Digital photography From Wikipedia, the free encyclopedia
Digital photography, as opposed to film photography, uses an electronic sensor to record the image as binary data. This facilitates storage and editing of the images on personal computers. Digital cameras now outsell film cameras and include features not found in film cameras such as the ability to shoot video and record audio. Some other devices, such as mobile phones, now include digital photography features.
There are two main types of sensors:
- charge-coupled device (CCD) - charge is shifted to a central charge-to-voltage converter
- CMOS sensors
There are also two main types of sensor mechanisms possible:
- Area array
- Linear array (in professional "scanning" camera backs)
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 studio cameras. With the advent of area array sensors, consumer digital cameras became available for considerably lower prices.
Multifunctionality and connectivity
Except for some linear array type 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 usually take pictures and additionally sound and video. Some can be used like 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.
Most digital cameras can connect directly to a computer in order to store pictures or to be used as a webcam. Digital cameras generally include a USB or FireWire port, and a card slot. There are also cameras available that can send pictures directly to a PictBridge compatible printer.
Some digital cameras can record movies but may be limited by storage capacity. A 1GB memory card will store approximately 1 hour's worth of video in an MP4 format. Newer digital cameras, such as the Canon PowerShot S1 IS, Canon PowerShot SD200/300 and the Pentax Optio MX/MX4 will capture continuous footage at a rate of 30 frames per second at a display resolution of 640 x 480 pixels (similar to a television screen). Some digital cameras can connect directly to a computer and store video on the computer's hard disk or DVD recorder.
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'.
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 (= 1,920,000 pixels) 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 6 MP Bayer filter sensor is roughly equivalent to a 10.2 MP Foveon X3 (3.4 MP*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.
Since the light sensitive component in a digital camera consists of discrete pixels, problems of Moiré, or interference patterns may occur when photographing fine patterns, such as textiles, geometric figures, and computer or TV screens. The example at left shows severe Moirés in a shot of a TV screen. However, this is not a problem for most real-life situations.
"Highlight burn-out" is also a potential problem. Depending on the contrast of the subject, the lightest parts of the image may be so over-exposed that there is no image information, other than total white, in these highlights. Also, the reverse may occur. Shadows parts of the image may become murky to totally black, because of the inability of the camera's sensor to cope with the contrast. The image at right shows both these conditions simultaneously. Some digital cameras can show these blown highlights in the image review, allowing the photographer to reshoot the picture with a modified exposure. Others compensate for the total ammount of contrast in an image by selectively exposing darker pixels longer.
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 lens, 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 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 specialized 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 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 (much higher amounts of) funds for supplies and developing. 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 science cannot offer. 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 frequently, and this means that a photographer needs access to electrical outlets. For this reason, photographers who work in remote areas, such as those who work for National Geographic overwhelmingly favor film SLR cameras.
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 have much more uniform and predictable response, and 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 generally monochrome and cooled to liquid nitrogen temperatures so as to reduce the noise which is caused by heat. Many astronomical instruments have arrays of many CCDs, sometimes totalling almost a billion pixels. Nowadays amateur astronomers also commonly use digital cameras, including the use of webcams for speckle imaging or "video astronomy".
Sensor size and angle of view
Cameras with digital sensors that are smaller than the typical 35mm film size will have a smaller field or angle of view when used with a lens of the same focal length. This is because angle of view is a function of both focal length and the sensor or film size used.
If a sensor smaller than the camera's original film format is used, such as the use of APS-C-sized digital sensors in 35mm format digital SLRs, then the image's field of view is cropped by the sensor giving the impression that the focal length of the lens has changed.
If the digital sensor has approximately the same resolution (effective pixels per unit area) as the 35mm film surface (24 x 36 mm), then the result is similar to taking the image from the film camera and cutting it down (cropping) to the size of the sensor. For an APS-C size sensor, this would be a reduction to approximately the center 50% of the image. The cheaper, non-SLR models of digital cameras typically use much smaller sensor sizes and the reduction would be greater.
If the digital sensor has a higher or lower density of pixels per unit area than the film equivalent, then the amount of information captured will differ correspondingly. While resolution can be estimated in pixels per unit area, the comparison is complex since most types of digital sensor record only a single colour at each pixel location, and different types of film will have different effective resolutions. There are various trade-offs involved, since larger sensors are more expensive to manufacture and require larger lenses, while sensors with higher numbers of pixels per unit area are likely to suffer higher noise levels.
