Sensor Design

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You can relate the film-frame size of a negative to the size of a sensor in a digital camera; however, there is a bigger difference in the variety of sensor sizes today than there was in filmframe sizes in the film days. Although most cameras from the film days took 35mm film, today only the more expensive dSLR cameras have sensors of that size. Sensors that are the same size as 35mm film (24x36mm) are called full-frame sensors. Sensors come in a variety of sizes, but all are rectangular (see Figure 7.1).

Medium Format Sensor

Figure 7.1

Examples of the sensor sizes available today

Figure 7.1

Examples of the sensor sizes available today

You must consider both sensor size and resolution to determine how good a sensor is. Resolution is measured in pixels. Today, cameras have so many pixels that the resolution is measured in megapixels. The more pixels you have, the higher the resolution of the image, which means that there will be a smoother gradation of color and increased sharpness when you are looking at the picture from a distance. A low number of pixels means each pixel has to make a decision as to what color it is, creating noise in the process.

The number of megapixels a camera has isn't the only consideration in its ability to take a sharp, colorful image. You also have to consider the size of the sensor. If a sensor is tiny, as it is on most point-and-shoot cameras, the image will deteriorate more as it is blown up larger because you're starting out small. All you have to do to observe the difference is look at an image from a point-and-shoot with a small sensor (no matter how many megapixels it has) and compare it to one taken with a dSLR camera with a larger sensor when the image is at 100 percent resolution. The former will break up the details, adding noise to them.

Sensors convert light into electrons. There are two kinds of sensors—CCD (charge-coupled device) and CMOS (complementary metal-oxide semiconductor). In both types of sensors, photodiodes convert light into electronic charges.

In a CCD sensor, the light is converted into electronic charge values, and the values are transferred non-digitally; then digital calculations are made in another location located off the sensor. In a CMOS sensor, each value is calculated digitally near the spot where the electronic charge from the light has been received.

CCD sensors are more expensive to produce than CMOS sensors because in a CCD sensor, the charges for each pixel are moved and then read collectively; whereas in a CMOS sensor, each pixel charge is read from where the charge is made. CCD sensors have more room on the sensor for light detection because there is no apparatus for making digital calculations on the sensor itself. According to Ken Rockwell, "Technically, CCD sensors are superior to CMOS."1

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