A digital camera is an electronic device used to capture
and store photographs electronically in a digital format, instead of using
photographic film like conventional cameras, or recording images in an analog
format to magnetic tape like many video cameras. Modern compact digital cameras
are typically multifunctional, with some devices capable of recording sound
and/or video as well as photographs. In the Western market, digital still
cameras now outsell their 35 mm film counterparts.[edit] Conversion of film
cameras to digital
When digital cameras became common, a question many photographers asked was if
their film cameras could be converted to digital. The answer was yes and no. For
the majority of 35 mm film cameras the answer is no, the reworking and cost
would be too great, especially as lenses have been evolving as well as cameras.
For the most part a conversion to digital, to give enough space for the
electronics and allow a liquid crystal display to preview, would require
removing the back of the camera and replacing it with a custom built digital
unit.
Many early professional SLR cameras, such as the NC2000 and the Kodak DCS
series, were developed from 35 mm film cameras. The technology of the time,
however, meant that rather than being a digital "back" the body was mounted on a
large and blocky digital unit, often bigger than the camera portion itself.
These were factory built cameras, however, not aftermarket conversions.
A notable exception was a device called the EFS-1, which was developed by
Silicon Film from ca. 1998–2001. It was intended to insert into a film camera in
the place of film, giving the camera a 1.3 MP resolution and a capacity of 24
shots. Units were demonstrated, and in 2002 the company was developing the
EFS-10, a 10 MP device that was more a true digital back.
A few 35 mm cameras have had digital backs made by their manufacturer, Leica
being a notable example. Medium format and large format cameras (those using
film stock greater than 35 mm), have users who are capable of and willing to pay
the price a low unit production digital back requires, typically over $10,000.
These cameras also tend to be highly modular, with handgrips, film backs,
winders, and lenses available separately to fit various needs.
The very large sensor these backs use lead to enormous image sizes. The largest
in early 2006 is the Phaseone's P45 39 MP imageback, creating a single TIFF
image of size upto 224.6 MB. Medium format digitals are geared more towards
studio and portrait photography than their smaller DSLR counterparts, the ISO
speed in particular tends to have a maximum of 400, versus 3200 for some DSLR
cameras.
[edit] History
[edit] Early development
The concept of digitizing images on scanners, and the concept of digitizing
video signals, predate the concept of making still pictures by digitizing
signals from an array of discrete sensor elements. Eugene F. Lally of the Jet
Propulsion Laboratory published the first description of how to produce still
photos in a digital domain using a mosaic photosensor.[1] The purpose was to
provide onboard navigation information to astronauts during missions to planets.
The mosaic array periodically recorded still photos of star and planet locations
during transit and when approaching a planet provided additional stadiametric
information for orbiting and landing guidance. The concept included camera
design elements foreshadowing the first digital camera.
Texas Instruments designed a filmless analog camera in 1972, but it is not known
if it was ever built. The first recorded attempt at building a digital camera
was by Steve Sasson, an engineer at Eastman Kodak. It used the then-new solid
state CCD chips developed by Fairchild Semiconductor in 1973. The camera weighed
8 pounds (3.6 kg), recorded black and white images to a cassette tape, had a
resolution of 0.01 megapixel (10,000 pixels), and took 23 seconds to capture its
first image in December of 1975. The prototype camera was a technical exercise,
not intended for production, and it still existed as of 2005.
[edit] The arrival of true digital cameras
The first true digital camera that recorded images as a computerized file was
likely the Fuji DS-1P of 1988, which recorded to a 16 MB internal memory card
that used a battery to keep the data in memory. This camera was never marketed
in the United States. The first commercially available digital camera was the
1991 Kodak DCS-100, the beginning of a long line of professional SLR cameras by
Kodak that were based in part on film bodies, often Nikons. It used a 1.3
megapixel sensor and was priced at $13,000.
The move to digital formats was helped by the formation of the first JPEG and
MPEG standards in 1988, which allowed image and video files to be compressed for
storage. The first consumer camera with a liquid crystal display on the back was
the Casio QV-10 in 1995, and the first camera to use CompactFlash was the Kodak
DC-25 in 1996.
The marketplace for consumer digital cameras was originally low resolution
(either analog or digital) cameras built for utility. In 1997 the first
megapixel cameras for consumers were marketed. The first camera that offered the
ability to record video clips may have been the Ricoh RDC-1 in 1995.
