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Wednesday, May 30, 2007

All About Image Sensors

At the heart of every digital camera is an electronic marvel

All About Image Sensors The cost of a sensor also increases with its surface area, negating any inherent price advantage for CMOS. Because full-frame circuitry is too large to be imaged on the chip at once, the technical challenges and price increase. And because fewer full-frame sensors can be made from a standard 8-inch raw silicon wafer (about 20, compared to more than 1000 LSI microchips), the chances are greater that a single imperfection will render the whole sensor useless.

Medium-format digital backs utilize even larger sensors. These huge CCDs deliver tremendous resolution—at a price. Kodak's 39-megapixel KAF-39000 sensor is the highest-resolution camera sensor available today. It's found in the Phase One P45 digital back and H2-compatible backs from Hasselblad—each with prices around $30,000. Medium-format backs are expensive and highly complex devices, and because they involve some different fundamental technology, we'll fully address them in a separate article in the March/April 2007 issue of Digital Photo Pro.

Bigger isn't always better when it comes to sensor size. Full-frame sensors will always be more expensive than smaller ones, but many manufacturers use smaller sensors for more than financial reasons. Large sensors are more susceptible to vignetting. Because individual photosites have depth, light must strike them very close to perpendicular to register a charge. At the edges of a large sensor, the light is traveling at a greater angle and is less likely to deliver its full charge to the photosite. Microlenses are employed for this too, redirecting light closer to perpendicular.

Camera manufacturers like Olympus found that smaller sensors could deliver high image quality in a smaller package, which translated into a smaller and lighter camera. According to Digital SLR Product Manager Rich Pelkowski, Olympus was free to build their digital cameras around the sensor.

“Why stay with a larger sensor and a larger lens mount size if you really don't have to?” he says. “We didn't have to because we didn't have a large, very popular system of autofocus film SLR cameras that we had to bridge over into the digital age.”

The smaller size of the Four-Thirds sensor in the Olympus E-series cameras (which also has been adopted by Fuji, Leica and Panasonic), as well as that of the APS-C and APS-H sensors in many digital SLRs, allows designers to use smaller mirror boxes and smaller pentaprisms, ultimately translating into smaller cameras. Ask a traveling pro about the importance of reduced size, weight and expense, and the advantages of smaller sensors are clear.

Pros wouldn't ever consider small sensors if the image quality wasn't up to par. The Nikon D2x, for example, uses an APS-sized sensor to deliver 12.4 megapixels. That's similar resolution to the full-frame 12.8-megapixel Canon EOS 5D. There's a risk, however, that as photosites get smaller and more of them are packed into smaller sensors, the chances of electronic interference and noise increase as dynamic range decreases.


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