Technical Details
- Understanding How Lenses Work
- Manipulating Exposure Controls
- Learning Workarounds for Camera Settings
- Caring for Digital Cameras
- Summary
This chapter is from the book
This chapter is from the book
This chapter is from the book
What You'll Learn in This Chapter:
- Understanding How Lenses Work
- Manipulating Exposure Controls
- Learning Workarounds for Camera Settings
- Caring for Digital Cameras
It's important to understand the differences between telephoto, wide-angle, and normal lenses, and how shutter speeds affect the photograph, even if your camera doesn't enable you to control these functions. At the very least, you'll know why your pictures come out the way they do, and at best you will learn how to use the factors you can control to make up for those factors you can't do much about.
Understanding How Lenses Work
The lens is the camera's eye. Like the human eye, some focus better than others. Some see farther away; others can read the fine print. Camera lenses can be made of many individual polished glass elements, or cast as a bubble of transparent plastic. Its function is to focus light rays onto the light-sensitive surface. The better it does the job, the clearer your picture is. The simplest type of lens is a single element convex lens, the same kind used in a magnifying glass. In profile, it is thick in the middle and tapered at the ends, as shown in Figure 3.1.
As you learned in the description of the pinhole camera, light travels in a straight line through a transparent medium. However, when light passes from one transparent medium to another, for example, through a lens, the light rays can be bent or refracted. This means that more light rays from each point on the subject are gathered and refracted toward each other so that they meet at the same point.
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Figure
3.1 The convex lens is thick in the middle and tapered at the ends.
In Figure 3.2, you can see some of the light rays from a specific point on the tree passing through the lens and converging to form that point on the image of the tree.
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Figure
3.2 Refraction happens when the light is slowed down by the density of the
glass.
This point of convergence is called the focal plane. Conventional cameras are designed so that the film is held in position precisely on the focal plane, to get the best possible picture. Digital cameras direct the light to the CCD (Charge Coupled Device—the "digital film"), which replaces the film at the focal plane.
Focusing
In a fixed focus camera, the ideal distances between the lens, the average subject, and the focal plane have been mathematically calculated. The lens and film holder are positioned according to this measurement and can't be changed, regardless of whether you're photographing a distant mountain or a nearby flower. In a variable focus camera, there's a mechanism to move the lens forward and backward, while still keeping it centered on the focal plane. This enables you to compensate for a subject that's closer to, or farther away from, the lens.
Typical autofocus snapshot cameras send out a beam of sound (sonar) or infrared light. Most digital cameras use the infrared light method, and nearly all of the current models have built-in autofocus.
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When a scene is brightly lit, it's easier to determine whether the depth of field (area in focus) is shallow. Use "flat lighting" (general over-all illumination) to lower the contrast if you are concerned about keeping foreground and background relatively in focus.
High-end professional digital SLR cameras use a system called passive autofocus, which "reads" the image, comparing the contrast of adjacent areas of the frame as it appears in the viewfinder. Focusing with this system is quite precise, and there's no problem switching lenses because the focus measurement is done through whatever lens is mounted.
Using autofocusing provides no guarantee that what you want in focus will be so. Autofocusing is difficult when the subject is very close. If the camera can't focus, you may need to move back. Focusing is also difficult if the subject has very low contrast, such as a white wall, sky, or the side of a building with very little detail.
Subjects that move fast or flicker, such as a fluorescent light or candle flame, can cause focusing problems. Subjects under low lighting conditions or strongly backlit, such as the statue in Color Plate 3.1, can cause problems for both focus and exposure metering. In this case, I exposed for the sky, allowing the statue and the branches to be in silhouette, and focused on the bridge of the statue's nose. Because it was a bright day, the small aperture provided acceptable focus on the branches to the right of the picture, which were about 50 feet away. The branches on the left were more than 100 feet distant, and are definitely less sharp.
Some autofocus cameras include a focus lock. If you can lock the focus, you can resolve many of the problems discussed here. Simply find something else that you can focus on that is the same distance away as the difficult subject. When it's in focus, enable the focus lock, recompose your picture, and shoot it.
