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Tuesday, April 29, 2014

The Medium Mainstay

Big telezooms get the glory and wide-angle models get the headlines, but the underappreciated middle child in a typical three-zoom kit may be your most useful lens. Learn how to choose the right one for you.

Labels: LensesGear
Image Stabilization
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.


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