One of the top selling points of the new iPhone 7 Plus is its telephoto lens, which lets you get twice as close to the action as the typical smartphone.

Other smartphones use a digital zoom to approximate the same effect, but that degrades the image quality.

The iPhone isn't the first to offer a telephoto lens. The Asus Zenfone Zoom has a 3x zoom, and the bulky Samsung Galaxy S4 Zoom was introduced in 2013 with a 10x zoom.

But that feature is still a rarity. Fortunately, consumers have a good alternative: a miniature camera lens that attaches to their phone.

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We tried out a number of different accessory lens kits earlier this year, and now we've tested their quality more rigorously in our labs.

The telephoto smartphone lenses we tested range in price from $20 to $155. We found that they're easy to use and work just fine. Some are compatible only with iPhones or certain other models. Be sure to check before you make your purchase.

How We Tested and What We Found

We bought four different telephoto smartphone lenses that would fit on an Apple iPhone 6s, which we had chosen as our test phone.

In the lab, we looked at the images the lenses produced for sharpness, distortion, color accuracy, and light falloff—higher-quality lenses tend to maintain more-even lighting from the center of an image to the edges.

Most of the lenses improved sharpness in the middle of the images, but the photos were noticeably blurry in the corners. All the lenses showed significant distortion, particularly in the corners of the images.

They also produced chromatic aberration, a defect in which a lens fails to focus different wavelengths of light on the same place, resulting in a purple or magenta “ghost” image. (The effect is also known as purple fringing, since it often appears as a purple outline around the subjects. It can generally be corrected with image-editing software.)

But that doesn't mean the telephoto smartphone lenses aren't worth buying. On balance, they outperformed the phone's built-in digital zoom and allowed us to take closeup photos that we liked.

We found all the lenses relatively easy to use, though they do require you to take your smartphone out of its protective case. Some of the lenses come with their own smartphone cases.

Schneider iPro Case and 2x Telephoto Lens, $155

Overall, this well-constructed telephoto lens (2x) and case had the best image quality in our tests. However, it was also the priciest.

To use this lens, you’ll first need to insert your iPhone into the hardcover case and then screw the telephoto lens onto the case. We like the protection this offers, as well as the case's tripod mount, which is great for shooting in low light or using long exposures.

Schneider also sells additional lenses, which we did not test, including a fisheye lens ($80) and a macro lens ($40). They are all compatible with the same case.

The Schneider iPro system has compatible cases for most iPhone models, in addition to the iPhone 6s, and one Android model, the Samsung Galaxy S4.

Schneider does not yet have a compatible case for the new iPhone 7 and 7 Plus.

Olloclip Active Lens Kit (With 2x Telephoto Lens), $100

This kit comes with two lenses that are permanently attached in a compact unit: a 2x telephoto lens and a wide-angle that we didn't evaluate. The telephoto lens provides very good sharpness in the center of the image and just slight falloff in the corners. Still, it didn't perform quite as well as the Schneider iPro.

Once you slide the lens mount over the corner of your phone, it’s fairly secure. And here's a nice detail: Line up the telephoto lens on the rear-facing camera, and the wide-angle lens lines up with the front-facing, or selfie, lens (and vice versa).

What this means, of course, is that the Olloclip is built specifically for the iPhone 6s—it won't work with any other phone. And even with this smartphone, the fit wasn't perfect: The bottom edge of the Olloclip bracket covers the edge of the iPhone display, which could interfere with some menu settings on a photo app.

Various Olloclip kits are compatible with most iPhone models, including the 5, 5s, SE, 6, 6s, 6 Plus, 6s Plus, and the new iPhone 7 and 7 Plus. Certain kits fit some iPad models, too. You can also buy some kits for the Samsung Galaxy S4 and S5.

Carson HookUpz 7x Telephoto Lens, $20

This model comes with a soft rubber case to protect the lens when you're not using it, a hard plastic adapter that slips over the top of your phone, and the lens itself.

This lens provides more than double the optical zoom (7x) found on the other models we evaluated, but in our tests we found that it had the lowest image quality. It also has a quirky problem with the image itself: When you use the telephoto lens, the image comes out in a circular format with a black rectangular frame around it.

Carson has several versions, sized to fit different iPhones. But aside from one version that will fit the Samsung Galaxy S4, there are no options for other phone brands.

Photojojo 2x Telephoto Lens, $20

To use the Photojojo lens, you attach a ring with an adhesive backing to your phone, and the lenses attach to the ring magnetically. That's good because it fits on a wide variety of cameras and it's easy to snap the lens on and off.

However, the connection isn't as secure as the screw-on lenses. And we think that with repeated, long-term use the adhesive might wear out.

As another drawback, the zoom images weren't too much sharper than what you'd get from the 2x digital zoom built into the iPhone 6s. Which sort of defeats the purpose.

On the positive side, there's the price: At $20, it tied with the HookUpz lens as the cheapest option. And if you're just looking to try out the technology, that makes the Photojojo an appealing option.

Further, once the magnetic ring is attached to your phone, you can experiment with other Photojojo lenses, including a super fisheye and a polarizing lens ($20 each).

You can buy Photojojo lenses that are compatible with several iPhones (5 and 5s; 6 and 6s; or 6 Plus and 6s Plus), or you can buy one that the manufacturer states is "Android/Universal," which should fit most any smartphone.

However, the company doesn't yet have compatible models for either the iPhone 7 or 7 Plus.

Bottom Line

Let's be realistic: An add-on lens can't turn your smartphone into an SLR. No matter how good the product might be, piling one lens on top of another lens is not the best way to enhance image quality. Especially when the two lenses aren't aligned precisely—and even the full-body Schneider case isn't the equivalent of an SLR lens mount.

But that's not the point. Our tests found that the best lenses, especially the Schneider telephoto iPro lens, can really expand the kinds of photography you can do with an iPhone. They're fun, they're fairly affordable, and the best of them do decidedly better than a phone's native digital zoom, which inevitably degrades image quality. If you understand what you're getting, these lenses can be a good investment.

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