Photography 1 ch 3

compound lens

A ______ ____, as this cutaway view shows, is usually made up of six or eight separate lenses. Each lens is added to correct some of the aberrations (focusing defects) in the others. This lens has six elements in four groups. An element is a simple lens with two curved sides or one curved and one flat side. A group may be a single element, like (a) or (d), or it may be two or more elements cemented together, like (b) and (c). Because lens aberrations are mathematically predictable, some of these focusing defects can be eliminated digitally from the picture after it has been made (see Profiles for Raw Files, p. 117), but using a well-designed lens is always the best start. How refraction works is shown in this diagram of light rays passing through four glass blocks. The rays, entering from the left, strike the first block head on (1) and therefore pass straight through. The next block (2) has been placed at an angle; the rays are bent, but as they exit the block they are bent back to resume their former direction. The concave surfaces of the third block (3) spread the rays apart, but the last block (4)—a convex lens like the basic light gathering lenses used in cameras—draws the rays back together so that they cross each other at the point of focus.

bokeh

Controlling Depth field Evaluating and controlling the depth of field is more important in some situations than in others. If you are relatively far from the subject, the depth of field (the distance between the nearest and farthest points in a scene that appear sharp in a photograph) will be greater than if you are up close. If you are using a short-focal-length lens, you will have more depth of field than with a long lens. If the important parts of the scene are more or less on the same plane left to right, they are all likely to appear sharp as long as you have focused on one of them. But when you photograph a scene up close, with a long lens, or with important parts of the subject both near and far, you may want to increase the depth of field so that parts of the scene in front of and behind the point on which you focused will also be sharp. Sometimes, though, you will want to blur a distracting background that draws attention from the main subject. You can accomplish this by decreasing the depth of field. You can use the aperture to control depth of field. To increase the depth of field so that more of a scene in front of and behind your subject is sharp, setting the lens to a smaller aperture is almost always the first choice. Select f/ 16 or f/22, for example, instead of f/2.8 or f/4. This is the case even though you have to use a correspondingly slower shutter speed to maintain the same exposure. A slow shutter speed can be a problem if you are photographing moving objects or shooting in low light. To decrease the depth of field and make less of the scene in front of and behind the subject sharp, use a wider aperture, like f/2.8 or f/4. There are other ways to control depth Of field. You can increase the depth of field by changing to a shorter focal length lens or stepping back from the subject, although both of those choices will change the picture in other ways as well (see opposite). To decrease the depth of field and make less of the scene in front of and behind your subject sharp, you can use a longer lens or move closer to the subject. These alternatives will also change the composition of the picture (see opposite). Most DSLRs and all compact digital cameras have sensors smaller than a 35mm frame. For them, a lens with a given angle of view (for example, its normal lens) will have a shorter focal length and give more depth of field at any given aperture than a lens with the same angle of view—in this case, a 50mm lens—on a 35mm camera. This is an advantage if you want everything in your picture to be in focus, but a limitation if you want to set a subject apart from its background (see pages 228 and 237). Why does a lens of longer focal length produce less depth of field than a shorter lens used at the same f-stop? The answer relates to the diameter of the aperture opening. The relative aperture (the same f-stop setting for lenses of different focal lengths) is a larger opening on a longer lens than it is on a shorter lens (see below). Different lenses go out of focus differently. The word ________ (Japanese for blurring) refers to the way an out-of-focus subject looks in a photograph, which depends on both the shape of the aperture and the design of the lens LENS FOCAL LENGTHS, APERTURES, AND LIGHT The two lenses below, both set at f/4, let in the same amount of light, even though the actual opening in the 100mm lens (below left) is physically smaller than the opening in the longer 200mm lens (below right). The longer the focal length, the less light that reaches the sensor or film, therefore a long lens will form a dimer image than a short lens unless more light is admitted by the aperture. The sizes of the aperture openings are determined so that at a given f-stop number the same amount of light reaches the film, no matter what the focal length of the lens. The f-stop number, also called the relative aperture, equals the focal length of the lens divided by the aperture diameter. f-stop = lens focal length/ aperture diameter

