|This Article Features Photo Zoom|
Telephoto zooms are popular with wildlife photographers and wide-angle zooms with landscapists, but the mid-range zooms also have their fans. The 24-70mm to 24-105mm zooms (for full-frame DSLRs and 35mm cameras; 16-50mm to 17-70mm for APS-C cameras) take you from the start of true wide-angle into portrait telephoto (yes, people can be great outdoor photo subjects, too!). Most will focus close enough to do good flower and spider-web still lifes and, of course, that focal-length range covers most of the classic landscape angles of view.
Mid-range zooms come in fast (and bulkier) ƒ/2.8 form and in slower (and more compact) ƒ/4 form. If you like to shoot in dim light, you'll find the ƒ/2.8 zoom more suitable; if traveling light is more important to you, you'll be better off with the ƒ/4s.
Wider apertures produce less depth of field, making the faster zooms better for selective-focus shots, where you want to concentrate the viewer's attention on a specific part of a subject or scene by minimizing depth of field and throwing everything else out of focus. Shooting wide open at ƒ/2.8 also helps blur a busy background so it's less distracting.
The ƒ/2.8 lenses also autofocus more quickly and accurately, in part, because they provide more light for the AF system to work with and, in part, because many mid- and high-end DSLRs have central AF points that provide added precision with ƒ/2.8 (and faster) lenses due to the wider beam of light provided to the AF system (much as wide-base rangefinders are more accurate than narrower-base ones). Within a given brand, the ƒ/2.8 lens also likely has a better AF motor.
On the other hand, ƒ/4 lenses are more compact and less costly, while still providing the focal-length range and delivering excellent image quality. As an example, the Canon EF 24-70mm ƒ/2.8 weighs 28.4 ounces, and the EF 24-70mm ƒ/4, 21 ounces. That's almost a half-pound difference—a lot when carrying the camera and lens on a neck strap all day. The ƒ/2.8 also costs 50% more.
Bottom line: If you specialize in handheld low-light photography or selective-focus work, it's likely you'll be happier with an ƒ/2.8 mid-range zoom. If light weight and low cost are more important to you, an ƒ/4 is a better choice.
Many photographers use a mid-range zoom as their "walking around" lens because these lenses are relatively compact and provide a versatile range of focal lengths. As "carry-around" lenses, they're most often used handheld.
Some of these zooms have built-in optical image stabilization—Canon IS, Nikon VR, Sigma OS and Tamron VC models. While it may seem as though stabilization isn't as important with these lenses as with supertelephoto focal lengths, stabilization is critical for the type of "ƒ/8 and be there" handheld shots you're likely to pursue with mid-range zooms. Pentax and Sony DSLRs have sensor-shift stabilization in the camera body, so their lenses don't have optical stabilization built in—all lenses mounted on these cameras are stabilized by the sensor-shift feature.
Minimum Focusing Distance
Most mid-range zooms will focus close enough to produce a reproduction ratio of around 0.2X. (At the minimum focusing distance and maximum focal length, the subject's image at the image plane will be 1/5 life-size.) Of course, you can blow it up much more than that when you size the image for printing or display, but this capability opens up the world of macro to you—flowers, dragonflies, spider webs and the like.
The wide end of these zooms allows you to move in very close to a subject to exaggerate its size relative to its surroundings, while the wide angle of view still includes much of those surroundings. The closer the minimum focusing distance at the wide end, the more you can exaggerate the subject's size in this manner.
Note that as you move closer, nearer subjects in the image grow more rapidly than farther ones, and the distance between them seems to expand—the perspective changes. This is different from the effect of changing focal lengths from a given spot. When you change focal lengths, the field of view changes, everything in the image grows or shrinks at the same rate, and the apparent distance between objects doesn't change.
You can prove this for yourself. Put your camera on a tripod and take a shot with a long focal length, then change to a shorter focal length (by changing to a shorter lens or zooming a zoom lens) and take another shot from the same spot. The wide-angle shot obviously will show more of the scene, and everything in the scene will be smaller in the frame. But if you blow up the center section of the wide-angle shot so it covers the same area as the long focal-length shot, you'll find that the perspective is identical—the sizes of the objects in the frame, their sizes relative to one another and their apparent distances from one another are the same in both images.
Moving closer expands perspective; moving farther away compresses it. The focal length changes the field of view and the magnification. "Wide-angle expansion" and "telephoto compression" are really due to the shooting distance, not the focal length. When we use a wide-angle lens, we generally move in on the subject, which expands perspective, and we generally shoot from farther away when using a long lens, which compresses perspective. That's the main reason why wide-angle landscapes with no important foreground object are often boring—a large area with distant flat perspective isn't very interesting unless there's a really colorful sky or great cloud formations. You can use camera position and focal length to create just the effect you want in your images, although the zoom lens provides nice framing control when you can't move closer or farther away—the mid-range zooms give you a lot of versatility.
|This Article Features Photo Zoom|
LEFT TO RIGHT: Pentax 17-70mm ƒ/4; Sigma 24-70mm ƒ/2.8; Zeiss Vario-Sonnar T* 24-70mm ƒ/2.8; Tamron 24-70mm ƒ/2.8; Nikon AF-S 24-70mm ƒ/2.8
Wide-angle lenses, especially fast ones, tend to suffer from spherical aberration and distortion, and longer lenses from chromatic aberrations. With spherical aberration, light rays coming through the edges of the lens aren't focused at the same point as rays coming through the center. With chromatic aberration, different colors (wavelengths of light) are focused at different planes. Both reduce image sharpness.
Today's lens makers combat these by using special lens elements. Aspherical elements counter spherical aberration and distortion, providing sharper images and also allowing for the design of more compact lenses. Low-dispersion and extra-low dispersion elements (called ED, UD, SUD, ELD, SLD, FLD and the like by the various manufacturers) counter chromatic aberrations, also delivering sharper images. All of the zooms in the accompanying chart feature special elements in their construction; the more costly lenses generally offer more effective ones (i.e., they do a better job of minimizing aberrations and distortion).
Larger filters cost more than smaller ones, so you may want to consider that when selecting a mid-range zoom if you use a lot of filters. The ƒ/2.8 lenses tend to take larger filters, as they must be larger in diameter to provide their faster maximum apertures.
If you have more than one lens, it's nice if they all take the same filter size so you can use the same filters on all of them—another consideration when buying a new lens if you have a lot of filters. Of course, you can use adapter rings to mount larger filters on smaller lenses, but if the new lens takes larger filters than the ones you currently own, you'll have to buy new filters for it.
Faster lenses generally cost more than slower ones, another consideration when selecting the right one for your needs. Mid-range zooms start at around $500 and go up beyond $2,000. The more costly ones tend to be faster, better optically, bulkier and more ruggedly built. If your budget doesn't permit purchasing a high-end lens, the lower-cost ones make good options.
A good mid-range zoom costs much less than separate wide-angle, normal and short telephoto prime lenses of equal speed (i.e., ƒ/2.8 or ƒ/4). This can save you a lot of money, along with the speed and convenience factor. You can change framing merely by rotating the zoom ring rather than having to mount another lens. And minimizing lens-changing also reduces the opportunities for dust to enter the camera body and settle on the sensor assembly.