For these reasons, it is possible to obtain cheap digital cameras with sensor sizes much smaller than 35mm film, but with high pixel counts, that can still produce high-resolution images. Such cameras are usually supplied with lenses that would be classed as extremely wide angle on a 35mm camera, and which can also be smaller size and less expensive, since there is a smaller sensor to illuminate. For example, a camera with a 1/1.8" sensor has a 5.0x field of view crop, and so a hypothetical 5-50mm zoom lens will produce images that look similar (again the differences mentioned above are important) to those produced by a 35mm film camera with a 25–250mm lens, while being much more compact than such a lens for a 35mm camera since the imaging circle is much smaller.
This can be useful if extra telephoto reach is desired, as a certain lens on an APS sensor will produce an equivalent image to a significantly longer lens on a 35mm film camera shot at the same distance from the subject, the equivalent length of which depends on the camera's field of view crop. This is sometimes referred to as the focal length multiplier, but the focal length is a physical attribute of the lens and not the camera system itself. The downside to this is that wide angle photography is made somewhat more difficult, as the smaller sensor effectively and undesirably reduces the captured field of view. Some methods of compensating for this or otherwise producing much wider digital photographs involve using a fisheye lens and "defishing" the image in post processing to simulate a rectilinear wide angle lens.
As of 2005, only a few high-end DSLR camera models from Canon and Kodak have sensor sizes that match a 35mm film frame. This is the ideal size for maximising the use of lenses designed for a 35mm camera, since it can reproduce the effect smaller sizes by cropping the image (assuming equal pixel density). Common values for field of view crop in DSLRs include 1.3x for some Canon sensors, 1.5x for Sony APS-C sensors used by Nikon, Pentax and Konica Minolta, 1.6 (APS-C) for most Canon sensors and Fujifilm sensors, ~1.7x for Sigma's Foveon sensors and 2x for Kodak 4/3" sensors currently used by Olympus. Crop factors for non-SLR consumer and prosumer cameras are larger, frequently 4x or more.
source : DPreview
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 not standardised.
A large variety of data storage device formats are used in consumer digital cameras:
- Secure Digital Card (SD)
- Compact Flash (CF-I and CF-II)
- Memory Stick
- Multi Media Card (MMC)
- xD-Picture Card (xD)
- miniSD card
- USB keydrive
Most manufacturers of digital cameras do not provide drivers and software to allow their cameras to work with Linux or other free software. Still, many cameras use the standard USB storage protocol, and are thus easily usable. Other cameras are supported by the gPhoto project.
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
- Linear array
- CCD (monochrome)
- 3-strip CCD with color filters
Linear array cameras are also called scan backs.
- 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 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 of 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, albeit expensive, camera back with over 7,000 pixel horizontal resolution in the mid-1990s. However, as of 2004, it is still difficult to buy a comparable CCD matrix camera of the same resolution. Rotating line cameras, with about 10,000 color pixels in its sensor line, are able, as of 2005, to capture about 120,000 lines during one full 360 degree rotation, creating this way a single digital image of 1,200 Megapixel.
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 x 37 mm (2.04 x 1.45 inch) 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 x 24 mm). In comparison, a consumer digital camera usually uses a much smaller 1/2.5 inch or 7.176 x 5.329 mm (~ 1/1.8 inch) CCD sensor. Further, the 1/2.5 or 1/1.8 inch 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 existing SLR camera and lens without much trouble. To some medium format camera users, the convenience of a bellows has no substitute.
Comparison with film cameras
Advantages of digital: consumer cameras
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, the photographer can immediately correct the problem and take another picture.
- Only successful pictures need to be printed. This means you can take many shots of the same scene but with slightly different settings, then choose the best one. Doing this with film would be too expensive.
- Minimal ongoing costs for those wishing to capture hundreds of photographs for digital uses, such as computer storage and e-mailing, but not printing.
- If one already owns a newer computer, permanent storage on digital media is considerably cheaper than film.
- Images may be copied from one media to another without any degradation.
- Pictures do not need to be scanned before viewing them on a computer.
- Ability to print your own pictures using a computer and consumer-grade printer.
- Ability to print your own pictures using printers that can communicate directly with the camera, or its memory card, for computer-less printing.
- Digital cameras can be much smaller than film cameras of equivalent quality.
- Ability to embed metadata within the image file, such as the time and date of the photograph, model of the camera, shutter speed, flash use, film speed, and other similar items, to aid in the reviewing and sorting of photographs. Film cameras have limited ability to handle metadata, though many film cameras can "imprint" a date over a picture by exposing the film to an internal LED array (or other device) which displays the date.
- Ability to capture and store hundreds of photographs on the same media device within the digital camera; by contrast, a film camera would require regular changing of film (typically after every 24 or 36 shots).