lens focal, focal length, Light

Lens All photographic lenses do the same basic job: they collect light emanating from a scene in front of the camera and project it as an image onto a light-sensitive surface (a piece of film or a digital sensor) at the back of the camera. This chapter explains how you can use _______ _______ (which adjusts the sharpest part of an image), lens _______ _______ (which controls the magnification of a scene), lens aperture, and subject distance to make the kinds of pictures you want. Lenses for both digital and film cameras follow the same principles of optics. _______ must be controlled if our eyes or our cameras are to form images of objects. You can't simply place a rectangle of sensitized film or an array of light-sensitive diodes in front of a subject and wait for an image to appear. Light reflecting from the subject would hit the surface in a random jumble, resulting not in a picture but in a uniform exposure over the entire surface. Light acts both like rays and like waves. For simplicity's sake, this drawing shows it as rays—only a few coming from just two points on the man. But their random distribution over the surface makes it clear that they are not going to produce a useful image. What is needed is some sort of light-control device between the subject and the digital sensor or film that will select and aim the rays, placing the rays from each part of the image where they belong, resulting in a clear picture.

focal point, focal length

Lens Focal Length The most important way lenses differ is in their focal length. Because the camera you are most likely to choose can be used with interchangeable lenses, you can also choose which lens or lenses to buy or use. A lens is often described in terms of its focal length (a 50mm lens, a 12-inch lens) or its relative focal length (normal, long, or short). Technically, focal length is the distance between the lens' rear nodal point and the focal (image) plane when the lens is focused at infinity. Theoretically, infinity is a distance immeasurably far away, beyond the edge of the universe. In photographic terms, infinity is a distance from which light enters the lens in parallel rays. Lens designers call the image point where those rays come together, behind the lens, the ______ _____. ______ _____ controls magnification, the size of the image formed by the lens. The longer the lens, the greater the size of objects in the image (see diagrams, right) that is projected on the film or sensor. Focal length also controls angle of view, the amount of the scene shown on a given size of sensor or film (see photographs, opposite). A long-focal-length lens forms a larger image of an object than a short lens. As a result, on a given size of sensor or film, the longer lens includes less of the scene in which the object appears. If you make a circle with your thumb and forefinger and hold it close to your eye, you will see most of the scene in front of you—the equivalent of a short lens. If you move your hand farther from your eye—the equivalent of a longer lens—the circle will be filled by a smaller part of the scene. You will have decreased the angle of view seen through your fingers. In the same way, the longer the focal length, the smaller, or narrower, the angle of view seen by the lens. A lens of short focal length bends light sharply. The rays of light focus close behind the lens and form a small image of the subject. A lens of longer focal length bends light rays less than a short lens does. The longer the focal length, the less the rays are bent, the farther behind the lens the image is focused, and the more the image is magnified. The size of the image increases in proportion to the focal length. If the subject remains at the same distance from the lens, the image formed by a 50mm lens will be twice as big as that from a 25mm lens. The effect of increasing focal length while keeping the same lens-to-subject distance is an increase in magnification and a decrease in angle of view. Because the photographer did not change position, the sizes of objects within the scene remained the same in relation to each other. The diagram below shows the angle of view of some of the lens focal lengths that can be used with a 35mm or "full-frame" digital camera. The lens focal length for other digital cameras is often stated as a "35mm equivalent."

