Understanding Magnification and Focal Length

Comparing zoom ranges of digital cameras can be confusing because the exact same lens can produce different magnifications on different cameras. That's why you usually see the zoom range of a digital camera presented either as absolute magnifications or in the equivalents of the 35mm camera lenses that the zoom settings correspond to.

Zoom range doesn't relate to lens quality. You'll find excellent 4:1 zooms and average 4:1 zooms. However, the longer the zoom range, the more difficult it is to produce a lens that makes good pictures at all zoom settings.

Until the advent of digital cameras, figuring the magnification of consumer camera lenses was relatively easy because, in recent years, most consumer (and the workhorse professional) film cameras used a standard film size — the 35mm film frame — which measures a nominal 24mm x 36mm.

The magnification of a particular lens with a standard film size is easily calculated by measuring the distance from the film the lens must be positioned to focus a sharp image on the film. (This is the focal length of the lens.)

By convention, in 35mm photography, a lens with a 45–50mm focal length is considered a normal lens. Lenses with a shorter focal length, such as 35mm, 28mm, 20mm, or less, are described as wide-angle lenses. Those with longer focal lengths (such as 85mm, 105mm, or 200mm) are described as telephoto lenses. Wide-angle and telephoto images are shown in Figure 1.

Then came the digital camera, and all the simple conventions about focal lengths and magnifications went out the window. For valid technical reasons, most digital camera sensors do not measure 24mm x 36mm. You wouldn't want a sensor that large (roughly 1 x 1.5 inches) anyway in a compact digital camera because the camera would have to be large enough to accommodate it. In addition, as with all solid-state devices, the larger a device such as a sensor becomes, the more expensive it is to manufacture. Sensors that are as large as the full 35mm film frame are available for an increasing number of digital SLR cameras, which means that the magnification effect that is discussed in the following paragraphs doesn't apply to those models.

Most sensors for non-SLR cameras are more likely to measure, say, 16mm x 24mm or less. Even the larger digital camera sensors might be no bigger than about 38mm x 38mm. So, a normal lens on one digital camera might be 8mm, whereas another normal lens on another camera might be 6mm. A 4:1 zoom lens can range from 8mm–32mm or 5.5mm–22mm. What a mess! How can you compare lenses and zoom ranges under those conditions?

Camera vendors have solved the problem by quoting digital camera lens focal lengths according to their 35mm equivalents. If you're already familiar with 35mm camera lenses, that's great. If you're not, at least you have a standard measurement to compare with. That's why you often see digital camera zoom ranges expressed as "35mm–135mm equivalent (roughly 4:1)" or some similar expression.

Because digital camera lenses have such short focal lengths in the first place, most models tend to be deficient in the wide-angle department. Expect to see most digital cameras with no better than a 35mm–28mm wide-angle equivalent. That's barely acceptable because 35mm isn't very wide. If you really need a wide field of view, consider a wide-angle attachment that fits on the front of your lens.

What focal length (equivalents) do you look for? Your preferred range will depend on what kind of photos you want to take. If you shoot architecture or indoor photos, you'll want the shortest focal length possible. An alarming number of digital cameras seem to have settled on a 38–39mm (equivalent) focal length as their widest setting. There are several reasons for that. First, a lens with that meager wide-angle field of view is easier to design (and usually more compact) than one with a broader perspective.

In addition, a lens with a particular magnification has a longer telephoto effect if you're starting from a base focal length that's narrower to begin with. Consumers tend to get more excited about long telephotos than they do about wider wide-angles, even though both perspectives have an important place. Table 1 shows how a 4:1 zoom lens gains a more impressive telephoto "look" when the vendor snips millimeters from the wide-angle portion of the range.

Table 1: 4X Zoom Lenses (35mm Equivalents)