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Digital photography

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File:CanonEOS300D.jpg
Canon EOS-300D digital single-lens reflex camera, a popular modern digital camera.

Digital photography, as opposed to film photography, uses an electronic sensor to record the image as a piece of electronic data rather than as chemical changes on film.

Sensors

There are two main types of sensors:

There are also two main types of sensor mechanisms:

  • Area array
  • Linear array (very rare, only limited to the highest-end)

An area array sensor reads the entire image plane at once, whereas a linear array sensor works more like a flatbed scanner. Since this technology predates area arrays, it was available earlier, in professionally-priced cameras. With the advent of area array sensors, consumer digital cameras became available for considerably lower prices. (The Ritz Dakota Digital is an extreme example.)

Except for some linear array type at the highest-end and web cams at the lowest-end, a digital memory device (usually flash memory; floppy disks and CD-RWs are less common) is usually used for storing images, which may then be transferred to a computer later. Many linear array type cameras and web cams connect directly to a computer.

Comparison with film cameras

Advantages of digital

The advantages of digital photography over traditional film include:

  • Instant review of pictures, with no wait for the film to be developed: if there's a problem with a picture, you find out immediately and can correct the problem and take another picture.
  • You only pay for the printing of successful pictures.
  • Colour reproduction and gamut is not dependent on film quality.
  • Permanent storage on digital media is considerably cheaper than film.
  • There's no need to scan the picture before using it in a computer.
  • Digital cameras can be smaller and lighter than film cameras of equivalent quality.
  • Image quality does not degrade with time.

Recent digital cameras from leading manufacturers such as Nikon and Canon have promoted the adoption of digital Single-lens reflex cameras (SLRs) by photojournalists. Images captured at 2+ megapixels are deemed to be of sufficient quality for small images in newspaper or magazine reproduction. Six megapixel images, found in modern digital SLRs, when combined with high-end lenses can match or even exceed the detail of film prints taken with 35 mm film based SLRs, and the latest 12-megapixel models can produce astoundingly detailed images better than almost all 35mm images. [1]

Some professional models can capture 20- to 40-megapixel images. These ultra high resolution pictures may not be very useful to ordinary users, but they are useful to artists, scientists and commercial uses.

Advantages of film

  • Film cameras are quicker to use: you don't have to wait for the camera to start up, for the photosensor to capture the image, or for the picture to be compressed and saved to the memory card.
  • Batteries last much longer in film cameras.
  • Film camera batteries are of standard types and are easier to replace.
  • A digital camera's LCD may become unusable in very bright light.
  • The user does not need to know how to handle a computer.
  • A picture on film is likely to last longer than a harddisk or CD-R used to store digital pictures.

Equivalent features

  • Image noise / grain: film grain is equivalent to image noise, at high ISO levels (film speed) the grain / nosie becomes more apparent in the final image. Althougth film iso levels can be lower that digital ISO levels (25 and 50 respectivly) digital settings can be changed quickly according to requirements whilst film must be physically replaced. Additionaly image noise reduction techniques can be used to remove noise from digital images whilst film grain is fixed. From an artistic point of view, film grain and image noise may be desirable when creating a specific mood for an image.
  • Frames per second: The maximum Number of frames per second (fps) achievable on digital and film cameras is 8fps (Nikon D2H digital SLR, Nikon F5 35mm film SLR). The F5 is limited to 36 continuous frames (the length of the film) whilst the D2H is able to take 40 images before it's buffer must be cleared and the remaining space on the storage media can be used.
  • Image quality: Although digital image data does not degrade per se (whilst film stock can fade) the media on which the digital images can decay or become corrupt leading to a loss of image integrity.

A comparison of frame aspect ratios

A typical digital camera's aspect ratio is 1.33 (4:3). The same ratio as today's NTSC or PAL/SECAM TVs or earliest movies. However, a 35 mm picture's aspect ratio is 1.5 (3:2). If you send your digital pictures to a lab to print, either the top and bottom parts of your pictures will be gone, or you will see ugly blanks on the right and left edges.

