Cell-Phone Cameras That Zoom

A new design could put the power of a telephoto lens into thin cameras.

While each generation of mobile-phone camera captures more megapixels, the images still can't match the quality of those taken with stand-alone cameras. The major reason: the lens. In a mobile-phone camera, the embedded lenses are frozen in place, without the ability to physically zoom in on a subject.

This novel lens design has the zooming capabilities of a telephoto lens 40 millimeters long, even though it’s only 5 millimeters thick. The trick is to collect light from the outer edge (the dark ring); reflect it within the lens eight times, using mirrors on the front and in the back; and focus it onto a camera sensor. Credit: UC San Diego

But now researchers at the University of California, San Diego (UCSD), working with Illinois-based optics company Distant Focus, have developed a new type of lens that could let mobile-phone cameras take close-up shots. Joseph Ford, professor of electrical and computer engineering at UCSD, and his group have developed a five-millimeter-thick lens that has the power of an optical system that is usually 40 millimeters long. The group's novel design collects light and reflects it within the lens to obtain the full 40-millimeter optical path, and then it focuses the light onto the camera's sensor. Ford says the lenses could be used in, in addition to mobile-phone cameras, any situation in which a small and lightweight but powerful camera, from a telescope to a military imaging system, is needed.The research is funded by the U.S. Defense Advanced Research Projects Agency as part of the "MONTAGE" imager program. 

The research is based on technology called a folded optical system, which can be found in some telescopes today. In these telescopes, a series of separate lenses and mirrors are used to increase the distance that light travels before it reaches the imaging sensor, a distance known as the focal length. Light is collected using a lens at one end, reflected between mirrors, and then focused onto a sensor. The longer the focal length of a system, the larger the final image will appear. Ford's group compressed this idea into a novel thin lens and designed it in such a way that light reflects inside the lens eight times before hitting the sensor.

To do this, the researchers made extreme modifications to a traditional lens. First, they used diamond machining to carve mirror surfaces out of a lens material called calcium fluoride. The mirrors steer the light, altering its path so that all the light converges onto the camera's sensor. Second, they coated both the front and the back of the calcium fluoride with mirrors so that the light reflects inside the lens.The key to making this lens work is precise allignment between the mirrors, which is accomplished by machining them all from a single piece.

The mirror on the front of the lens blocks roughly 90 percent of the light from entering, says Ford, which can reduce the contrast in an image. However, even with so much light blocked, he says, the group's camera was able to perform almost as well as a conventional lens almost ten times as long, producing images that are only slightly less crisp than those created using a traditional camera, in which 100 percent of the light from an image passes through the lens.