Most digital SLR users own at least a couple of lenses for their "serious" photography, but it's impractical to drag along the full camera bag on every outing. Especially during vacation trips or when hiking or boating, a heavy camera bag can become a real pain in the .... shoulder. These are the times when an extremely versatile 18-200mm or similar zoom would be a fine choice for most any subject that you're likely to encounter.
As the wide-angle photo at the top of the story and the telephoto photo of the same scene below illustrate, multi-purpose zooms offer great versatility in a relatively compact package.
Multi-purpose zooms have become increasingly popular since their introduction in the late 1980s. The latest models are significantly better than their predecessors and even smaller/lighter. They often provide superior optical elements and closer focusing ability. In many cases, autofocus is also faster, especially with the lenses featuring an Ultrasonic or Silent Wave AF motor. Available in several brands, 18-200mm, 18-250mm or 18-270mm zooms cover the full range of focal lengths, from true wide-angle to strong telephoto.
In order to get the best possible results with a multi-purpose zoom lens, consider the following recommendations.
Maximize optical potential: For the best results overall, shoot at f/8 or f/11, the optical "sweet spot," to improve sharpness and to minimize darkening at the corners of your photos.
Use higher ISO: A tradeoff of these versatile zooms feature is small maximum apertures, typically f/3.5-5.6 or even f/3.5-6.3. Because of this aspect, shutter speed will not be very fast at ISO 100 when use the widest aperture or especially the recommended f/8 or f/11. But set an ISO of 400 in daylight, or ISO 800 on a dark overcast day, and the shutter speed will be faster. That will minimize the risk of blurring from camera shake and it will usually "freeze" the motion of an action subject.
Mount the lens hood: Nearly all multi-purpose zooms include a lens hood that you can attach to the front. (If yours did not come with a hood, purchase one as an accessory.) A lens hood can prevent stray light from striking the front element and creating flare: an effect resembling a bright veil over the image, decreasing contrast and sharpness.
Minimize distortion: Some test reports emphasize the fact that multi-purpose zooms can produce distortion. At wide-angle settings, lines at the edges of the frame can bow outward. And at telephoto, lines at the edges may bow inward. Frankly, neither type of "linear" distortion will be obvious in your photos unless you take pictures of a brick wall.
However, there is another type of distortion that becomes obvious when a lens is tilted upward, to include an entire building, for example. This "perspective distortion" can make the structure appear to lean backward. As well, vertical lines can seem skewed while the edges of the horizon bow upward. These effects are caused by shooting technique, not by any optical flaw. When you want to include a tall subject in a single photo without distortion, try this. Switch to a vertical camera format and move further back from the subject so you do not need to tilt the lens upward.
Take it along: Don't lose sleep over the fact that a multi-purpose zoom does not score as high in test reports as some other types of lenses. Instead, take advantage of these hints to get the best possible images. Whenever size/weight and convenience are most important, a multi-purpose zoom would be a fine choice with a compact DSLR. And as a bonus, you'll get less gunk on your camera's sensor because you won't be changing lenses. That's a real plus especially when traveling in locations with a lot of dust, sand or water spray.