- Many digital cameras now include an AV-out function (and cable) to allow the reviewing of photographs to an audience using a television.
- Digital image files can be backed up to CD-ROM or DVD-ROM, although, it is believed that negatives will probably last at least as long as, if not longer than CD-ROMs.
- Digital photography enables you to experiment with the camera settings, different styles of images can be tried out, learnt from and techniques improved all without the expense of film processing.
Alcatraz Island taken with a Canon Digital Rebel XT. It was stitched together using Panorama Tools (software) called PTGUI
Advantages of digital: professional cameras
- Immediate image review and removal, lighting and composition can be assessed without wasting storage space.
- The ability to shoot in RAW format (images that contain tagged data directly from the sensor). However, as of this writing, there are a number of proprietary RAW formats, some of which require specific software to manipulate.
- Faster workflow: Management (colour and file), manipulation and printing tools are more versatile than conventional film processes. However, batch processing of RAW files can be time consuming, even on a fast computer.
Recent manufacturers such as Nikon and Canon have promoted the adoption of digital Single-lens reflex cameras (dSLRs) by photojournalists. Images captured at 2+ megapixels are deemed to be of sufficient quality for small images in newspaper or magazine reproduction. Six to 14 megapixel images, found in modern digital SLRs, when combined with high-end lenses, can approximate the detail of film prints taken with 35 mm film based SLRs, and the latest 16 megapixel models can produce astoundingly detailed images which are believed to be better than 35mm film images and the majority of medium format cameras. 
Disadvantages of digital cameras
- Equivalent film cameras are much less expensive than digital cameras.
- Film cameras can potentially work without batteries, as the image is stored on film rather in memory using CCD/CMOS sensors and associated electronics which require power to operate.
- There are special types of film, such as for infrared light, that have no equivalent in digital (CCD's are sensitive to Near Infrared).
- A film camera does not require a computer to download images to. However, almost all one-hour photo labs can now make prints of digital images using the camera's memory card. In addition, newer photo printers that use PictBridge technology can make prints of digital images without a computer.
- Film and prints can be easily stored in a file cabinet. Digital images stored on a computer can be lost if a hard drive fails; CD-ROMs may be a good alternative, but no one knows if computers that can read them will be widely available 100 years from now.
- Film remains more admissible as evidence in court, as it is much harder to manipulate than digital.
- Film is capable of much greater resolution than digital photographs. Estimated equivalent film resolutions are debated.
For most consumers in prosperous countries such as the United States and Western Europe, the advantages of digital cameras outweigh their disadvantages. However, the professional photography community is split on the issue. Much of the post-shooting work once done in the past by a photo lab is now done by the photographer himself. Problems some professional photographers have voiced include: editing and post-processing of RAW files can take longer than 35mm film, downloading a large number of images to a computer can take away from valuable shooting time, shooting in remote sites requires the photographer to carry a number of batteries and add to the load she/he must carry, all cameras break from time to time — film cameras can often be fixed on the spot but digital cameras often can not. As time passes, it is expected that more professional photographers will switch to digital.
- Image noise / grain: Film grain is equivalent to image noise. At high ISO levels (film speed) the grain/noise becomes more apparent in the final image. Although film ISO levels can be lower than digital ISO levels (25 and 50 respectively), digital settings can be changed quickly according to requirements, while film must be physically replaced. Additionally, image noise reduction techniques can be used to remove noise from digital images and 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. Modern digital cameras have comparable noise/grain at the same ISO as film cameras. Some digital cameras though, do exhibit a pattern in the digital noise which is not found on film.
- Speed of use: Current digital and film cameras can be switched on and take images instantly. Saving images to disk takes no longer than winding on the film (see Frames per second).
- Frames per second: The Canon EOS-1D Mark II can achieve 8.5 frames per second which makes it the fastest digital SLR in the world; the fastest film SLR could shoot 10 frames per second. The Nikon F5 is limited to 36 continuous frames (the length of the film) while the Nikon D2H is able to take 40 images before its buffer must be cleared and the remaining space on the storage media can be used.
- Image longevity: Although digital image data does not degrade (film stock can fade), the media on which the digital images are stored can decay or become corrupt, leading to a loss of image integrity. Both formats should be stored under archival conditions for maximum longevity. Perfect copies of digital images can be made on fresh media whereas copying negatives or transparencies incurs additional noise and loss of detail.
- Colour reproduction: Colour reproduction (gamut) is dependent on the type of film / sensor used and the quality of the capture media, lens group and processing. Different films and sensors are sensitive to differing subsets of colour thus the photographer needs to have an understanding of the light conditions and the media used to ensure accurate colour reproduction. Many digital cameras offer RAW format (sensor data) which makes it possible to choose color space on development stage regardless of camera settings.