Focal Length
You've heard lenses discussed in terms such as normal, telephoto, and wide-angle. What do these terms really mean? Lenses vary in focal length. A "long" lens is a telephoto lens, and a "short" lens is a wide angle lens. Physically, the telephoto is anywhere up to a foot long or more, while a wide angle lens is anywhere from one to three inches long. Focal length is an important concept, because the focal length determines the size of the image projected on the film or CCD at a given distance from the camera, and it also determines the width of the area in front of the camera included in the picture.
In conventional camera terms, a normal lens is one that has a focal length approximately equal to the diagonal of the picture's negative area. Normal on a 35mm camera, which uses a 24x36mm negative, is when the focal length is approximately 43mm, which could be determined from the Pythagorean Theorem, if you were so inclined. (Trust me. It works.) On a typical digital camera, the area of the CCD is much smaller than the area of a 35mm negative, so normal is much smaller, too. The standard lens on the Ricoh RDC-2 is a 5.6mm lens (equivalent to a 55mm lens on a 35mm camera). The lens on the Casio QV100 has a 4.9mm focal length.
To determine the focal length of a lens, the lens designer or optical engineer sets the lens focus to (theoretical) infinity and measures the distance from the optical center of the lens to the focal plane (see Figure 3.3). (For practical purposes, the horizon at sea level is assumed to be infinity although it's really only a few miles away. Optical infinity is not a measurable distance, but rather the furthest point at which the lens can focus.) Fortunately, you don't need to worry about this. If you have a camera that uses interchangeable lenses, they're labeled. Some low-end (inexpensive) cameras don't have a normal lens, but instead let you flip back and forth between a wide-angle and telephoto. Because the wide lens isn't extremely wide, and the telephoto lens isn't extremely long, one or the other suffice for most normal shots. If your camera has only one lens built-in, assume it's a "normal" lens. A few special purpose cameras, such as the disposable panoramic ones, have a wide-angle or moderate telephoto lens instead.
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Figure
3.3 This is the way it's done in physics class.
Visual Angle
A normal lens is curved to a degree that admits light from about the same angle of view, or visual angle, that our eyes do. A wide-angle lens has a greater curve and sees more of the scene than we do. The peep-holes sometimes found in doors, especially in hotel rooms, are a good example of a wide-angle lens. Objects within the scene are smaller than they would be if seen through a normal lens. A telephoto lens has less of a curve and sees a narrower angle of view. Because it's seeing less of the scene, however, objects appear much larger. They are magnified as if seen through a telescope, hence the name. Doubling the focal length of the lens actually doubles the size of the image. Figure 3.4 shows examples of all these lenses.
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Figure
3.4 Wide, normal, and telephoto lenses and how their visual angles
vary.
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The most important rule to remember about focal lengths is that a "short" lens (that is, any lens with a focal length less than normal for the camera format) is a wide-angle lens. A "long" lens (that is, any lens with a focal length greater than normal for the camera format) is a telephoto lens.
Zoom Versus Prime Lenses
A lens that has a single focal length is called a prime lens. A zoom lens is a lens that can shift its focal length. Because most modern lenses are made up of several individual lens elements, it's a fairly easy matter to build the lens in such a way that rotating the barrel moves one of the elements back and forth, thus changing the focal length. The barrel itself may even telescope in and out becoming longer or shorter as needed. Zoom lenses cover a range of focal lengths. Common zoom lenses for 35mm applications are 30–70mm (wide-angle to slight telephoto); 70–300mm (slight to definite telephoto); and so on. Of course, you're not limited to the ends of the spectrum here. The 30–70mm lens can be used as a 30, 35, 50, 70, or anywhere in between. Instead of carrying a set of prime lenses and changing them, you can carry just one and zoom in and out as you shift your attention from the distant forest to the nearby tree. Figures 3.5 and 3.6 show the same scene as photographed with a wide-angle and telephoto lens.
