The ability of many 35mm cameras to accept interchangeable lenses is one of the features that makes these cameras so versatile. Since the lens plays a vital role in determining what the camera can do for you, knowing how to take full advantage of the various lenses for your camera pays dividends in terms of better, more exciting pictures.

The normal lens is the one that comes with most 35mm SLR cameras. Because it gives such natural-looking pictures, you'll probably use it more than any other lens. The normal lens receives its name because it provides a natural perspective and an angle of view similar to the central vision of the eye.

The focal length of the normal lens for most 35 mm cameras falls between 45 to 55 mm.

The focal length of a camera lens is the distance from the film plane of the camera to the center of the lens when the lens is focused on infinity.

The normal lenses on 35mm cameras are usually fast, having maximum apertures of f/2.8, f/2, or as large as f/1.2. With today's fast films, a normal lens with a maximum aperture of about f/1.8 will usually suffice. You will pay much more to get a lens with a maximum aperture of f/1.4 or f/1.2. A lens with a large maximum aperture lets you handhold the camera to take pictures in dim light. Using a large aperture, such as f/1.8, also enables you to use high shutter speeds to stop action. However, the depth of field is quite shallow at such maximum apertures. Focus carefully. Otherwise you risk out-of-focus blur with all but distant subjects.

Certainly the most popular lenses in use today are zoom lenses. Though early zooms suffered from the reputation of being heavy, awkward to use, and less sharp than fixed-focal-length lenses, today's zoom lenses are compact, easy to use, and sharp. Zoom lenses are available for both manual-focus and auto-focus cameras.

Zoom lenses offer you the flexibility of many different focal lengths along with the convenience of having to carry only one lens. For example, instead of carrying a wide-angle, a normal, and a telephoto lens, you could take along a single zoom lens with a range of 35 to 135mm. Such a zoom not only replaces several fixed-focal-length lenses, but also offers you all the focal lengths that fall within that particular zoom range. Instead of having to decide between a normal and a telephoto lens, for instance, you could use a focal length anywhere between the two.

Equally enticing is that a zoom lens lets you fine-tune your compositions without having to change your shooting position. Instead of walking closer to or farther from your subject, you can remain in place and adjust the focal length to fit the composition you have in mind. With a wide-angle-to-telephoto zoom, for instance, you could take a group shot around the picnic table at a wide-angle setting and then zoom in close to take a full-frame portrait without taking a step.

Zoom lenses are available in a variety of focal-length ranges. Among the most common are those that cover a wide-angle-to-normal range (28 to 50mm or 21 to 50mm), a wide-angle-to-medium telephoto range (35 to 105mm or 35 to 135mm), and a medium-telephoto-full-telephoto range (80 to 200mm). The zoom lenses with the greatest ranges - 28 to 210mm and 50 to 300mm, for example - tend to be heavier and less sharp than those with more moderate ranges. They're also more expensive. Their close-focusing distances may be as far as 10 feet.

If you are thinking of buying your first zoom, those in the wide-angle-to-medium-telephoto group (35 to 135mm, for example) make a good choice; they offer a great range of focal lengths in a lightweight package and have relatively fast maximum apertures. Also very useful are the medium-to-long-telephoto zooms (around 70 to 210mm). They provide you with substantial telephoto capability, but still allow you to broaden the composition when necessary. Many zooms in this range also have macro capability, which lets you take close-ups of small objects.

Most zooms in use today are called "one touch" lenses. You focus the lens and change focal lengths by pushing or pulling on the focusing collar. A few zoom lenses are "two touch;" they have separate controls for focusing and zooming. Auto-focus zooms are available too, and they're even easier to use; they do the focusing and leave you to concentrate on adjusting the focal length to get the composition you want.

What are the disadvantages of zooms? Weight is one disadvantage (especially among longer telephoto zooms); zooms are heavier and bigger lenses. Flare is another; especially in backlit situations, zoom lenses may flare noticeably, scattering light throughout the picture. But when you compare carrying one zoom lens to carrying several fixed-focal-length lenses, the zoom ends up being the lighter choice. Zooms also cost a bit more than fixed-focal-length lens of similar focal lengths, but again, one zoom is cheaper than several fixed-focus lenses and considerably more flexible.

