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Wednesday, May 30, 2007

All About Image Sensors

At the heart of every digital camera is an electronic marvel

Understanding CMOS

Unlike CCDs, CMOS sensors convert electronic charge into voltage at the pixel itself. Electrodes are therefore freed up for use in other purposes, such as transferring charges faster and dealing with noise. Multi-channel transfer is fast, and when multiplied by the millions of pixels on a given sensor, it translates into measurable impact for photographers.

Although they use CCDs in point-and-shoots, Canon pioneered the use of CMOS in digital SLRs. The first SLR with a CMOS sensor was the EOS D30 in 2000, and today it's found throughout the ranks of professional SLRs. This couldn't happen if photographers were unhappy with the image quality delivered by CMOS, and according to Canon's Westfall, it's because technology utilizes the on-chip abilities of CMOS to record the noise of each pixel prior to exposure and subtract that noise from the final image.

With every advantage also comes a trade-off. Additional transistors make CMOS fast and flexible, but they also mean more likelihood of heat and failure. It's a camera's off-chip processing abilities that then step in to improve the signal/noise ratio.

A major impact of additional on-chip electronics is decreased fill factor. Fill factor represents the amount of area on any sensor that's actually covered by light-sensitive photosites. While it's easy to think of pixels as directly abutting one another, there are spaces between them. On a CMOS chip, these spaces are used for other circuitry and additional functionality, so CCDs have greater natural fill factor. While the spacing means better heat dissipation, it also increases chances for moiré and artifacts because more of the light will be wasted if it hits the sensor in the dead space between pixels. Microlenses are then used to redirect light from the dark areas of a sensor into the light-sensitive photodiodes.