A zoom lens is a time and equipment saver for photographers, providing the ability to frame a subject in different ways from one vantage point with a single lens. A zoom lens does this by providing variable focal length between its shortest and longest settings. The zoom ratio for a camera or a lens is simply the comparison between these settings, though a camera's software may go beyond what a lens can provide optically.
Understanding Focal Length
Focal length indirectly determines the field of view that a lens covers. Short focal lengths, such as 18 or 24 mm, cover wide angles, while lenses of 35 to 70 mm correspond roughly to the normal field of human vision. Lenses over 70 mm provide telephoto views. These values are for cameras that use 35 mm film or have a similar sized image sensor. When the sensor or film is smaller, for example, a 50 mm lens acts more as a telephoto lens. Zoom lenses have variable focal lengths expressed as the shortest and longest values, such as 18-55 mm or 75-300 mm.
Zoom ratio is simply the ratio of the shortest and longest ranges of a zoom lens. For example, an 18-55 mm lens has a ratio of 0.33, or about 1:3, while a 75-300 mm lens ratio is 0.25, or 1:4. Used with 35 mm film or an equivalent sensor, the 18-55 mm lens covers the wide-angle to normal range, while the 75-300 mm lens covers telephoto views. Note that the zoom ratios have no connection to field of view, but merely express the range between short and long.
Compact Digital Cameras
While some understanding of zoom ratio is handy for cameras with interchangeable lenses, zoom ratio is more commonly applied for comparison of compact point-and-shoot cameras with zoom functions. Zoom ratio is often expressed using 2x or 10x format. The lenses from the example would be described as 3x and 4x with this method. A camera with a 5x zoom ratio will have only half the field of vision range as a camera with a 10x zoom.
Many compact cameras with zoom features boast both optical and digital zoom. Any zoom function resulting from a zoom lens is optical. The resulting image uses all the pixels on the image sensor to create a photograph. Digital zoom enlarges the data from a portion of the image sensor, creating a virtual zoom effect. In essence, the camera throws away data around the edge of the sensor and uses only the pixels from the middle. While this makes it appear that the subject is closer to the camera, it is actually a crop performed in the camera's electronics. Extreme digital zoom ratios reduce image resolution, and will reduce the quality of your images.