A comparison of frame aspect ratios
A typical digital camera's aspect ratio is 1.33 (4:3), the same as today's NTSC or PAL/SECAM TVs or earliest movies. However, a 35 mm picture's aspect ratio is 1.5 (3:2). Several new digital cameras will take photos in either ratio and nearly all digital SLRs take pictures in a 3:2 ratio as they usually use lenses designed for 35mm film (Olympus digital SLRs are a notable exception). Some photo labs also offer the option of printing photos on 4:3 ratio paper, as well as the existing 3:2. In 2005 Panasonic launched the first consumer camera with a native aspect ratio of 16:9, matching HDTV.
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 were similar to those of prints made from film negatives. However, because digital images have a different aspect ratio than 35 mm film images, people have started to realize that 4x6 inch prints crop some of the image off the print. Some photofinishers have started offering prints with the same aspect ratio as the digital cameras record.
In July 2003, digital cameras entered the single-use market with the release of the Ritz Dakota Digital, a 1.2 megapixel (1280 x 960) 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. When the pre-programmed 25 picture limit is reached, the camera is returned to the store, and the consumer receives back prints and a CD-ROM with their photos. The camera is then refurbished and resold. Since the introduction of the Dakota Digital, a number of similar single-use digital cameras have appeared. Most of the various single-use digital cameras are nearly identical to the original Dakota Digital regarding specifications and functionality, although a few include superior specifications and more advanced functions (such as higher image resolutions and LCD screens). Most, if not all, of these single-use digital cameras cost less than $20 (USD), not including processing fees.
The price of 35mm compact cameras have dropped with manufacturers further outsourcing to countries such as China. Kodak announced in January 2004 that they would no longer sell Kodak-branded film cameras in the developed world . In January 2006 Nikon followed suit and announced that they will stop the production of all but two models of their film cameras, they will continue to produce the low-end Nikon FM10, and the high-end Nikon F6. In the same month, Konica Minolta announced that it was pulling out of the camera business altogether. The price of 35mm and APS compact cameras have dropped, probably due to direct competition from digital and the resulting growth of the offer of second-hand film cameras. Pentax have reduced production of film cameras but not halted it. . The technology has improved so rapidly that one of Kodak's film cameras was discontinued before it was awarded a "camera of the year" award later in the year. Since 2002, digital cameras have outsold film cameras. However, the use of 35mm cameras is greater in developing countries.  It remains to be seen if 35 mm film will remain the medium of choice as countries like China advance rapidly.
The decline in film camera sales has also led to a decline in purchases of film for such cameras. In November 2004, a German division of Agfa-Gevaert, AgfaPhoto, split off. Within six months it filed for bankruptcy . In addition, by 2005, Kodak employed less than a third of the employees that it had twenty years earlier. It is not known if these job losses in the film industry have been offset in the digital image industry.
Throughout the history of photography, technological advances in optics, camera production, developing, and imaging have had an effect on the way people view images. Prior to the 1970s, most people in the United States used slide or chrome film and viewed the images with a slide projector. After that, people began to make prints from color negatives. The simultaneous increased use of the Internet and email, relatively cheap computers and digital cameras led to a tremendous increase in the number of photographic images in digital formats.
In the early part of the 21st century, the dominant method of viewing still images has been on computers and, to a lesser extent, on cellular phones (although people still make and look at prints). These factors have led to a decrease in film and film camera sales and film processing, and has had a dramatic effect on companies such as Fuji, Kodak, and Agfa. In addition, many stores that used to offer photofinishing services or sell film no longer do, and those that do have seen a tremendous decline.
Photographic images have always been prone to fading and loss of image quality due to sun exposure or improper storage of film negatives, slides, and prints. Since digital images are stored as data on a computer, the image never loses visual quality, detail, or fidelity as long as the digital media upon which it is stored remains intact. The only way to ruin a digital image is to delete the image file, to corrupt or re-write some of the image file's data, or to damage or destroy the electronic storage media (hard drive, disk, CD, flash card, etc.) upon which the file resides. As with all computer files, making backups is the most effective way of ensuring that a copy of a digital image can always be recovered.
It is likely that film will never again be purchased and used on the scale it was for most of the 20th century. However, it probably will not disappear altogether. At its advent at the start of the 20th century, many believed photography would supplant the painting of portraits and landscapes. In the same way that acrylic and oil paint are still dominant media in use by artists and hobbyists, it's likely that photographic film and equipment will continue to be an option for enthusiasts. It is also important to note that the difference between film and digital photography is far less than the difference between painting and film photography.
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