considered 'fast,' because wide f-stops allow for short exposure times (faster shutter speeds). This can be useful when photographing hand-held without a tripod in low light, or achieving sharp images of moving subjects. Photographing wide open also blurs anything distracting, such as a background or other people, which are not in the subject's field of focus.
Aperture also affects a lens's 'bokeh'—a Japanese term referring to the quality of the out-of-focus elements in an image. Optics considered having good bokeh render blur—particularly in the highlights—as soft and unobtrusive. Usually a prime (non-zoom, fixed focal length) lens built with an aperture diaphragm using many blades (typically between 8-13) and wide f-stops (1.8 or wider) will produce aesthetically pleasing bokeh. The out-of-focus highlights appear round and uniform, rather than irregular pentagonal shapes which are caused by the shape the aperture takes.
Many of these fast, bokeh-producing lenses command a high price, due to their complex design. Fast prime lenses, when used wide open, naturally produce distortion, vignetting and other distracting effects or aberrations. Manufacturers correct these through the use of multiple internal optical elements placed in groups and layers. Unfortunately, these costly lenses may still exhibit at least one of these flaws despite their optics (made from crystal that costs around $200 a gram) and distortion-reducing aspherical glass.
Most often, stopping the lens down a stop or so, or converting to black and white in post production will correct these effects. Yet, stopping down too much may cause diffraction (which is the bending of light at the aperture blades) and decrease overall sharpness. Photographers should take the time to experiment with their lens's full range of f-stops to see how it performs in terms of sharpness.
How Do I Use Them?
Comprehending how aperture works and where it falls short gives a photographer more creative tools and control. Using f-stops effectively will help get the most out of the camera and lens you may already own. As your skills develop, you may find that having one or two more f-stops makes the difference between getting or missing the shot. Understanding aperture not only can improve one's photography, but also make them a better photo-consumer.
Using Apertures to Create the Miniature Effect
Some manufacturers, like Canon and Nikon, have created special tilt-shift lenses, which allow the front of the lens to swing left, right, up or down. This allows a photographer to move the plane of focus around the image area. A 'Miniature Effect' can be created in-camera with the right combination of wide aperture and lens tilt. The result is a small narrow area of sharp focus surrounded by blur, drawing a viewer's eye directly to the focal point.
Demonstration of Apertures
The following images were shot with the Canon 7D, 85mm f/1.8, and a 430 EX II being triggered wirelessly to provide sufficient illumination. The focusing point was the same throughout the series of photos taken. This was done in aperture priority, with the ISO setting on Auto. Take notice of how more and more becomes in focus as the aperture stops down, and also notice how the images become darker.