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Digital Photography



Digital photography is a form of photography that uses cameras containing arrays of electronic photodetectors to capture images focused by a lens, as opposed to an exposure on photographic film. The captured images are digitized and stored as a computer file ready for further digital processing, viewing, digital publishing or printing.

Until the advent of such technology, photographs were made by exposing light sensitive photographic film, and used chemical photographic processing to develop and stabilize the image. 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 other photographic images.

Since the 2000s, digital photography has largely superseded all other forms of photography.

The first flyby spacecraft image of Mars was taken from Mariner 4 on July 15, 1965 with a camera system designed by NASA/JPL. It used a video camera tube followed by a digitizer, rather than a mosaic of solid state sensor elements, so it was not what we usually define as a digital camera, but it produced a digital image that was stored on tape for later slow transmission back to earth.

The first recorded attempt at building a digital camera was in 1975 by Steven Sasson, an engineer at Eastman Kodak. It used the then-new solid-state CCD image sensor 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 megapixels (10,000 pixels), and took 23 seconds to capture its first image in December 1975. The prototype camera was a technical exercise, not intended for production.

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 internationally, and has not been confirmed to have shipped even in Japan.

The first commercially available digital camera was the 1990 Dycam Model 1; it also sold as the Logitech Footman. It used a CCD image sensor, stored pictures digitally, and connected directly to a computer for downloading images.

Sensors

Image sensors read the intensity of light, and digital memory devices store the digital image information as RGB color space or as raw data.

The two main types of sensors are charge-coupled devices (CCD), in which the photocharge is shifted to a central charge-to-voltage converter, and CMOS or active pixel sensors.

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 a memory card; floppy disks and CD-RWs are less common) is used for storing images, which may 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 memory cards with the same functionality as digital cameras.

Performance metrics

The quality of a digital image is a composite of various factors, many of which are similar to those of 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 figure of merit. Digital camera manufacturers advertise this figure because consumers can use it to easily compare camera capabilities. It is not, however, the major factor in evaluating a digital camera for most applications. The processing system inside the camera that turns the raw data into a color-balanced and pleasing photograph is usually more critical, which is why some 4+ megapixel cameras perform better than higher-end cameras.

Resolution in pixels is not the only measure of image quality. A larger sensor with the same number of pixels generally produces 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 (so-called point and shoot cameras) of the same resolution.

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).

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 as well as some DSLR 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 CCDs 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-color pixels. For cameras using single-chip image sensors the number claimed is the total number of single-color-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 have different numbers of RGB pixels: Bayer-sensor cameras produce as many RGB pixels as photosensors via demosaicing (interpolation), while Foveon sensors produce uninterpolated image files with one-third as many RGB pixels as photosensors. Comparisons of megapixel ratings of these two types of sensors are sometimes a subject of dispute.

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, megapixels in the entire image.

Dynamic range

Practical imaging systems both digital and film, have a limited "dynamic range": the range of luminosity that can be reproduced accurately. Highlights of the subject that are too bright are rendered as white, with no detail; shadows that are too dark are 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 problem 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.

Storage

Many camera phones and most digital cameras use memory cards having flash memory to store image data. The majority of cards for separate cameras are SD format; many are CompactFlash and the other formats are rare. XQD card format was the last new form of card. Most modern digital cameras also use internal memory for a limited capacity for pictures that can be transferred to or from the card or through the camera's connections; even without a memory card inserted into the camera.

Memory cards can hold vast numbers of photos, requiring attention only when the memory card is full. For most users, this means hundreds of quality photos stored on the same memory card. Images may be transferred to other media for archival or personal use. Cards with high speed and capacity are suited to video and burst mode.

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