How Digital Cameras Work: Vital
information you must know before you buy
By: Drew Laughlin
 Digital
cameras are basically the same as film cameras in
their handling. They both have a lens to focus the
specific image, a shutter to let light inside the
camera, and an aperture to control the amount of
light which enters the camera.
The differences between digital and traditional
photography happen to be 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
calculate light intensity. The most universally
recognized 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 built-up 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
harvested the resulting image is stored on a memory
card.
Photosites can only measure intensity of
light -- not color. In order to produce a colour
image, each photosite must be covered with a colored
filter which can be red, blue, or green. These are
the three primary colors which can be combined to
produce any other colour including white.
The colored 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 colored image. This is done by
analyzing a certain 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 undeniably 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 information
cannot be recovered, so JPEG is called a 'lossy'
format.
Several 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.
About the Author
Drew Laughlin is a successful Webmaster and
publisher of Learn-Digital-Cameras.com. He provides
more information on Digital Photography, Digital
Cameras, Digital Camera Accessories and how to
purchase your digital camera for less on his
website.
Learn-Digital-Cameras.com.
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