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Figure
3.5 Photograph taken with a wide-angle view.
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Figure
3.6 The same scene, through a telephoto lens.
Most of the current digital cameras have zoom lenses. However, many of these have a fairly limited zoom ratio, often 3:1. If you intend to use your camera for more than one kind of photography—for instance, travel pictures, which really need a wide angle lens to capture as much as possible, and portraits, which are best done with a medium telephoto—look for a camera with a higher zoom ratio. It's also important to make sure that you're using optical zoom, rather than digital zoom, for reasons which I'll explain in a moment.
If you're fortunate enough to be using one of the camera systems that use interchangeable lenses, such as the Nikon D-1, you have literally dozens of lenses to choose from—all the way from an 8mm fisheye (ultra wide angle), to a 1,000mm extreme telephoto.
It used to be true that zoom lenses were not as sharp as prime lenses, but testing shows the differences in the current crop of lenses to be so minor that it's not worth worrying about.
Distortion
There's one difficulty that all photographers must face. When you send light, which wants to travel in a straight line, through a piece of curved glass, you inevitably get distortion. This is particularly obvious when you use a wide-angle lens on a very close subject (see Figure 3.7). Objects at the edge of the scene may look as if they'd been stretched. Objects closest to the camera are enlarged.
You can work around this by either cropping the picture to remove the distorted parts or by switching to a normal lens. Wide-angle lenses shouldn't be used for portraits, because they tend to make noses and chins appear grotesquely large, but they're good for catching the action at a sporting event. When you shoot you can be fairly certain that the ball or hockey puck is somewhere within the picture, and you can crop until you have a reasonable composition.
Telephoto lenses have their own set of problems. They must be held steady when you shoot, as they accentuate any camera movement. They appear to compress perspective, producing a flat, two-dimensional effect. This works to your advantage in portraiture and wildlife photography because you can stand a bit farther away from your subject and not crowd him or her.
Depth of Field
Depth of field is sometimes called depth of focus, which is perhaps a more accurate term. It refers to the extent of the scene, from close to far, that is in focus in the picture (see Figure 3.8). You can see every bump and paint flake in the stern of the dory, but the texture of the rope in the bow is barely visible.
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Figure
3.7 Notice how large the hand is, compared to the face.
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Figure
3.8 Depth of field is the extent of the scene in focus in the
photograph.
Several factors influence depth of field, but the most important is the viewer's tolerance for fuzziness. When the lens is focused to provide a sharp image of a particular object within the scene, objects closer and farther away are less sharp. Because the decline in sharpness is gradual, when you look at the photo you may not be conscious of the blur. Some lenses, notably wide-angle lenses, provide sharp focus over a greater depth of field than others. Any well-made normal lens should also have acceptable depth of field.
Of course, sharp focus is, itself, a relative term. The lens reproduces an image of a point as a small circle. Pictures are made up of millions or even billions of these circles. They're called circles of confusion. When the lens is focused on an object, it means that the circles that make up the object are as small as they can possibly be. The unassisted eye can't discern fine detail.
Even a person with excellent close vision can't see circles smaller than 1/100th of an inch in diameter. They look like dots, and they merge to form the image. At the sharpest point of focus the circles are the smallest. As you get farther away from that point, the circles get bigger, so that eventually you see them as circles rather than as points. At this point, you're aware that the image is blurred. Depth of field, therefore, is the area in which the circles of confusion aren't large enough to be noticed. Figure 3.9 shows a greatly magnified (theoretical) view.
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Figure
3.9 Circles of confusion. In a real camera, there'd be an infinite
number of overlapping ones.
In general, the depth of field is divided in thirds so that one third of the area in focus is in front of the focal point (object in focus), and two-thirds of the focused area is behind it. As the camera moves nearer to the object, the depth of field narrows and becomes more evenly divided, up to halfway on either side of the focal point, when the object is close enough to give an image equal to the size of the original object, or at a distance equal to twice the focal length of the lens. At this point, the depth of field is extremely shallow, on the order of an inch or less on each side of the point of focus. This is important to remember if you are using a macro lens to photograph small objects such as stamps or coins.
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Macro is shorthand for macrofocus, and indicates a lens that is able to focus on very close objects. Many digital cameras have a macro setting, for close-ups.