Thursday, November 12, 2009

Digital photography

Digital photography is a form of photography that utilizes digital technology to make images of subjects. Until the advent of such technology, photography used photographic film to create images which could be made visible by photographic processing. By contrast, digital photographs can be displayed, printed, stored, manipulated, transmitted, and archived using digital and computer techniques, without chemical processing.

Digital photography is one of several forms of digital imaging. Digital images are also created by non-photographic equipment such as computer tomography scanners and radio telescopes. Digital images can also be made by scanning conventional photographic images.

Sensors and storage

Sensors read the intensity of light as filtered through different color filters, and digital memory devices store the digital image information, either as RGB color space or as raw data.

There are two main types of sensors:

  • charge-coupled device (CCD) – photocharge is shifted to a central charge-to-voltage converter
  • CMOS sensors ("Active pixel sensor")

Nearly all digital cameras now use built in and/or removable solid state flash memory. Digital camcorders that double as a digital still camera use flash memory, discs and internal hard disks. For a time floppy disks and mini-CDs were used in early digital cameras such as the Sony Mavica range.

Multifunctionality and connectivity

Except for some linear array type of cameras at the highest-end and simple 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.

Digital cameras can take pictures, and may also record sound and video. Some can be used as webcams, some can use the PictBridge standard to connect to a printer without using a computer, and some can display pictures directly on a television set. Similarly, many camcorders can take still photographs, and store them on videotape or on flash memorycards with the same functionality as Digital Cameras.

Performance metrics

The quality of a digital image is the sum of various factors, many of which are similar to film cameras. Pixel count (typically listed in megapixels, millions of pixels) is only one of the major factors, though it is the most heavily marketed. Pixel count metrics were created by the marketing organizations of digital camera manufacturers because consumers can use it to easily compare camera capabilities. It is not, however, the major factor in evaluating a digital camera. The processing system inside the camera that turns the raw data into a color-balanced and pleasing photograph is the most critical, which is why some 4+ megapixel cameras perform better than higher-end cameras.

  • Lens quality: resolution, distortion, dispersion (see Lens (optics))
  • Capture medium: CMOS, CCD, negative film, reversal film etc.
  • Capture format: pixel count, digital file type (RAW, TIFF, JPEG), film format (135 film, 120 film, 5x4, 10x8).
  • Processing: digital and / or chemical processing of 'negative' and 'print'.

Pixel counts

The number of pixels n for a given maximum resolution (w horizontal pixels by h vertical pixels) is the product n = w × h. This yields e. g. 1.92 megapixels (1,920,000 pixels) for an image of 1600 × 1200. The majority of compact (not SLR) digital cameras have a 4:3 aspect ratio, i.e. w/h = 4/3. . According to Digital Photography Review, the 4:3 ratio is because "computer monitors are 4:3 ratio, old CCD's always had a 4:3 ratio, and thus digital cameras inherited this aspect ratio."

The pixel count quoted by manufacturers can be misleading as it may not be the number of full-colour pixels. For cameras using single-chip image sensors the number claimed is the total number of single-colour-sensitive photosensors, whether they have different locations in the plane, as with the Bayer sensor, or in stacks of three co-located photosensors as in the Foveon X3 sensor. However, the images will have different numbers of RGB pixels: the Bayer-sensor cameras produce as many RGB pixels as photosensors via demosaicing (interpolation), while the cameras with Foveon sensors produce uninterpolated image files with one-third as many RGB pixels as photosensors. It is difficult to compare the resolutions based on the megapixel ratings of these two types of sensors, and therefore sometimes subject of dispute.


Resolution provides an indication of the amount of detail that is captured, but, like the other metrics, resolution is just another factor out of many in determining the quality of an image. Furthermore, different methods of creating an image make it impossible to compare the resolutions of cameras simply based on the number of pixels produced by the image sensor. For example, the Sigma SD14 camera uses Foveon technology, which is quite different from most other digital cameras. It claims to be a 14 megapixel camera, but is generally considered to have detail-capturing capabilities roughly equivalent to 9 megapixels in terms of Bayer sensors.