magnification

Lens Focal Length: Long focal length A long-focal-length lens provides greater image _____ and a narrower angle of view than a normal lens. For a 35mm camera (or a DSLR camera that uses a full-frame sensor) a popular and useful long focal length is 105mm. For a camera using 120 film for 6 x 7cm negatives, a popular comparable focal length is 150mm; for a 4 x 5 view camera, it is about 300mm. Most DSLR and compact digital cameras have sensors smaller than a 35mm frame; there is a multiplier factor you can use to compare angles of view to lenses for 35mm use. A digital camera with a 22.5 x 15mm sensor has a multiplier of 1.6; any lens on that camera will have the same angle of view as a lens with a 1.6 times longer focal length on a 35mm film camera. For example, a 65mm lens used with a 22.5 x 15mm sensor will be comparable to a 105mm lens used on a 35mm film camera. Long lenses are excellent when you cannot or do not want to get close to the subject. In the photograph opposite, the photographer seems to be in the middle of the action even though he is not. Long lenses make it possible to photograph birds and animals from enough distance that they are not disturbed. Medium-long lenses are excellent for portraiture; most people become self-conscious when a camera is too close to them so their expressions are often artificial. A long lens used at a moderate distance also avoids the kind of distortion that occurs when shorter lenses used close to a subject exaggerate the size of whatever is nearest the camera—in a portrait, usually the nose (see below). There are subtle qualities that can be exploited when you use a long lens. Because a long lens has less depth of field, objects in the foreground or background can be photographed out of focus so that the sharply focused subject stands out clearly. (See page 13, top.) Also, a long lens can be used to achieve an unusual perspective in which objects seem to be closer together than they really are (see opposite page). Long lenses have some disadvantages, and the longer the lens the more noticeable the disadvantages become. Compared to lenses of normal focal length, they usually are heavier, bulkier, and more expensive, especially telephotos with wide apertures. Because they have relatively shallow depth of field, they must be focused accurately. They are difficult to use for hand-held shots because they magnify lens movements as well as subject size. The shutter speed for a medium long lens, such as an unstabilized 105mm lens on a 35mm camera, should be at least 1/125 second if the camera is hand held. For an unstabilized 200mm lens, you will need at least 1/250 sec. Otherwise, camera movement may cause blurring. A tripod or other support is your best protection against blurry photos caused by camera movement. Photographers commonly call any long lens a telephoto, or tele, although not all long lenses are actually of telephoto design. A true telephoto has an effective focal length that is greater than the actual distance from lens to focal plane. This design makes the lens shorter and easier to handle. A tele-extender or teleconverter contains an optical element that increases the effective focal length of any lens. It attaches between the lens and the camera body and magnifies the image from the lens onto the film. With these devices, the effective length of the lens increases, but less light reaches the film. A converter that doubles the lens focal length, for example, loses two f-stops of light. Long lenses often produce better portraits. A moderately long lens such as an 85mm or 105mm lens (35mm equivalent) used at least 6 ft from the subject (near right) makes a better portrait than a shorter lens used close to the subject (far right). Compare the size and shape of nose and chin in the two pictures of the same subject. Photographing a person at too close a lens-to-subject distance makes features nearest the camera appear too large and gives an unnatural looking dimension to the head. Athletes at sporting events, like these oarsmen, can be at a considerable distance. Photographers often rely on long lenses—like the 500mm telephoto used here—to come in tight on the action. Jones set his lens to a medium aperture so he could use a high shutter speed and still get the side-by-side boats in focus. The sense that space in the picture is being compressed comes from the long distance between photographer and subject, not the lens. See page 61 for more.