Performance metrics

The quality of a digital image is the sum of various factors. Pixel count (typically listed in megapixels) is only one of the major factors, though it's the most heavily marketed.

  • Analog
    • Lens quality
      • Lens resolution
      • Geometric distortion
      • Color dispersion
      • Inner reflection
    • CCD/CMOS noise
  • Digital
    • Data gathering
      • Pixel count
      • Color depth
      • Bayer filter calculation (for cameras with Bayer filter sensors)
      • Color correction
    • Data storage
      • File format


Pixel counts

The number of pixels n for a given maximum resolution (w horizontal pixels by h vertical pixels) can be found using the formula: n = wh. This yields e. g. 1.92 megapixels for an image of 1600 x 1200. The majority of digital cameras have a 4:3 aspect ratio, i.e. w/h = 4/3.

The megapixel or pixel count quoted by the manufacturers is misleading because it is not truly representative of the number of full colour pixels. For cameras using a Bayer sensor it is the number of single coloured photosites (light sensitive areas) on the sensor. For the Foveon X3 sensor the number currently (Feb 2004 - Sigma SD-10) presented by Sigma is the number of photosites times three (multiplied because each photosite records three colours), however the images that result will have a number of pixels equivalent to the number of photosites - not the tripled number quoted. It is not possible to directly compare the megapixel ratings of these two sensors but in many people's opinions a 6MP Bayer filter sensor is roughly equivalent to a 10.2MP Foveon X3 (3.4MP*3). Some hold the opinion that the Foveon is worse than this and the ratio is more like one Bayer to two Foveon. It is largely a matter of personal opinion so prints from the two sensors should be inspected by interested parties.

Applications and considerations

With the acceptable image quality and the other advantages of digital photography (particularly the time pressures, of vital importance to daily newspapers) an increasing number of professional news photographers use these devices.

It has also been adopted by many amateur snapshot photographers, who take advantage of the convenience of the form when sending images by email and to place on the World Wide Web.

Some commercial photographers, and some amateurs interested in artistic photography, tend to avoid digital photography at this stage, as they believe that the image quality available from a digital camera of a given price is still inferior to that available from a film camera, and the quality of images taken on medium format film is near-impossible to match at any price with a digital camera. Some have expressed a concern that changing computer technology may make digital photographs inaccessible in the future while printed images have a very long lifespan. A related concern in a specialised application is the use of digital photographs in court proceedings, with the perceived difficulty of demonstrating an image's authenticity.

Other commercial photographers, and many amateurs, have enthusiastically embraced digital photography, as they believe that its flexibility and lower long-term costs outweigh its initial price disadvantages. Almost all of the cost of digital photography is capital cost, meaning that the cost is for the equipment needed to store and copy the images, and once purchased requires virtually no further expense outlay. Film photography requires continuous expenditure of (much higher amounts of) funds for supplies and developing.

Digital photography was used in astronomy long before its use by the general public and had almost completely displaced photographic plates by the early 1980s. Not only are CCDs more sensitive to light than plates, but the information can be downloaded onto a computer for data analysis. The CCDs used in astronomy are similar to those used by the general public, but are cooled to liquid nitrogen temperatures so as to reduce the noise which is caused by heat.

Market impact

In late 2002, 2 megapixel cameras were available in the United States for less than $100, with some 1 megapixel cameras for under $60. At the same time, many discount stores with photo labs introduced a "digital front end," allowing consumers to obtain true chemical prints (as opposed to ink-jet prints) in an hour. These prices competed with prints from negatives.

In July 2003, digital cameras entered the "single-use" market with the release of the Ritz Dakota Digital, a 1.2 megapixel (1280x960) CMOS-based digital camera costing only $11 (USD). Following the familiar single-use concept long in use with film cameras, the Dakota Digital was intended to be used by a consumer one time only, be returned to the store, and the consumer would receive back prints and a CD with the digital photos, but not the camera (The camera gets refurbished and resold). Since the introduction of the Dakota Digital, a handful of similar single-use digital cameras have appeared, most of which are functionally identical to the original Dakota Digital, and others that have superior specifications, such as higher resolutions and LCD displays that show the previous picture taken. Most, if not all, of these single-use digital cameras cost less than $20 (USD), not including processing.

