Wednesday, May 30, 2007
All About Image Sensors
At the heart of every digital camera is an electronic marvel
Power, Noise, Price And Size
CCDs typically require more power to operate than CMOS sensors—as much as 100 times more. This can lead to more noise and less battery life. A CMOS sensor doesn't draw significant power until the transistors are switching on and off, which helps keep heat, and therefore noise, to a minimum.
Westfall says that the primary noise difference between CCD and CMOS is that designers can utilize the flexibility of CMOS to virtually eliminate it. Supporting technology can make them appear less noisy, so CMOS today tends to deliver better signal/noise ratios than CCDs in demanding high-ISO and long-exposure situations.
Convincing professional photographers to abandon film required digital sensors to be more than just as good. They had to offer other advantages. CMOS sensors can be made on the same assembly lines that make chips for things like computer RAM, and on-chip functionality means fewer external components. These economies of scale gave CMOS an immediate price advantage over CCD, whose components must also be higher grade to withstand the greater voltage demands of a power-hungry chip.
Cheaper is well and good, but the real advantage it offers for professionals is that cheaper to manufacture means easier to make bigger. One way to increase resolution is to make a bigger chip. Pixel count directly correlates to the size of the image file, so manufacturers began building chips the size of a 35mm film frame with higher total pixel counts. (The number of physically present photosites on a chip is the total pixel count; active pixels are the ones directly involved in image capture. Approximately five percent of pixels on a given chip are inactive due to defects or because they're used for other purposes.)
A larger sensor often means a higher dynamic range because photosites themselves can be bigger to collect more photons. Smaller sensors create a focal-length magnification factor when used with 35mm-film-format lenses, effectively cropping into the image and altering the anticipated depth of field.
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