1999 saw the introduction of the Nikon D1, a 2.74 megapixel camera that was the
first digital SLR developed entirely by a major manufacturer, and at a cost of
under $6,000 at introduction was affordable by professional photographers and
high end consumers. This camera also used Nikon F-mount lenses, which meant film
photographers could use many of the same lenses they already owned.
2003 saw the introduction of the Canon Digital Rebel, also known as the 300D, a
6 megapixel camera and the first DSLR priced under $1,000, and marketed to
consumers.
[edit] Image resolution
The resolution of a digital camera is often limited by the camera sensor
(usually a charge-coupled device or CCD chip) that turns light into discrete
signals, replacing the job of film in traditional photography. The sensor is
made up of millions of "buckets" that collect charge in response to light.
Generally, these buckets respond to only a narrow range of light wavelengths,
due to a color filter over each. Each one of these buckets is called a pixel,
and a demosaicing/interpolation algorithm is needed to turn the image with only
one wavelength range per pixel into an RGB image where each pixel is three
numbers to represent a complete color.
The one attribute most commonly compared on cameras is the pixel count. Due to
the ever increasing sizes of sensors, the pixel count is into the millions, and
using the SI prefix of mega- (which means 1 million) the pixel counts are given
in megapixels. For example, an 8.0 megapixel camera has 8.0 million pixels.
The pixel count alone is commonly presumed to indicate the resolution of a
camera, but this is a misconception. There are several other factors that impact
a sensor's resolution. Some of these factors include sensor size, lens quality,
and the organization of the pixels (for example, a monochrome camera without a
Bayer filter mosaic has a higher resolution than a typical color camera). Many
digital compact cameras are criticized for having too many pixels, in that the
sensors can be so small that the resolution of the sensor is greater than the
lens could possibly deliver.
Excessive pixels can even lead to a decrease in image quality. As each pixel
sensor gets smaller it is catching fewer photons, and so the signal-to-noise
ratio will decrease. This decrease leads to noisy pictures, poor shadow region
quality and generally poorer-quality pictures.
As the technology has improved, costs have decreased dramatically. Measuring the
"pixels per dollar" as a basic measure of value for a digital camera, there has
been a continuous and steady increase in the number of pixels each dollar buys
in a new camera consistent with the principles of Moore's Law. This
predictability of camera prices was was first presented in 1998 at the
Australian PMA DIMA conference by Barry Hendy and since referred to as "Hendy's
Law".[2]
[edit] Methods of image capture
Since the first digital backs were introduced, there have been three main
methods of capturing the image, each based on the hardware configuration of the
sensor and color filters.
The first method is often called single-shot, in reference to the number of
times the camera's sensor is exposed to the light passing through the camera
lens. Single-shot capture systems use either one CCD with a Bayer filter mosaic
it, or three separate image sensors (one each for the primary additive colors
red, green, and blue) which are exposed to the same image via a beam splitter.
The second method is referred to as multi-shot because the sensor is exposed to
the image in a sequence of three or more openings of the lens aperture. There
are several methods of application of the multi-shot technique. The most common
originally was to use a single image sensor with three filters (once again red,
green and blue) passed in front of the sensor in sequence to obtain the additive
color information. Another multiple shot method utilized a single CCD with a
Bayer filter but actually moved the physical location of the sensor chip on the
focus plane of the lens to "stitch" together a higher resolution image than the
CCD would allow otherwise. A third version combined the two methods without a
Bayer filter on the chip.
The third method is called scanning because the sensor moves across the focal
plane much like the sensor of a desktop scanner. Their linear or tri-linear
sensors utilize only a single line of photosensors, or three lines for the three
colors. In some cases, scanning is accomplished by rotating the whole camera; a
digital rotating line camera offers images of very high total resolution.
The choice of method for a given capture is of course determined largely by the
subject matter. It is usually inappropriate to attempt to capture a subject that
moves with anything but a single-shot system. However, the higher color fidelity
and larger file sizes and resolutions available with multi-shot and scanning
backs make them attractive for commercial photographers working with stationary
subjects and large-format photographs.
Recently, dramatic improvements in single-shot cameras and RAW image file
processing have made single shot, CCD-based cameras almost completely
predominant in commercial photography, not to mention digital photography as a
whole. CMOS-based single shot cameras are also somewhat common.
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