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Advanced Imaging Magazine

Updated: January 12th, 2011 09:49 AM CDT

Camera Phones Zoom in on Fixed Lenses

A software-enhanced lens provides 3x optical zoom with no moving parts
Figure 1A. While optical zoom (Figure 1B) is bulky, fragile and doesn’t meet the ruggedness requirements of a camera phone, digital zoom (Figure 1A) is radically inferior to optical zoom.
Figure 1B. While optical zoom (Figure 1B) is bulky, fragile and doesn’t meet the ruggedness requirements of a camera phone, digital zoom (Figure 1A) is radically inferior to optical zoom.
Figure 2. Tessera’s OptiML combines its lens design with specialized algorithms to enable true 3X optical zoom capabilities without moving parts.
Figure 3A. Images of a city: The raw image (3A) vs. the one taken with a fixed lens zoom. (3B)
Figure 3B. Images of a city: The raw image (3A) vs. the one taken with a fixed lens zoom. (3B)
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By Eran Kali and Giles Humpston

Despite consumer demand, few camera phones provide optical zoom capabilities, instead relying on the more technologically inferior digital zoom. Traditional optical zoom involves either changing the position of lenses on the optical axis of a camera or altering the shape of fixed-position lenses. While both approaches are perfectly acceptable, they tend to be quite large, power hungry, and not particularly robust. Therefore, they are both incompatible with cell phone cameras, where the current trend is toward a thinner footprint, more durable cases, and a longer battery life. A potential solution is available in the form of a specially-designed lens combined with a simple algorithm.

TRADITIONAL ZOOM

Before getting into solutions, however, it’s important to understand camera zoom, which permits the photographer to get closer to the subject in a much more convenient manner than moving the camera. It’s not surprising that consumers rate zoom high on the list of “must-have” features when purchasing a digital still camera or camera phone. Camera manufacturers have responded to this demand. Most digital still cameras come standard with optical zoom, while camera phones have digital zoom.

While optical zoom is bulky, fragile and doesn’t meet the ruggedness requirements of a camera phone, digital zoom is radically inferior to optical zoom. A 3x optical zoom magnifies an area of the scene, increasing the optical resolution in the zoomed frame by three. With optical zoom, information quantity is preserved as the magnification varies. In contrast, digital zoom includes the process of cropping an image and expanding the remaining portion to fill the original pixel count so the image appears magnified. With digital zoom, the optical resolution is fixed at image capture and the information quantity in the zoomed image degrades with the square of the magnification. As a result, a digitally-zoomed photograph quickly becomes visually unacceptable. For a 3x digital zoom, 91 percent of the information quantity in the captured image is decimated. Nevertheless, digital zoom has the singular advantage of being based on software and is, therefore, physically compact, rugged, virtually instantaneous, consumes negligible power and can be implemented at relatively low cost.

SOFTWARE-ENHANCED LENSES

The sole purpose of conventional camera optics is to replicate a scene onto the imager as closely as possible. Extreme effort is invested in achieving the best possible chromatic and illumination uniformity, frequency-contrast response, and other optical metrics so that the scene presented to the image sensor pixels has the highest possible optical fidelity.

The relatively new software-enhanced lens combines a special lens design with an algorithm that processes the images produced by this specialty lens. The lens manipulates the optical rays to provide an intensity distribution on the camera sensor with desired features. These can include predefined compensation distortion or robust point-spread function behavior for an extended depth of field. In most cases, the manipulated image is not used as is; it needs further software correction. However, because the image was manipulated in a known manner, it can be digitally restored so high-quality output can be extracted. Software-enhanced lenses provide access to some very desirable lens functions, of which optical zoom from fixed lenses is one example.

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