More than just megapixels—what you need to know about your digital camera’s core component
CCD vs. CMOS
Two types of image sensors are used in D-SLRs: CCD and CMOS. Canon has been making its own CMOS sensors for years, while most other D-SLR manufacturers have gone with CCDs. However, today, most manufacturers are using CMOS for their higher-end cameras: Nikon, Pentax, Samsung and Sony have recently introduced top-of-the-line CMOS models, while Olympus, Panasonic and Leica's Digilux 3 employ related Live MOS sensors, and the Foveon X3 sensor is also a CMOS.
CCD stands for "charge-coupled device," a term that refers to the method used to move the charges. During exposure, the photodiodes collect photons and convert them into electrons. Once the exposure has been made, the sensor moves the charges from the first row to a shift register, then to an amplifier, then on to the A/D converter. Then the sensor moves the charges from the second row and so on until the last row has been transferred. This method of transfer is accurate, but relatively slow. It also requires a lot of power, thus reducing camera battery life.
CMOS stands for "complimentary metal oxide semiconductor," a term that refers to the manufacturing process, not the sensor design. Like CCD sensors, CMOS sensors contain millions of photodiodes that collect photons and convert them to electrons. However, on CMOS sensors, each photodiode has its own amplifier and converts the charge to voltage right on the pixel. Transferring voltage requires much less power than transferring a charge, and multiple channels of sensor data can be read out simultaneously at high speed. Thus, CMOS is faster than CCD and uses less power (for longer battery life).
The state of the art is such that today, top-quality images can be produced with both CCD and CMOS sensors, although the trend now is toward CMOS for higher-end D-SLRs.
Canon EOS-1Ds Mark III Sensor Assembly: Because sensors are quite sensitive to infrared as well as visible wavelengths, the sensor assembly includes an IR-cutoff filter. Sensors using a Bayer array also include a low-pass filter to reduce aliasing (color artifacts and moiré patterns).
• Lower inherent noise
• Better inherent image quality
• Less power consumption
• One-chip Live View
• Lower cost
The Sensor Assembly
The image sensor in a D-SLR isn't just a sensor chip. For example, CCD and CMOS sensors are quite sensitive to infrared, so an infrared-blocking filter must be added. And sensors using a Bayer array require a low-pass filter to reduce aliasing (color artifacts and moiré patterns).