A more serious drawback of zoom lenses is that they are generally slower, i.e., they have smaller maximum apertures, than equivalent fixed-focal-length lenses. Also, most zooms have a variable maximum aperture that gets smaller (letting in less light) as you move toward the longer focal lengths of the lens. A 70 to 210mm f/4.5-5.6 lens has a maximum aperture of f/4.5 at 70mm, for example, but a maximum aperture of only f/5.6 at its longer focal lengths. The smaller aperture means slower shutter speeds, which can increase the likelihood of blur from camera shake or from moving subjects. Because it lets in less light, a smaller aperture makes focusing more difficult, too, particularly in low light levels.

Finally, remember that all of the information we're about to discuss for wide-angle and telephoto lenses also applies to zoom lenses that have wide-angle or telephoto focal lengths.

A wide-angle lens has a shorter focal length than the normal lens for the camera. A wide-angle lens takes in a greater angle of view than the normal lens. From any given spot a picture made with a wide-angle lens includes more than a picture made with the normal lens. 

When do you use a wide-angle lens? Some of its uses are rather obvious. A wide-angle lens is helpful for taking pictures in places where space is limited. Without enough space, you just can't move back far enough to include everything you want with the normal lens. When you're photographing such subjects as all the in-laws around the Christmas tree at home or a brand-new sports car on display at a crowded auto show, a wide-angle lens is a very handy item to have. 

A wide-angle lens is often a good friend to have outdoors, too. When you take pictures in narrow city streets or crowded public markets or photograph sweeping scenic vistas, a wide-angle lens will let you get it all in when this isn't possible with the normal lens. 

Another situation where a wide-angle lens may help you is in public places when people or other objects are between you and your subject. You may be able to eliminate them from the picture by using a wide-angle lens which allows you to move closer to your subject and frame the picture the way you want it. 

Perspective is determined by camera-to-subject distance. Whether you have a normal, wide-angle, or telephoto lens, perspective is the same for all of them if the camera-to-subject distance remains the same. When you get close to a subject, as you might with a wide-angle lens, nearby objects look unusually large, and distant objects in the same picture look small and far away. This is because the distance between the near and far subjects is great compared to the distance from the camera to the near subject. The wide-angle lens exaggerates space relationships by expanding the apparent distance between nearby and distant objects. You'll increase the feeling of vastness in scenic pictures by using a wide-angle lens and including a nearby foreground object, such as a person, tree, or automobile, for size comparison. 

For the same reason-exaggerated perspective-a close-up picture of a person's face made with a wide-angle lens gives the features a distorted appearance. The nose, since it is closer to the camera, looks bulbous, while the more distant ears look exceptionally small. If you use a wide-angle lens to take a picture of an automobile from a front angle, it will look especially long and sleek. A welcoming hand stretched toward a wide-angle lens looks as large as or larger than the head of the person offering the greeting. 

When you use a wide-angle lens to photograph entire buildings or similar subjects with prominent parallel vertical lines, try not to tilt the camera up or down. If you do, the vertical lines will converge, or keystone, in your picture. While keystoning is usually undesirable, there may be times when you want to create this effect-to make a building look taller, for example, or to exaggerate perspective for creative composition. 

Photography with a wide-angle lens offers the bonus of increased depth of field. For example, with a 28mm wide-angle lens on a 35 mm camera, if the lens is set at f/11 and is focused on a subject 10 feet away, everything from about 4 1/2 feet to infinity will be in focus. In the same situation with a 50mm lens, the depth of field would extend from about 7 feet to 17 feet.

Depth of field is actually the same for all lenses, no matter what their focal length, if you adjust the subject distance to give the same image size. For a particular camera and particular subject distance, however, we can say that depth of field increases as focal length decreases.

A telephoto lens has a longer focal length than the normal camera lens. Technically, the term telephoto refers to a particular kind of optical arrangement that has a positive front element and a negative rear element. This allows the physical length of some telephoto lenses to be shorter than the focal length. However, it has become common practice to call any lens with a focal length that is longer than normal a telephoto lens, so that's what we do.

Telephoto lenses do just the opposite of what wide-angle lenses do. They include a narrower angle of view than the normal lens, so they take in a smaller area of the scene. Consequently distant subjects photographed through a telephoto lens appear closer than they do when photographed through a normal lens. A telephoto lens magnifies the image similarly to the view you see through binoculars or a telescope.

