Digital Versus Optical Zoom

With so many new cameras available at great prices for the holidays, you'll undoubtedly be making comparisons on all sorts of features. One question I recently got was about the difference between an optical and a digital zoom.

First, the camera basics: A lens focuses the light from the scene onto the sensor. The sensor used to be a piece of film, but in digital cameras it's a light-sensitive circuit. People are often comparing cameras by their sensors (it's the sensors that are measured by their megapixels), but the lens is an equal partner in producing an image, and lenses haven't received much critical attention.

With a zoom lens, you can change the field of view without having to walk closer to or farther from the scene. When a zoom lens is in its "wide" setting, you get the largest area of the scene, and as you zoom "in" you continually look at smaller and smaller areas of the scene, revealing more detail.

Most of today's consumer digital cameras have an optical lens zoom. The range of the lens is the ratio of the length of the lens when zoomed in all the way versus its length on the widest setting. Zoom lenses with 3X or 5X range are very common. The greater the range of the lens, the more options you have each time you take a picture—zoom out to show the whole family group, or zoom in for a head shot of the wailing baby. Zoom out for a mountain scene, or zoom in on the wildlife.

Because today's cameras work with digital images (they actually contain a small computer), it's possible to use digital image processing to zoom in a bit more once the optical zoom has reached its limit. Typical digital zooms give you an additional 2X of range. The problem is that the image processing has to work hard to create more information than was originally present in the sensor. Digital zooms often make images look pixilated since they are putting in "the zoom" with digital effects and not a real lens.

The zoom you really want to pay money for is the optical zoom. And don't be misled by thinking the two are cumulative. A 3X optical zoom with a 2X digital zoom does not mean that you're getting a 6X zoom. My advice: When comparing cameras, only pay attention to the optical zoom—it's the one that you'll be using 99.9 percent of the time.

If the optical zoom didn't go quite far enough, you can do a digital zoom afterwards, using a photo retouching program. Doing it afterwards gives you more control and more time. You can see the tradeoffs of detail versus quality on your computer's large screen, whereas you can't tell what you are losing in the tiny screen of the camera.