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Digital
cameras are quite similar to traditional cameras in their operation.
They both have a lens to focus the image, a shutter to allow light
inside the camera, and an aperture to control the amount of light
which enters the camera.
The
differences between digital and traditional photography occur after
the light enters the camera. A traditional camera captures the images
on film, while a digital camera captures the image on an
image sensor.
Image
sensors are electronic devices made up of an array of electrodes
(or photosites) which measure light intensity. The most common type
of image sensor for digital cameras is the CCD
(Charge-Coupled Device) although others such as CMOS
and Foveon are sometimes used.
The
number of photosites in the image sensor gives
the digital camera its megapixel (millions of pixels) rating. Each
photosite corresponds to a pixel in the final image, so a camera
which is rated at six megapixels, for example, has an image sensor
which is 3008 pixels wide by 2000 pixels high.
When
light hits the image sensor it is converted into electrical signals
which are amplified and fed to an analog-to-digital (A/D)
converter. The A/D converter changes the electrical signal
into binary numbers which are processed by a computer housed in
the camera body. Once the numbers have been processed the resulting
image is stored on a memory card.
Photosites
can only measure intensity of light -- not colour. In order to produce
a colour image, each photosite must be covered with a coloured filter
which can be red, blue, or green. These are the three primary colours
which can be combined to produce any other colour including white.
The
coloured filters are arranged in a grid so that there are twice
as many green filters as there are red or blue. This is because
the human eye is twice as sensitive to green light. Filters are
arranged in a pattern called the Bayer pattern - one row of red,
green, red, green (etc.), and the next row of blue, green, blue,
green (etc).
Since
each photosite can only be covered with one coloured filter, computer
processing is necessary to produce a full coloured image. This is
done by analyzing each individual pixel and its immediate neighbors
and producing a composite colour from these calculations. For example,
if a bright red pixel is surrounded by bright green and bright blue
pixels, the bright red pixel must actually be white, because white
is the combination of red, blue, and green. This process is called
demosaicing.
After
demosaicing the image is adjusted according to the settings on your
camera. Most cameras have settings for brightness, contrast, and
colour saturation. After these adjustments are made some cameras
may also apply a sharpening algorithm to make the image clearer.
The
final step before saving the image on the memory card is to compress
it. Most cameras use JPEG as a compression format.
This reduces the size of the file by eliminating excess data. This
data cannot be recovered, so JPEG is called a 'lossy' format.
Many
cameras have the ability to save uncompressed images as TIFF
files or raw data. Raw data is the original
photosite data even before demosaicing. It can be transferred to
a computer for processing with special software that will perform
all of the processing functions of the camera but with much greater
control.
Photo
Soren
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