Hardware Reference
In-Depth Information
RR
B
RR
BB
G
G
One pixel is made
up for four CCDs,
one red, one blue,
and two green
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G
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G
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Lens
Diaphragm
CPU
RAM
Flash memory
CCD array
Digital camera
Figure 2-43.
A digital camera.
the other two. Two greens are used because using four CCDs to represent one
pixel is much more convenient than using three, and the eye is more sensitive to
green light than to red or blue light. When a digital camera manufacturer claims a
camera has, say, 6 million pixels, it is lying. The camera has 6 million CCDs,
which together form 1.5 million pixels. The image will be read out as an array of
2828
2121 pixels (on low-end cameras) or 3000 times 2000 pixels (on digital
SLRs), but the extra pixels are produced by interpolation by software inside the
camera.
When the camera's shutter button is depressed, software in the camera per-
forms three tasks: setting the focus, determining the exposure, and performing the
white balance. The autofocus works by analyzing the high-frequency information
in the image and then moving the lens until it is maximized, to give the most detail.
The exposure is determined by measuring the light falling on the CCDs and then
adjusting the lens diaphragm and exposure time to have the light intensity fall in
the middle of the CCDs' range. Setting the white balance has to do with measur-
ing the spectrum of the incident light to perform necessary color corrections in the
postprocessing.
Then the image is read off the CCDs and stored as a pixel array in the camera's
internal RAM. High-end digital SLRs used by photojournalists can shoot eight
high-resolution frames per second for 5 seconds, and they need around 1 GB of in-
ternal RAM to store the images before processing and storing them permanently.
Low-end cameras have less RAM, but still quite a bit.
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