Technically speaking, a zoom lens is one that allows you to change its focal length within a given range. For most people, however, the results of the zooming process are more important. Zooming lets you change the size of the image in your viewfinder and in your files. You zoom in to make things bigger, as if you had stepped closer to whatever you're photographing. Zooming out makes things smaller so you can fit more of your subject into the frame. Zooming also has aesthetic effects, such as changing the depth of field and compressing distance.
The classic zoom is optical. The lens creates the entire zoom effect by changing the path light beams take through the lens. The optical principle is actually simple. A zoom lens has an extra internal element that acts as a magnifier, increasing the size of the image by apparently increasing the focal length of the original lens. The magnifying effect changes as the magnifier moves with respect to the other elements of the lens. Therefore, by sliding the magnifier, you can continuously change the focal length of the original lens.
In practice, zoom lenses are not so simple. They are among the most complex optical devices in common use. They have multiple lens elements to correct for image problems and distortions added by the zoom process—for example, maintaining that constant f-stop setting as the focal length of the lens changes.
Maintaining the quality of the image produced by a lens throughout its zoom range has always been a challenge for optical designers. Thanks to modern computers, however, they have become adept at creating superb zoom lenses. The designs are complex, however, and that means they can be costly to produce. That's the chief disadvantage of the optical zoom.
The quality—meaning the sharpness, contrast, and color saturation—of the image produced by a modern zoom lens does not change as you zoom through the entire range of the lens. Keep that in mind. It's the chief advantage of the optical zoom.
Optical zooming works on the image before it is actually formed on the image sensor inside a digital camera. Digital zooming, in contrast, manipulates the image after it is registered by the sensor. In effect, digital zooming is nothing more than interpolating the image to a different size. The process is exactly the same as you can do with image-editing software inside your computer. It's called digital zooming only because it takes place within the digital camera with the results showing in the camera's viewfinder.
As with any interpolation, quality can suffer. The interpolation process cannot create image data that's not there to start with. Because the zooming process typically fills the entire frame with an image taken from a fractional part of the frame and discards the rest, the zoomed image inevitably is of lower quality than the full-frame image.
The zooming range of a lens is measured in its own x-factor. The "x" number represents the longest focal length of the zoom lens divided by its shortest focal length. For example, a 50 to 150 mm zoom lens is described as a 3x zoom (150 divided by 50). At that, the x-factor describes the power of the zoom to increase the linear dimensions of the images it produces. With a 3x zoom, for example, the greatest magnification of an image will be three times taller and three times wider than at its least magnification.
A larger zoom range is better than a small one and, of course, typically more costly. Be alert that sometimes camera-makers publish the total zoom capabilities of their cameras as a single figure that's a product of both optical (useful) and digital (doubtful) zoom capabilities. For example, a camera advertised with 6x zoom capabilities often has a 3x optical zoom and a 2x digital zoom. For most practical purposes, you can consider such a camera to have merely 3x capabilities.
Just as modern lenses are made from multiple elements, you can add additional elements to a lens to change its fundamental characteristics. For example, put a positive (convex) element in front of a camera lens, and you will change its focus—it will focus nearer. Additional elements that you can attach to the basic lens of your camera are called supplemental lenses. Some digital camera makers call them conversion lenses.
Four types of supplemental lenses are sometimes available. These include the following:
Which of these four types of converters are available to you depends on what your camera-maker offers or whether aftermarket products fit the camera you choose. If you want to experiment with additional telephoto reach or wide-angle panoramas, check to see which options you can buy before you purchase a new digital camera.