You can't always get right up to what you photograph — be it a band onstage, a kid on a soccer field or animals in nature. That's why cameras have optical-and digital-zoom capabilities — but these are two very different technologies, so it’s important to know how they work, and when you should use them.
Optical zoom uses lenses to magnify a far-off image. It's possible to find cameras priced under $150 with lenses that zoom by a factor of 10X; and zoom ranges of 30X and even 60X are becoming common.
However, if you read the product description for these cameras, you'll often see a second, even bigger, zoom number. This is digital zoom: The camera's processor crops into the center of the photo.
Optical zoom is ultimately better, as it magnifies an image to fill the entire image sensor — say, 10 megapixels worth. Digital zoom takes just the center portion of what the lens threw on the sensor, capturing fewer pixels, say 6MP. The camera's processor scales up the smaller image, stretching out those 6 megapixels and "interpolating" (making up stuff to put between them) in order to get the photo resolution back to 10MP. Made-up pixels won't show the same, accurate detail as the real thing.
"There was a day when [digital zoom] really sucked," says Steve Heiner, Nikon's senior technical manager. "There wasn't a lot of thought put into it beyond simply cropping the image."
But with today's sky-high pixel counts, powerful image processors and much smarter algorithms, augmenting your optical zoom with digital zoom up to a 2X factor produces results almost as good as pure optical zoom. That’s especially good for cellphones, which almost never have zoom lenses.
Digital Zoom: Not What It Used to Be
We were initially skeptical to hear how good digital zoom has gotten. But after testing one Sony and two Nikon compact cameras, we were convinced. The digital zoom of an iPhone 7 was not as impressive, but still usable for photos and very good for video close-ups. (We shot all photos as JPEGs, the most-common format used.)
We began with two 20MP Nikons. The $400 Coolpix A900 has a 35x zoom lens, which extends to a focal length equivalent to 840mm on a full-size DSLR. The $450 Coolpix B700 has a 60X optical zoom extending to the equivalent of 1440mm. That allowed us to compare photos and videos of the same subjects using only optical zoom for the B700 and optical plus digital zoom for the A900. For subjects within a few hundred feet, such as the intricate wiring atop a telephone pole, there was virtually no difference between optical and digital zoom, even when I enlarged the photos to 100 percent on the 2560 x 1600 Retina display LCD of a MacBook Pro.
To really push it, I stood atop Corona Heights hill in the center of San Francisco and photographed the tower of Mission Dolores basilica, about 3,500 feet away. The B700's photo, taken at a 1440mm focal length, was sharp enough to show frilly baroque ornamentation and individual color tiles that cover the tower's bell-shaped dome. Boosting the A900’s shorter optical zoom with 1.8 digital magnification, resulted in slightly blurred details, but not bad for such an extreme case.
For most video situations, adding digital zoom was fine. I shot 1080p HD movies of leaves rustling in a tree about 400 feet away; aside from variations in how each camera captured color and sound, there is essentially no difference in the quality of combined optically and digitally zoomed video from the A900 and optically zoomed video from the B700.
That’s not surprising. Both cameras have 20MP sensors, but 1080p video is only about 2MP (4K/UHD video is about 8MP). So even using just the center portion of the sensor, the camera still has far more pixels than it needs — and thus eliminates most of them — to produce video. Cameras also apply less sharpening to video frames, and motion causes some blurring, says Heiner, so any quality loss from digital zoom would be hard to make out.
The Cropping Alternative
There's another way to "digitally" zoom into a photo: Shoot with only optical zoom and then crop into the center of the photo yourself. Any photo-editing software can do this, as can smartphone camera apps and often software installed in compact cameras. The difference here is subtle, but important. Both processes start by cropping into the center of the photo, but in-camera digital zoom takes the extra step of adding made-up pixels to retain the same number of megapixels as a full-sensor image.
That may have been important a dozen years ago, when camera sensors had only 5- or 6-megapixel resolution. But with 12-20 MP (and higher) sensors today, just a small center portion of a photo may still be plenty. Consider this: A giant 4K TV has a resolution of only about 8MP. Photos on Snapchat, Facebook or Instagram are smaller. Even cropping aggressively into a photo, you'll probably still have more pixels than you need, without requiring the camera's image processor to make up additional fillers. Cropping is certainly the better way to go when shooting highly detailed RAW images.
But for JPEGs, once again, the compact cameras surprised us. We compared photos shot with digital zoom with ones from the same camera shot with only optical zoom and then cropped to match.
When we took pictures of a telephone pole about 50 feet away, there was barely any difference with photos taken using the Nikon A900 at the end of its optical zoom (840mm focal length) that was cropped to match with a shot taken using its optical plus digital zoom for a 1512mm focal length (1.8X digital magnification).
The same pattern was true with the 20PM Sony Cyber-shot RX100 III. We shot a stilllife from about 4 feet using the Sony’s max optical zoom (70mm focal length), and cropping it to match a photo taken using the optical plus digital zoom of 105mm (1.5X digital magnification). Sony is cagey about explaining how its technology, called ClearImage Zoom, works. But it appears to draw on an internal database of common objects in order to intelligently put back details lost in the digital-zoom process. And it really works.
Photo results were less impressive for the iPhone 7. Using the same method of shooting close-up without digital zoom (28mm focal length) and farther away with it (48mm), we saw more blurring of detail in digitally zoomed photos. There may be several reasons for this.
The iPhone 7 has fewer megapixels —12 versus 20 or more for larger cameras. And perhaps its zoom algorithms aren't as sophisticated, at least using the built-in Apple Photos app we tested. Manually cropping is a better strategy here, and it's a simple matter of dragging a box around a portion of the photo with your finger. Cropping a video would be much harder, but fortunately, digital zoom for video works as well with the iPhone as with the Nikon cameras.
The other option would be to buy a phone like the iPhone 7 Plus. It has two cameras, one with a wide 28mm-equivalent lens and one with a zoomier 56mm lens. (See how the iPhone compared against the Samsung Galaxy S7). A few other smartphones, such as the Asus ZenFone 3 Zoom, also have dual cameras.
Purists vs. Realists
In absolute terms, optical zoom will almost certainly be better than digital zoom. In a perfect world, you would use only top-notch optical-zoom lenses — preferably shooting RAW on a professional DSLR or cinema-grade video camera. In the real world, when your camera's zoom lens (if it even has one) just can't stretch far enough — and you don't want to spend extra time editing photos — digital zoom is a now a handy fix that works in certain situations.
Photo credit: Sean Captain/Tom's Guide. Featured image: Shutterstock