You've been framedYour next holiday snaps could be with
a camera that stores images digitally. Jon Mainwaring reports
Advanced Photo System (APS), the latest photographic film format
for stills cameras, may also be the last because chip makers are
taking the film out of cameras. Digital cameras are now at a stage
where they offer a number of benefits over conventional
photography.
The cameras use internal memory or removable pc cards to store
images rather than the traditional film. They can transmit those
images in digital format over various kinds of media and have the
potential to be more compact. In addition, some cameras offer an
LCD view allowing users to check the image they are about to
capture before taking the picture.
The basic architecture of a digital camera consists of four
components: a charged coupled device (CCD) chip, a camera body, an
analogue to digital converter (ADC), and a medium for image
storage. These components are integrated into a system for
recording images digitally by using a controlling device. Such
devices are becoming increasingly dedicated to this task.
The CCD consists of a matrix of light reactive sensors that each
produce an electrical charge that is directly proportional to the
amount of light that strikes them. The charges need a mechanism by
which they can be transmitted to the storage medium. It would be
too complicated to connect a link to each sensor, so charges are
removed from rows of sensors serially, and recorded as they come
off. This enables an image to be stored without using a complex
network of connections.
Philip Fennessy is digital and applied imaging specialist at Kodak
and co-author of Electronic Imaging for the Photographer. He uses
the analogy of a stadium full of people each possessing a number,
representing the charge, to explain CCDs: 'Being a fully automated
stadium, removing the people is achieved by having the seats on a
belt system which moves all the seats in a row one step nearer the
aisle at a time. The stadium is emptied row by row starting at the
top. As each person reaches the aisle they hand their number to an
official.'
CCDs will only record images in shades of grey. Colour film, which
uses silver-based material, allows an image to be recorded in a
number of colour sensitive, transparent, layers. For the CCD,
however, the chip is a single, non-transparent layer. There are two
main methods that can be used to record colour in a digital camera.
One method involves using a prism to split light received by the
camera into red, green and blue, before sending the split beams to
three CCD chips that represent each colour component for recording
purposes.
The other method uses a mosaic of red, blue and green filters built
on the surface of the chip. Each sensor in the CCD has its own
filter and they operate in groups of three, one of each colour, to
measure the colour balance at a particular point in the image.
A housing is needed for the CCD with a lens that can focus light
onto the chip. The lens system can have either a fixed focal length
or can zoom to vary the field of view. The size of the CCD has an
effect upon the focal length of the lens, a CCD that is smaller
than the original film will require a shorter focal length lens for
a wider angle of view.
Digital cameras have shutters. The timing of them depends upon the
technique used to record the image. 'Block array' recording
captures the whole image at once, just like a photographic film, so
the shutter operates like an ordinary camera. However, for
'scan-back' recording, where the image is scanned from top to
bottom, the shutter must be held open long enough for the whole
image to be captured.
The CCD delivers information in the form of varying electrical
charges. before this information can be read by a processor, it
needs to be converted into a digital format. An analogue to digital
converter (ADC) is used for this task. A storage medium will then
record the signal digitally. This can take various forms, including
DRAM, PC memory cards and floppy discs.
Controlling technologies are needed to co-ordinate the various
components within the camera. Apple's Image Capture Platform is a
reference design that has been developed to achieve this. Apple
calls its image processing technology QuickTime IC (image capture).
This is at the heart of the Image Capture Platform and makes use of
application programming interfaces (APIs) dedicated to the
platform. The APIs functions include: in-camera time lapse
photography, direct connection to the Internet to share and
transmit images, and in-camera filters.
Motorola has developed a chip, in conjunction with Apple, which
incorporates technologies that are part of the image capture
platform. Called the MPC823, it has some of the APIs built-in. The
MPC823 also includes a PowerPC processor core, serial and I/O
functionality through an integrated communications processor, a 24
bit colour LCD controller and an interface for PC Cards.
Jim Bridgwater, a spokesman for Motorola's advanced processor
division, said: 'MPC823, together with Apple's Image Capture
technology will allow camera manufacturers to decrease development
time. We think the MPC823's feature set will establish PowerPC as
the microprocessor architecture for digital cameras.'
These kind of technologies incorporated within a digital camera are
invaluable to anyone who may need to record an image and send it
across the world immediately without having to go through the
rigmarole of developing a photograph and scanning it into a
computer. Press photographers could certainly make use of such a
camera. At the moment, however, the resolutions available would
probably not appeal to a wedding photographer.
The amount of pixels that can be recorded at the moment is only of
the order of half a million. But it should not be too long before
manufacturers provide enough pixels to make large-sized digital
pictures indistinguishable from ordinary photographs, even to the
keenest eye.