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Tuesday, January 30, 2007

Understanding Image Sensors

The sensor is the soul of your digital camera and knowing how it works will help you to compose better images

Labels: Learning Center

Sensitivity. If you examine each photosite of a CMOS, you'll see that the power-saving and speed-enhancing circuitry of converters and amplifiers takes up space. That extra space can't be used for capturing light. On the other hand, a CCD sensor might capture 100 percent of the light hitting the photosite.

Noise. CCDs essentially have a single amplifier through which all of the data travels. That and the fact that extra processing is done off the image sensor means the noise on a CCD is largely kept at bay.

A CMOS chip may contain millions of tiny converters and amplifiers in each photosite. In a perfect world, all of those converters and amplifiers would work exactly alike, no matter the operating environment. But this isn't a perfect world. As all of the amplifiers begin working at different efficiencies, they induce fixed-pattern noise into the data that they're amplifying. (What's fixed-pattern noise? Imagine taking a picture with the lens cap on and having some of the pixels register black and some register a little lighter than black. That's an approximation of fixed-pattern noise.) Also, having that additional circuitry all on the same chip can add noise.

Blooming. Blooming occurs when the electron charge leaks from one photosite to the other. It can be seen in the image as streaks anchored at small high-brightness areas. CCDs are prone to blooming when individual photosites are oversaturated.

Since CMOS sensors convert the charge to a voltage right at the photosite, it's more difficult for the charge to leak to adjacent photosites. CMOS chip builders also can add pixel-reset circuitry right at the photosite to minimize overloading (remember all that extra circuitry they can add directly to the photosite?). So by its inherent design, CMOS sensors generally are immune to blooming.




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