When do you use a telephoto lens? As a rule, you use a telephoto lens when you can't get as close to your subject as you'd like-for example, when you're photographing a baseball game, an alligator in Okefenokee Swamp, or a chalet perched on a distant hillside. You can't climb into a cage at the zoo to get a close-up of a lion or stand in the middle of the Hudson River to get a close-up of a luxury yacht, but a telephoto lens can produce a big image of such subjects by bringing them closer to you optically.

Like wide-angle lenses, telephoto lenses offer advantages that aren't so obvious, as well. For example, lenses in the 75mm to 105mm focal-length range are great for making head-and-shoulder portraits of people. You can be six or more feet from your subject and still get a nice-size head-and shoulder image on the film with a moderate telephoto lens. This means that the nose-to-ears distance is very small in relation to the total subject distance, so the exaggerated perspective we mentioned in the section on wide-angle lenses doesn't exist. Being six or more feet from your subject also makes it easier to take pictures of people. You can move around your camera when it's on a tripod and your subject without stumbling over them.

We mentioned that wide-angle lenses expand distances. As you might suppose, telephoto lenses have the opposite effect-they compress distances. Distant objects look closer to each other than they actually are. You've probably seen extreme telephoto pictures of a large city or rows of buildings on a distant hillside. In such pictures, the distant subjects appear squashed together. This effect is caused by the very narrow angle of view, which eliminates from the picture all the nearby objects that help us judge distances.

Because a telephoto lens has a narrow angle of view, it can help eliminate distracting elements in the composition of a picture. If there is a water tower next to a picturesque country church, you can crop the tower out of the picture by using a telephoto lens with its narrower angle of view. The lens sees the church, but not the tower. In the same way, you can photograph between the heads of spectators at a sports event or parade.

Telephoto lenses have shallow depth of field. The longer the focal length of the lens, the more shallow the depth of field. This means that accurate focusing is much more important with telephoto lenses than with normal and wide-angle lenses.

As mentioned earlier, you can use shallow depth of field as a creative tool for throwing a distracting background out of focus or for de-emphasizing foreground objects, both of which help concentrate interest on the main subject.

While a wide-angle lens is forgiving, the telephoto lens is demanding. In addition to focusing very carefully, you must hold your camera extremely steady to get pictures that are free of blur. This is because a telephoto lens magnifies camera movement as well as image size. Consequently, it's a good idea to use a tripod and a cable release with a telephoto lens. As a general rule, don't try to handhold the camera when you're taking pictures with a lens that has a focal length longer than about 400mm.

To minimize the effects of camera motion, it's essential to use a high shutter speed. A good rule to follow is to use a shutter speed approximately equal to: 1/Focal Length in Millimeters second. For example, with a 200mm lens, you should use 1/200 second or higher. Since 1/200 second is not available on your camera, use the next higher shutter speed setting of 1/250 second. If you still have difficulty getting sharp pictures, use a shutter speed twice as fast-in this case 1/500 second. Be especially watchful with automatic cameras that determine the shutter speed. If the shutter speed indicated by the camera drops to a speed that's too slow for sharp pictures, use a larger lens opening so the camera will adjust for a sufficiently high shutter speed.

A telephoto lens increases the effect of subject movement, too. Since the subject appears to be nearby and relatively large in the picture, any movement of the subject will be quite noticeable. This means that when you photograph action with a telephoto lens, you should use higher shutter speeds than you would to photograph the same action with a normal lens.

               So, you may want to use a telephoto lens -

                   to produce a large image of a distant subject by
                   bringing it closer to the camera optically. 
                   to make portraits. 
                   to compress distances. 
                   to eliminate distracting elements from a picture. 
                   to get shallow depth of field. 

By using a teleconverter with a telephoto lens, you can double or triple the effective focal length. A teleconverter, sometimes called a teleextender, is an inexpensive way to increase the focal length of a telephoto lens. A teleconverter will also work with a normal-focal-length lens to produce the effects of a telephoto lens. These converters fit between the camera body and the lens. They extend or multiply the focal length of the lens by 2 or 3 times depending on the power of the converter. A 2X teleconverter converts a 100mm-focal-length lens to 200mm focal length, and a 3X teleconverter will convert the same lens to 300mm focal length.

Another advantage of a converter is that it will give you a larger image for close-up photography. You get a larger image because of the increased focal length of the lens with the converter and because you can focus at the same close focusing distance as the original lens when it's used without the converter.