The relative increase in detail resulting from an increase in resolution is better compared by looking at the number of pixels across (or down) the picture, rather than the total number of pixels in the picture area. For example, a sensor of 2560 × 1600 sensor elements is described as "4 megapixels" (2560 × 1600 = 4,096,000). Increasing to 3200 × 2048 increases the pixels in the picture to 6,553,600 (6.5 megapixels), a factor of 1.6, but the pixels per cm in the picture (at the same image size) increases by only 1.25 times. A measure of the comparative increase in linear resolution is the square root of the increase in area resolution, i.e., megapixels in the entire image.

Resolution in pixels is not the only measure of image quality; a larger sensor with the same number of pixels will generally produce a better image than a smaller one. One of the most important differences is an improvement in image noise. This is one of the advantages of digital SLR cameras, which have larger sensors than simpler cameras of the same resolution.

Dynamic range

Practical imaging systems, digital and film, have a limited "dynamic range": the range of luminosity which can be reproduced accurately. Highlights of the subject which are too bright will be rendered as white, with no detail; shadows which are too dark will be rendered as black. The loss of detail is not abrupt with film, or in dark shadows with digital sensors: some detail is retained as brightness moves out of the dynamic range. "Highlight burn-out" of digital sensors, however, can be abrupt, and highlight detail may be lost. And as the sensor elements for different colors saturate in turn, there can be gross hue or saturation shift in burnt-out highlights.

Some digital cameras can show these blown highlights in the image review, allowing the photographer to re-shoot the picture with a modified exposure. Others compensate for the total contrast of a scene by selectively exposing darker pixels longer. A third technique is used by Fujifilm in its FinePix S3 Pro digital SLR. The image sensor contains additional photodiodes of lower sensitivity than the main ones; these retain detail in parts of the image too bright for the main sensor.

High dynamic range imaging (HDR) addresses this issue by increasing the dynamic range of images by either

  • increasing the dynamic range of the image sensor or
  • by using exposure bracketing and post-processing the separate images to create a single image with a higher dynamic range.

HDR images curtail burn-outs and black-outs.

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) the majority of professional news photographers have begun capturing their images with digital cameras.

Digital photography has also been adopted by many amateur snapshot photographers, who take advantage of the convenience of the form when sending images by email, placing them on the World Wide Web, or displaying them in digital picture frames. Digital cameras have also been integrated into many cell phones, although, because of the small, poor quality lenses and sensors in most of these phones, the quality of these pictures makes them unsuitable for making even moderate size prints.

Some commercial photographers, and some amateurs interested in artistic photography, have been resistant to using digital rather than film cameras because they believe that the image quality available from a digital camera 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. A related concern in a specialized application is the use of digital photographs in court proceedings, with the added difficulty of demonstrating an image's authenticity. Some high-end film can also still be projected for viewing at a much higher optical resolution than even the best digital projectors.

Other commercial photographers, and many amateurs, have enthusiastically embraced digital photography because 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 funds for supplies and developing, although the equipment itself does not outdate so quickly and has a longer service life. Some commercial photographers have also begun moving to digital technology because of the tremendous editing capabilities now offered on computers. The photographer is able to color-balance and manipulate the image in ways that traditional darkroom techniques cannot offer, although film users can utilize the same technology with a film scanner. With fully color-balanced systems from the camera to the monitor to the printer, the photographer can now print what is actually seen on the screen.

However, digital cameras require batteries that need to be recharged or replaced frequently, and this means that a photographer needs access to electrical outlets. Digital cameras also tend to be much more sensitive to moisture and extreme cold. For this reason, photographers who work in remote areas may favour film SLR cameras, though many higher-end DSLRs are now equipped with weather-resistant bodies. Medium- and large-format film cameras are also still preferred by publications insisting on the very highest detail and resolution.

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 they have a much more uniform and predictable response. The CCDs used in astronomy are similar to those used by the general public, but are generally monochrome and cooled with liquid nitrogen so as to reduce the noise caused by heat. Many astronomical instruments have arrays of many CCDs, sometimes totaling almost a billion pixels. Nowadays amateur astronomers also commonly use digital cameras, including the use of webcams for speckle imaging or "video astronomy".