Chuck Gardner's Photography Class
Part Three - Digital Photography Basics
by Chuck Gardner
Digital Image capture

The human eye has color receptor "cone" cells which are sensitive to red, green, and blue wavelengths of light. The brain combines the signals from these three receptors to form the perception of all colors. For example, the perception of a bright yellow is actually caused by equal stimulation of both the red and green cone cells.

Digital image capture devices such as cameras and scanners follow the model of human vision and record the original scene through red, green, and blue filters, recording it as three separate red, green, and blue channels of information. Most digital still cameras divide the brightness of each channel into 256 discrete steps numbered from 0 (darkest) to 255 (lightest). This 256 step brightness level information can be stored in 8 binary "bits" (e.g. 10101111 ) which a computer can easily process. In most cameras the recording device is a Charged Coupled Device (CCD) about the size of your thumbnail which is divided, chessboard fashion, into cells overlaid with either a red, green, or blue filter gel. Each cell records 8 bits of data representing on a scale of 0 to 255, how much light was detected. The combined signals from three adjacent red (R), green(G), and blue(B) cells are combined into a single 24 bit binary data string (e.g., 111111110000000011111111). This 24 bit / 3 byte data point is called a pixel, and is the basic unit of digital image storage and manipulation.

Image resolution: Pixels = Detail

Because the image is broken into a chessboard pattern of pixels, the amount of detail which is recorded is limited by the number of pixels which can be packed onto the tiny CCD recording chip. Entry level digital cameras typically have a CCD array of 1600 x 1000 pixels, or 1.6 megapixel resolution. The high-end consumer models now have 2500 x 1500 pixels, or 3.45 megapixels. 5 megapixel cameras will hit the market 2001. More pixels is better, and each new generation of cameras raises resolution of the CCD array.

Many digital cameras offer the option of several image resolutions. For example my DC290 offers: High (1792x1200), Medium (1440x960), Standard (720x480), and Ultra (2240x1500). The CCD image sensor in this camera is actually 1792x1200. For the smaller sizes software in the camera scans or "resamples" the recorded image data and then stores it in fewer pixels. For the Ultra (2240x1500) format it resamples the recorded data adding new "interpolated" pixels containing values computed from the adjacent pixels recorded when the picture was taken.

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