Kodak announced in January 2004 that they would no longer sell Kodak-branded film cameras in the developed world, an indication that the transition to digital photography for amateurs is well underway. The technology was improving so rapidly that one camera already went out of production before it was awarded "camera of the year" award.

Nowadays, sales of digital cameras are more than triple those of traditional film cameras.

File types and data storage formats

Exchangeable image file format (Exif) is a set of file formats specified for use in digital cameras. This specifies the use of TIFF for the highest quality format and JPEG as a space-saving but lower quality format. Many low-end cameras can deliver only JPEG files. Another format that may be encountered is CCD-RAW, which is unstandardised.

A large variety of data storage device formats are used in consumer digital cameras:

Digital camera backs

Most digital cameras are built to operate as a self-contained unit. This is especially so at the lower-end, for these cameras usually include zoom lens and flashes that cannot be changed. However, at the highest-end, some digital cameras are nothing but a sophisticated light-sensing unit. Experienced photographers attach these digital "camera backs" to their professional medium format SLR cameras, such as a Hasselblad.

  • Area array
    • CCD
    • CMOS
  • Linear array
    • CCD (monochrome)
    • 3-strip CCD with color filters

Linear array cameras are also called scan backs.

  • Single-shot
  • Multi-shot (three-shot, usually)

These camera backs are originally used only in a studio to take pictures of still objects. Most earlier digital camera backs were using linear array sensors which could take seconds or even minutes for a complete high-resolution scan. The linear array sensor acts like its counterpart used in a flatbed image scanner by moving vertically to digitize the image.

Many of these cameras could only capture grayscale images. To take a color picture, it requires three separate scans done with a rotating colored filter. These are called multi-shot backs. Some other camera backs are using CCD arrays similar to typical cameras. These are called single-shot backs.

Since it is much easier to manufacture a high-quality linear CCD array that has only thousands pixels than a CCD matrix that has millions of them, very high resolution linear CCD camera backs were available much earlier than their CCD matrix counterparts. For example, you could buy an expensive camera back with an over 7,000-pixel horizontal resolution in the mid-1990s. However, as of 2004, you can still hardly buy a comparable CCD matrix camera of the same resolution.

Many modern digital camera backs are using very large CCD matrices. This eliminated the need of scanning. For example, Fujifilm produces a 20-million-pixel digital camera back with a 52 mm x 37 mm (2.04" x 1.45") CCD in 2003. This CCD array is a little smaller than a frame of 120 film and much larger than a 35 mm frame (36 mm x 24 mm). In comparison, a consumer digital camera usually uses an itsy-bitsy so-called 1/2.5" or 7.176 mm x 5.329 mm (so-called 1/1.8") CCD sensor. Beware, the 1/2.5" or 1/1.8" diagonal measurement is the size of the entire CCD chip. The actual photo-sensitive area is much smaller.

A digital camera back is a good idea to smooth the transition from film to digital. A photographer can reuse his beloved SLR camera and lens without much trouble. To some medium format camera users, the convienence of a bellows has no substitute. However, there's a catch.

Only a few very expensive high-end digital cameras or camera backs could use film-sized CCDs. Since a camera back's CCD is seldom as large as the film it replaces, either one has to use a smaller CCD, or to use multiple smaller CCDs that form a larger piece. For example, various cameras of the late 1990s based on the "building block" CCDs of Philips use of four to six 12 x 12 mm 1-megapixel CCDs to form larger light sensors.

If a smaller sensor is used, the focal length of the lens shall be adjusted to the actual size of the CCD. For example, when you're using a 35 mm SLR camera with a standard 50 mm lens and a 4/3" digital camera back with a 18.0 mm x 13.5 mm CCD (about half as wide as a 35 mm frame), your 50 mm normal lens becomes a 100 mm telephoto lens. If you want a 50 mm lens, you have to buy a 25 mm wide-angle lens.

See also

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