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Slimming Mirrors: The reflective rings on this crystal increase the optic’s focal length to enable high-quality zoom.

Cellphone designers strive for sleekness, a quality that makes it nearly impossible to include a quality zoom lens on your phone. The thin, wide-angle lenses found in today’s phones work fine for panoramic shots, but forget about crisp close-ups. To zoom in, cellphone cams simply stretch pixels, which kills image quality.

Now researchers at the University of California at San Diego have borrowed a mirror trick from reflective-telescope makers to cram sharp telephoto capability into a package just a few millimeters thick. The technique uses mirrors to bounce light back and forth, lengthening the path light travels (which increases the potential for magnification) without bulking up the length of the optic.

To make the lens, engineering professor Joseph Ford and graduate student Eric Tremblay carved an array of concentric reflective rings into a single optical crystal, creating a miniature hall of mirrors. When light enters the camera’s aperture, it bounces from ring to ring and eventually lands on a central sensor that interprets the information and produces close-up telephoto images on your screen. The new optic is seven times as thin as a traditional, 35-millimeter refractive lens, with nearly equivalent image quality.

The researchers have applied for a patent on the technology and are working on a version of the optic that’s one fifth the current size-which could be good news for your cameraphone in a couple years.