The aperture, or f/stop as it is commonly called, is used to regulate the diameter of the lens opening. That controls the luminance on the film plane. Besides controlling the luminance on the film plane, the f/stop also controls image sharpness by partially correcting various lens aberrations.

The most commonly used aperture control device is the iris diaphragm. An iris diaphragm is an adjustable device that is fitted into the barrel of the lens or shutter housing. It is called an iris diaphragm because it resembles the iris in the human eye (fig. 4-12). An iris diaphragm is a series of thin, curved, metal blades that overlap each other and is fastened to a ring on the lens barrel or shutter housing. The size of the aperture is changed by turning the aperture control ring. The blades move in unison as the control ring is moved, forming an aperture of any desired size. The control ring is marked in a series of f/stops that relate to the iris opening. The aperture controls the intensity of light that is allowed to pass to the film and the parts of the image that will appear in sharp focus.

Depth of Field

Depth of field is that zone both in front of and behind your subject that are in acceptably sharp focus. The focusing controls on most cameras are easy to use, providing you understand the factors that effect depth of field. To produce professional quality photographs, you must know how to control the depth of field.

Aperture, or f/stop, is the most important factor in controlling the depth of field. The smaller the f/stop opening, the greater the depth of field; for example, at f/16, a normal lens focused on a subject 16 feet from the camera may show everything in focus from 8 feet to infinity. At f/5.6, depth of field may range from about 3 feet in front of the subject to about 6 feet behind the subject. At f/2, only the subject focused on is sharp. As shown in figure 4-3, a shallow depth of field results in a blurry foreground and background, whereas greater depth of field results in more overall sharpness.

Camera-to-subject distance also has an effect on the depth of field. In general, the closer you are to the subject, the shallower the depth of field. Even at f/16 with a normal lens, if you focus on a subject only 3 feet from the camera, the depth of field may only be about 1 foot. At f/2, the subject's eyes may be in sharp focus, but the nose and ears are blurred. As you increase the camera-to-subject distance, the depth of field increases rapidly. Using an aperture of f/16 and focusing at 6 feet, the depth of field may extend from a foot in front of the subject to about 3 feet in back of the subject. Still using

Table 4-1. How to Control Depth of Field

Use a larger f/stop (lower number). Use a longer focal length lens. Move closer to the subject. Use a filter to reduce the amount of light allowed to be transmitted and use a larger f/stop.

Use a smaller f/stop (higher number). Use a shorter focal length lens. Back up from the subject. Use a faster film or a slower shutter speed and use a smaller f/stop. Focus at the hyperfocal distance.

f/16 but focusing now at about 16 feet, the depth of field is almost at infinity. Most normal lenses for 35mm cameras produce these maximum ranges of sharpness at about 16 feet. Focusing any farther from the camera only reduces foreground sharpness. You must remember this point when attempting to get the greatest possible depth of field.

Lens focal length is also a factor in depth of field. The shorter the lens focal length, the greater the depth of field at a given aperture. In other words, a wide-angle lens provides more depth of field at f/8 than a normal lens, and a normal lens provides more depth of field at f/8 than a telephoto lens.

You know that a small aperture like f/16 provides more depth of field than a wide aperture like f/2. With experience, you can predict the best aperture for the depth of field desired. Even with experience, you do not always have to guess the aperture setting or calculate the hyperfocal distance, near distance, and far distance by using formulas. Most lenses have a depth-of-field scale to guide you (fig. 4-13). The depth-of-field scale indicates the distance range from the camera that the subject(s) appear in acceptably sharp focus. The depth of field on an SLR is marked between the aperture ring and the focusing scale. Use the depth-of-field scale as follows:

1. Focus on the subject.

2. Select the f/stop.

3. Look at the depth-of-field scale and locate the marks that correspond to your chosen f/stop. The f/stop appears twice, once on either side of the scale center line.

4. Read the two distances on the focusing scale that are adjacent to the two f/stops on the

depth-of-field scale. You may have to estimate the distances.

You can see in figure 4-13 that the lens is focused at a distance of 30 feet with the aperture set between f/16 and f/22. You can see from the depth-of-field scale that the depth of field extends from approximately 11 feet to beyond infinity. If the aperture is opened up to f/8, the depth of field will range from about 16 feet to infinity.

At any given aperture, depth of field is maximized by focusing the lens at the hyperfocal distance. That is the closest point of acceptable sharp focus shown on the depth-of-field scale when the lens is focused at infinity. When you are changing the focus setting to the hyperfocal distance, the zone in front of the subject that is sharp is increased, and infinity is still the farthest point

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Figure 4-13. Depth-of-field scale.

Figure 4-14. When the lens is focused at 20 feet and set at f/22, the depth of field ranges from about 10 feet to infinity

in sharp focus. In figure 4-14, when the lens is set at f/22 and focused at infinity, the depth of field ranges from about 20 feet (the hyperfocal distance) to infinity; however, when you change the lens focus to 20 feet, the depth of field ranges from about 10 feet to infinity.

The lenses of modern SLR cameras stay at their maximum aperture until the shutter is tripped. These lenses provide a bright image in the viewfinder to focus. As a result, when you look through the viewfinder, you only see the depth of field for the maximum aperture and not the working f/stop. Most SLR cameras have a depth-of-field preview button to compensate for this. When you press it, the aperture closes down to the set f/stop. Although the viewfinder becomes darker, you can see the actual depth of field at the selected aperture.

Image Sharpness

The outer edges of a lens are least likely to produce a well-defined or aberration-free image; therefore, proper use of the diaphragm, aperture, or f/stop can improve image sharpness by blocking off light rays that would otherwise pass through the outside edges of a lens.

There is a limit to how far the aperture can be stopped down and still increase image sharpness. When the aperture is very small, it causes diffraction of light rays striking the edge of the diaphragm. Diffraction results in a loss of image sharpness. This loss of image sharpness is especially noticeable in copy work

Physical limitations in the design of lenses make it impossible to manufacture a lens of uniform quality from the center to the edges; therefore, to obtain the best quality with most lenses, you can eliminate the edges of the lens from being used by closing down the aperture about two f/stops from wide open This recommended adjustment is called the optimum or critical aperture. The optimum aperture for a particular lens refers to the f/stop that renders the best image definition.

When a lens is stopped down below the optimum aperture, there is an actual decrease in overall image sharpness due to diffraction. Although the depth of field increases when a lens is stopped down below the optimum aperture, image sharpness decreases; therefore, increased depth of field should not be confused with image sharpness. For example, the image formed by a pinhole camera has extraordinary depth of field but lacks image sharpness. When the lens aperture is closed down to the size of a pinhole, it behaves like one. This is an important factor for subjects in a flat plane (such as copying) where depth of field is not needed.