human

Lens Focal Length: Normal Focal Length A normal-focal-length lens, also called a standard-focal-length lens, approximates the impression ______ vision gives. One of the greatest of modern photographers, Henri Cartier-Bresson, who described the camera as an extension of my eye," often used a normal lens. His picture opposite includes as much of the scene as you would probably be paying attention to if you were there, the angle of view seems natural, and the relative size of near and far objects seems normal. A lens that is a normal focal length for one camera can be a long or short focal length for another camera. Sensor or film size determines what will be a normal focal length. The larger the size, the longer the focal length of a normal lens for that format; it corresponds roughly to the measurement of a diagonal line across the sensor surface or film frame (see below). A camera using 35mm film takes a 50mm lens as a normal focal length (50mm is about two inches). It is also normal for a 24 x 36mm (full-frame) digital sensor, the same size as a 35mm film frame. For a camera using 4 x 5-inch film, a 150mm lens is normal. The sensors in most digital cameras are smaller than a 35mm frame, so their normal lenses are shorter than 50mm. Usage varies somewhat: for example, lenses from about 40mm to 58mm can be referred to as normal focal lengths for a 35mm camera. A lens normal focal length has certain advantages over lenses of longer or shorter focal length. Most normal lenses are faster; that is, they open to a wider maximum aperture, so can be used with faster shutter speeds or in dimmer light than lenses that do not open as wide. They often are less expensive, more compact, and lighter in weight. Choice of focal length is a matter of personal preference. Some photographers habitually use a shorter focal length because they want a wide angle of view most of the time; others prefer a longer focal length that narrows the angle of view to the central objects in a scene. If you aren't sure, start with a normal focal-length A lens of a given focal length may be considered normal, short, or long, depending on the size of the film you are using or the sensor in your digital camera. If the focal length of a lens is about the same as the diagonal measurement of the light-sensitive surface (broken line), the lens is considered "normal." It collects light rays from an angle of view of about 500. The photograph on the right was taken with a 4 x 5 view camera using a 15 degree mm lens. The diagonal measurement Of 4 x 5-inch film is about 150mm, so a 150mm lens is a normal focal length for that size film. But 150mm is much longer than the diagonal of a 35mm film frame or a sensor the same size, called full frame. A 150mm lens is a long-focal-length lens for these cameras and— because most digital sensors are smaller than a 35mm frame—it is very long for most DSLRs (see bottom photos, right). Using a normal-focal-length lens is a way for the photographer to let the subject speak. Wide and long lenses make a stronger statement about the technical decisions a photographer must make, such as about perspective or depth of field. Although Cartier-Bresson resisted discussing technique, it is known that he frequently used a normal (50mm) lens on his 35mm Leica camera. In this case, the size relationships in the frame—clues that give us our sense of perspective—are not unusual. We are drawn to what the photographer wants us to notice, the poetry of an instant snatched from the fabric of time itself Cartier-Bresson called this a "decisive moment."

convex lens

Most modern photographic lenses are based on the ______ ______. Thicker in the middle than at the edges, a convex lens collects a large number of light rays from any single point on an object in front of the lens and refracts, or bends, them toward each other so that they converge at a corresponding single point (diagram, bottom left) behind the lens. Each point in the scene (object point) corresponds to a point behind the lens (image point). Inside a camera, a strip of film is stretched flat (or a flat digital sensor is positioned) across the image plane, sometimes called the film plane. All the points in a corresponding object plane in the scene will focus as points on that surface, so the picture will be sharp for everything in that plane. How does a lens refract (bend) light to form an image? When light rays pass from one transparent medium, such as air, into a different transparent medium, such as water or glass, the rays refract, or bend. Look at the shape of a spoon half submerged in a glass of water and you will see a common example of refraction: light rays reflected from the spoon are bent by the water and glass so that part of the spoon appears displaced. For refraction to take place, light must strike the new medium at an angle. If light rays are perpendicular to the surface when they enter and leave the medium (diagram left, l), the rays will pass straight through. But if they enter or leave at an oblique angle (2), the rays will be bent to a predictable degree. The farther from the perpendicular they strike, the more they will be bent. When light strikes a transparent medium with a curved surface, such as a lens, the rays will be bent at a number of angles depending on the angle at which each ray enters and leaves the lens surface. They will be spread apart by concave surfaces (3) and directed toward each other by convex surfaces (4). Rays coming from a single point on an object and passing through a convex Jens (the simplest form of camera lens) will cross each other—and be focused—at the image point.