However, there are some disadvantages in using a teleconverter. A converter makes the effective lens opening smaller-a 2X teleconverter reduces the effective lens opening by 2 stops; a 3X teleconverter by 3 stops. For example, if your lens is set at f/5.6, the effective lens opening with a 2X converter is f/11; with a 3X converter it is f/16. Through-the-lens exposure meters in cameras will automatically compensate for the reduced effective lens opening. With a separate exposure meter or for flash pictures you will have to determine the exposure based on the smaller effective lens openings.

The reduced lens openings make the viewfinder dimmer for viewing. Also you may have difficulty using a high enough shutter speed or getting enough exposure under dimmer lighting conditions. In addition, with a converter it's generally better to use a smaller lens opening to increase the image quality.

You may notice some darkness and a loss of the image around the corners and top edge of your viewfinder when you use a teleconverter. This happens because the mirror in a single-lens-reflex camera is not designed to reflect all the light from lenses when teleconverters are used with them. However, this affects only the viewfinder. The film should receive a complete image with no darkness around the edges when you take the picture.

In selecting a teleconverter, make sure that the mechanical linkage between the camera body and the lens will still work properly.

Off-center subjects can cause focusing errors for many auto-focusing cameras. If the subject is outside the focusing target in the viewfinder, either manually focus on the subject or use the focus lock to obtain correct focus and then recompose the picture. 

Perhaps the most exciting technological development in 35mm cameras in recent years has been the advent of fast, accurate, and affordable auto-focus cameras. Although auto-focus technology made its debut in relatively simple compact cameras, today the most sophisticated auto-focus lenses are being made for 35mm SLR cameras.

Auto-focus lenses for SLR cameras are available in a variety of fixed-focal lengths and zoom ranges. Some things to consider when buying an auto-focus lens are its weight, size, and focus response. To some degree each of these is related partly to the focal length of the lens and partly to the design of the camera and lens system. As with manual-focusing lenses, a wide-angle auto-focus lens will be much lighter and focus faster than a telephoto autofocus lens. Auto-focus lenses are available in many of the same focal lengths as manual-focus lenses. 

The focusing mechanisms of autofocus lenses fall into two categories: those that use a focusing motor in the lens and those that use a focusing motor in the camera body. Lenses with a built-in motor may focus slightly faster than those that use a motor in the camera body, but they are generally more expensive because you're buying a focusing motor each time you buy a lens. Lenses used on auto-focus cameras that have the focusing motors built into the camera body, on the other hand, are lighter and usually less expensive though the initial investment in the camera body may be a bit higher. There is debate among manufacturers (and users) about which design truly focuses faster, but to the average photographer the difference in speed is negligible.

Auto-focus lenses with built-in motors will not auto-focus on a non-auto-focus camera. This is because the focusing sensors that activate the focusing motor are in the camera body-regardless of where the focusing motor is located. You can, however, use an auto-focus lens as a manual focusing lens on any camera body that it's designed to fit. Similarly, a non-auto-focus lens used on an auto-focus body won't focus automatically, although you may be able to use it as a manual-focus lens. (Be sure to check your camera instruction booklet to see what lenses are safe to use on your body before attaching any lenses to it.) Also, a few camera manufacturers make auto-focus adapters so that older non-auto-focus lenses of the same brand can be used on a newer AF body.

The only exceptions to this discussion are a few "universal" auto-focus lenses that have both built-in focusing motors and auto-focus sensors; these lenses are specifically designed for use as auto-focus lenses on non-autofocus cameras. With these lenses, you choose the lens you want and then buy an adapter to fit it to your specific camera body.

One major difference between autofocus lenses and most manual-focus lenses is that most auto-focus lenses focus internally. In other words, as the lens focuses, the elements shift position internally, but the outer barrel neither rotates nor extends in length. With manual-focus lenses, the lens barrel turns, and the lens becomes longer as you focus on closer objects. On some autofocus lenses, the very front ring of the lens (particularly on zooms) does turn during focusing, which can be a problem if you're using a polarizing filter (or cross-screen filter) which has to be oriented in a certain direction for best effect. The only solution is to focus first, then mount the filter and rotate it to the correct position. (You may have to switch your lens to manual to mount the filter to keep from damaging the focusing motor; see your manual.).

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