Zoom is a classification of lenses in the field of photography, or cinematography, or what-have-you. Zoom lenses have a variable focal length, which simply means that they can make the subject matter appear closer or farther away without actually having to move your feet. What actually happens is that you change the angle of vision and the magnification.

Here are some examples of zoom lenses used with a 35mm camera. The focal lengths to get the same area of coverage would change if you are using a medium format or large format camera. A 17-35mm zoom lens would be considered a wideangle zoom and would be great for scenic landscapes or photographing a large group of people. Conversely, a 200-400mm zoom lens would be used to take pictures of sports or animals or other things that are difficult to get close to. This would be a telephoto lens. A 35-70mm zoom lens would fall in the normal focal length, and would be good for everyday shooting.

Zooms are a relatively recent advancement in the field of photography. They have been around for quite some time, actually, but only in recent years has the optical quality become good enough for them to be considered indespensable tools by professional photographers. The nice thing about zoom lenses is that you can "get it all in one package". One zoom lens could do the job of 2,3,4 or more fixed-focal length lenses. This will save considerable weight and cost. Some of the drawbacks are slight loss of optical quality and generally smaller apertures AFAIK.

Many budding photographers forget that convenience and flexibility frequently come with a cost, and zoom lenses are far from being an exception.

Here are some disadvantages of zooms that you may not be aware of:

  • Zoom lenses are slow - No, I don't mean the amount of time it takes to zoom in or out. We're talking the amount of light that's allowed in. Whereas a 50mm non-zoom (prime) lens may open all the way up to f/1.8 or more, typical mid-priced zoom lenses in that focal-length range aren't any faster than f/4 to f/5.6, and many get worse as you zoom in further.
  • The image quality isn't as good - A cheap prime lens will almost always beat out a mid-priced zoom lens when it comes to sharpness and image detail. The reason for this is that zoom lenses typically have up to twice as many elements (individual pieces of glass that comprise a lens) as a non-zoom lens within the same focal length range, and it's harder to engineer a lens with this much glass to produce a very good picture.
  • Zoom lenses are heavier - More glass in the lens and more mechanics to move it means a heavier lens.

Of course, these disadvantages can be greatly outweighed by the flexibility that being able to zoom offers. However, if you own an SLR, either film or digital, it is usually good to own at least one or two prime lenses, such as a 50mm f/1.8 or better and a 28mm for those wide-angle shots. You'll thank yourself when you end up in a low-light situation and don't want to or can't use flash; museums don't really care how slow your film is and won't bend their policies to accomodate you.

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